This seminar is presented as a part of weekly journal club and seminar presented in Apollo Hospital,Kolkata Department of Radiation Oncology.This seminar is moderated by Dr Tanweer Shahid.
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ICRU 83
1. INTERNATIONAL COMMISSION ON
RADIATION UNITS AND
MEASUREMENTS (ICRU) REPORT 83
DR. JOYDEEP BASU
REGISTRAR
DEPARTMENT OF RADIATION ONCOLOGY
APOLLO GLENEAGLES HOSPITALS
2. HISTORY
Originally known as International X-Ray Unit Committee & later as International Committee
for Radiological Units .
Conceived at 1st International Congress of Radiology (ICR) at London in 1925.
Officially came into being at ICR – 2 at Stockholm in 1928.
Meetings were initially held every 3 years.
Post 1950 meetings are held annually.
3. OBJECTIVES OF ICRU
Development of internationally accepted recommendations regarding:
(1) Quantities and units of radiation and radioactivity;
(2) Procedures suitable for the measurement and application of these quantities in diagnostic
radiology, radiation therapy, radiation biology, nuclear medicine, radiation protection, and
industrial and environmental activities;
(3) Physical data needed in the application of these procedures, the use of which assures
uniformity in reporting.
5. ICRU REPORT 29
• Target Volume : Volume containing those tissues that are to be irradiated to a specified
absorbed dose according to a specified time dose pattern.
Takes into account a) expected movements (eg :- breathing), b) expected variations in
shape & size of the target volume during course of treatment (eg :- urinary bladder,
stomach), c) inaccuracies in treatment set up.
• Treatment Volume : Volume enclosed by the isodose surface representing the minimal target
dose.
• Irradiated Volume : Volume receiving a dose considered significant in relation to normal tissue
tolerance.
• Organ at Risk : Radio-sensitive organs in or near the target volume that influences treatment
planning and/or prescribed dose.
• Hot – Spot : Tissues outside the target area that receives > 100% of specified target dose. Only
clinically meaningful if the area is atleast > 2 cm2 in a section.
6. ICRU REPORT 50
VOLUMES
• Volumes defined prior to treatment planning :
a) GTV
b) CTV
• Volumes defined at the time of treatment planning :
a) PTV
b) OAR
• Volumes as a result of treatment planning :
a) Treated volume
b) Irradiated volume
7. ICRU REPORT 50 CONTD.
• Gross Tumor Volume : The GTV is the gross palpable or visible/demonstrable extent and location of the malignant
growth.
• Clinical Target Volume : The CTV is a tissue volume that contains a GTV and/or subclinical microscopic malignant
disease, which has to be eliminated. This volume has to be treated adequately in order to achieve the aim of the
therapy: cure or palliation.
• Planning Target Volume : The PTV is a geometrical concept, and it is defined to select appropriate beam sizes and
beam arrangements, taking into consideration the net effect of all the possible geometrical varaitions and
inaccuracies in order to ensure that the prescribed dose is actually absorbed in the CTV.
• Organs at Risk : The OAR are normal tissues whose radiation sensitivity may significantly influence treatment
planning and/or prescribed dose.
• Treated Volume : The TV is the volume enclosed by an isodose surface, selected and specified by the radiation
oncologist as being appropriate to achieve the purpose of treatment (e.g., tumor eradication, palliation).
• Irradiated Volume : The Irradiated Volume is that tissue volume which receives a dose that is considered
significant in relation to normal tissue tolerance.
8. ICRU REPORT 50 CONTD.
• Dose Variations in PTV
When a dose to a given volume is prescribed, then the corresponding delivered dose should be as homogeneous as
possible.
In the best technical and clinical conditions the degree of heterogeneity must be kept within + 7% and – 5% of the
prescribed dose.
• Maximum Dose (Dmax)
One can identify the maximum dose within the PTV & outside the PTV.
Maximum dose to normal tissues is of importance for limiting and for evaluation of side effects of treatment.
A Hot Spot represents a volume outside the PTV, which receives a dose larger than 107 % of the specified PTV dose.
Generally considered significant only if the minimum diameter exceeds 15 mm. If it occurs in small organs, a dimension
smaller than 15 mm has to be considered.
• Minimum Dose (Dmin)
Minimum dose is the smallest dose in a defined volume.
9. ICRU REFERENCE POINT
CRITERION
• The dose at the point should be clinically relevant and representative of the dose
throughout the PTV.
• The point should be easy to define in a clear an unambiguous way.
• The point should be selected where the dose can be accurately determined. (Physical
accuracy.)
• The point should be selected in a region, where there is no steep gradient.
These recommendations would be fulfilled if the ICRU reference point is located firstly at the
center or the central part of the PTV and secondly on or near the central axis of the beam.
The dose at the ICRU reference point is the ICRU Reference Dose and should always be reported.
10. ICRU 62
• Published five years after ICRU Report 50.
• Defines additional concepts and formulates more accurately some definitions related to volumes,
margins, organs at risk, dose variations and uncertainties.
• Introduces the concept of conformity index.
• Classifies Organs at Risk.
• Introduces planning organ at risk volume.
• Gives recommendations on graphic.
• Does not contradict recommendations of Report 50, but rather reflects the developments that have
occurred since 1993.
11. INTERNAL MARGIN AND SET UP MARGIN
• Internal margin : A margin must be added to the CTV to compensate for expected physiological
movements & expected variations in size, shape & position of the CTV during therapy.
EXAMPLE : Respiration, swallowing, rectal/bladder filling
• Set up margin : It accounts for the uncertainties in patient positioning and aligning of
therapeutic beams.
Example : Uncertainties in patient positioning, mechanical uncertainties, dosimetric
uncertnities, human factor etc.
12. CLASSIFICATION OF ORGAN AT RISK
• Serial Organ : Functional subunits (FSUs) are arranged in a chain like pattern. Damage to one
FSU will lead to total dysfunction of the organ. eg : spinal cord
• Parallel Organ : Functional subunits are independent of each other. Damage to certain
proportion of FSUs are required for the morbidity. eg : liver, lung
• Serial – Parallel Organ : Have features of both. eg : kidney
• Planning Organ at Risk Volume (PRV) : An integrated margin must be added to the OAR to
compensate for variations including the movement of organ as well as setup uncertainties
13. GRAPHICS
• In this report, the following convention is recommended for the use of colours to depict the
volumes, beam geometry & the distribution of absorbed dose.
• GTV - Dark Red
• CTV - Light Red
• ITV - Dark Blue
• PTV - Light Blue
• OR - Dark Green
• PRV - Light Green
• Landmarks - Black
16. CONVENTIONAL VS 3D – CRT VS IMRT
• Conventional Radiotherapy
1) Uses a number of coplanar beams.
2) Beam shaping by using customized blocks.
3) Use of wedges for producing desired dose distribution.
• 3D – Conformal Radiotherapy
1) Uses 3D planning techniques & special delivery systems to shape the fields – to reduce normal tissue
damage close to the target volume.
2) Uses a larger number of beams.
3) Beam shaping done by MLC s.
• Intensity Modulated Radiotherapy
1) Large number of beams are used from different directions.
2) Each beam divided into number of beamlets whose intensity can be modulated.
3) Intensity modulation & beam shaping done by MLC s.
17.
18. EVOLUTION OF IMAGING IN RADIOTHERAPY
X – Ray simulator based planning (based on bony landmarks)
CT scan based planning (contouring of different volumes on CT slices)
Functional imaging ( MRS, PET to delineate the functional GTV.
Extra dose can be delivered to the sub GTV)
4-D CT & Adaptive planning (account for physiological tumor movement,
Change in shape, size & position of tumor & OAR s over the course of treatment)
19. In 3D – CRT planning beam modification done by
1) Changing Beam Boundaries
2) Changing beam Directions
3) Using Beam Modifiers
Quality of the absorbed dose distribution & the
resultant DVH depends on experience of the planner.
20. INVERSE PLANNING
• Word Inverse derived from mathematical inverse problem solving technique
Inverse treatment planning starts from a set of descriptors ie. Desired absorbed dose to the PTV & OARs
Optimization procedure is an iterative search for the solution that minimizes the cost & maximizes the goodness
Planner adjusts the values of the descriptors to achieve the goal.
OPTIMISED PLAN
Thus the term optimized planning is adopted instead of inverse planning in this report
22. CONSTRAINTS
• Hard Constraints : Restrict the solutions to those that are feasible. They cannot be violated
eg : Forbidding negative beam intensities, restriction of beam size & beam direction
• Soft Constraints : These are malleable and allows to achieve clinical goals.
eg : Dose volume uniformity and other dose volume criteria
Thus hard constraints bound the solution space while the soft constraints define a global
minimum or ‘ best solution ‘ for a given objective function. When global minimum cannot be
established in a realistic time, a local minimum is accepted.
23. The weights for the two beams define a 2 D
solution space.
The optimal solution resulting from the use of
objective function are superposed upon the region
of feasible solution. Two minima are indicated, only
one of which is the best solution (“ global minima”).
24. OBJECTIVE FUNCTIONS
• Objective Functions used for IMRT are based on soft constraints obtained from dose and
volume criteria using least-square minimization.
• Both target tissue and normal tissue can use the same function.
• Idea is to minimize the squared differences between the assumed administered absorbed dose
and the user defined constraints for both PTV and normal tissues.
• Weighting factors reflect the relative importance of a tissue type.
• Weighting factors are normalized to the number voxels that make up the tissue type, so that
the small important structures are not under represented.
25. OBJECTIVE FUNCTION
Where
• IPTV & IPRV are relative importance of the PTV & PRV respectively
• TPTV & TPRV are the number of voxels contained within the PTV & PRV
respectively
• di – Assumed administered dose.
• D-
PTV & D+
PRV are the minimum dose to the PTV & maximum dose to the
PRV respectively.
• c-
PTV & C+
PRV are set to zero when a voxel has met a constraint & set to
unity when a voxel has not met the constraint
• The summations are over only those voxels (labelled i) that are contained
within the PTV or the PRV.
26. OBJECTIVE FUNCTION AND OPTIMIZATION
• Higher values assigned to PTV importance will prevent low absorbed dose in the target areas
at the expense of normal tissue.
• It is recommended that dose constraints in OAR should be kept low & OAR importance be
increased till dose homogeneity in PTV is compromised.
• Multiple Dose – Volume constraints can be used for a single structure.
• For more than one constraints for a structure, “Constraint Importance” factor should be
included in the summation.
• Multiple constraints for each structure provide control on the shape of cumulative DVH.
27. ITERATION
• Iteration : Act of repeating a process , either to generate an unbound sequence of outcomes , or with the aim of
approaching a desired goal, target or result. Each repetition of the process is also called iteration and the results of one
iteration are used as the starting point for the next iteration.
Optimization starts from an initial set of parameters.
Change in relative importance of an organ & penalty on the absorbed dose optimizes dose distribution of the organ.
Several cycles of iteration produces an acceptable plan.
Additional iteration cycles with changes in optimization parameters might provide overall improvement of absorbed dose
distribution.
This procedure stops when there is no further improvement.
Thus IMRT optimization is highly dependent on the experience of the planner.
28. Two snapshots from a treatment-
planning system of the
optimization process after the first
and second cycles of iteration for
optimized planning for prostate
cancer.
29. ITERATION APPROACHES
Deterministic Method
• With the same set up & initial condition,
same solution will always be found. At
each step the value of the objective
function is smaller than the last.
• Faster process.
• If there are multiple local minima, there is
no assurance that global minima will be
reached.
Stochastic Method
• Randomly selected iterative steps test areas of
the solution space that are less optimal than
previous one.
• Slower process.
• Stochastic method is capable of finding the
global minimum if there is unlimited time to
search the parameter space.
30. BEAMLET OPTIMIZATION
• Each field or beam is subdivided into a grid of sub – beams or beamlets,
each with a characteristic intensity.
• Best suited for dynamic MLCs .
• Optimal intensity map is generated.
• Information translated to instruction for delivery – Leaf Segmentation.
• Final absorbed dose calculation and MU calculation.
• Difference between optimized plan and final calculation – Convergence Error.
31. APERTURE BASED OPTIMIZATION
• Apertures are the projection of the PTVs.
• Apertures are modified iteratively or new apertures are created to elevate the dose in the
target & reduce the dose in the normal tissue.
• No leaf segmentation step.
• Same dose calculation algorithm is used for optimization & final absorbed dose calculation –
avoidance of Convergence Error.
• Similar process used in 3-D CRT planning system (Forward Optimization).
32. LEVELS OF PRESCRIBING AND REPORTING
• Level 1 : Minimum standard required for performing radiotherapy. Includes knowledge of
absorbed dose on the central beam axis & simple 2 D absorbed dose distribution at the central
axis.
• Level 2 : Prescribing & reporting state-of-the-art techniques. Includes delineation of different
volumes on CT scan & MRI, availability of 3 D absorbed dose distribution including
heterogeneity correction, DVH of all volumes & a complete QA program.
• Level 3 : Optional research and development reporting – Development of new techniques
and/or approaches for which reporting criteria are not yet established. Eg :Tumor Control
Probability, Normal Tissue Complication Probability, Equivalent Uniform Dose etc.
33. ICRUREFERENCE POINT AND ICRUREFERENCE DOSE
In ICRU Report 50 (ICRU, 1993), the process for the selection of an ICRU Reference Point
was specified as follows:
• The absorbed dose at the point should be clinically relevant;
• The point should be easy to define in a clear and unambiguous way;
• The point should be selected so that the absorbed dose can be accurately determined;
• The point should be in a region where there is no steep absorbed-dose gradient.
These recommendations will be fulfilled if the ICRU Reference Point is located:
• Always at the centre (or in a central part) of the PTV and
• When possible at the intersection of the (treatment) beam axes.
34. DRAWBACKS OF POINT DOSE REPORTING IN IMRT
• The absorbed-dose distribution within a PTV for IMRT can be less homogeneous than in conventional
radiation therapy. Dose reporting at a point in a region of high or low absorbed dose could be
misrepresentative.
• IMRT produces a larger dose gradient within a PTV than a wedge.
• In IMRT there is a sharp dose fall off at the boundary. So a small shift in the field delivery affect the
reliability of a single point dose reporting.
• In Monte Carlo simulation, the statistical fluctuations in the results for small volumes make it difficult
and uncertain to determine an absorbed dose at a point.
• Modern treatment-planning systems have sufficient evaluation tools for Level 2 reporting to be the
standard for use in IMRT.
35. DOSE – VOLUME HISTOGRAM
• DVH is a concise although simplified way to present a dose volume relationship within a volume of interest.
• Visual inspection lead to identify presence of high or low dose or other absorbed dose heterogeneity (not the location).
• DVH – a) Differential
b) Cumulative – i) Absolute, ii) Relative
• Differential DVH – Gives the volume receiving a particular dose.
• Cumulative DVH – Histograms of the volume elements receiving at least a given absorbed dose.
They are usually expressed as absolute volume or volume relative to the total structure volume, receiving
at least a given absorbed dose.
• Absolute cumulative DVH – Relative cumulative DVH x Volume.
36. PLOTTING IN A DVH
• Dmin - The first defined point located at the intersection along the horizontal line representing
100 % volume and the vertical line representing the minimum calculated absorbed dose.
• Dmax – The last point in the DVH curve located at the intersection along the horizontal line
representing 0 % volume (along the abscissa) and the point of maximum calculated absorbed
dose.
• Dmedian – Absorbed dose received by 50% of the volume.
• Mean absorbed dose – Amount of energy imparted to the PTV divided by the PTV mass.
38. DOSE VOLUME SPECIFICATION FOR TREATMENT PLAN
• DV – Absorbed dose that covers a specified fractional volume (V).
• Minimum Absorbed Dose (D100%) – Absorbed dose in single or few voxels. Often difficult to
determine as
a) Located in High Gradient region at the edge of the PTV.
b) Highly sensitive to accuracy of calculation, delineation of CTV & PTV.
• Near Minimum Absorbed Dose (D98%) – More accurately determined & should replace D100% .
Clinically significant if located well within the PTV margin.
• Near Maximum Absorbed Dose (D2%) – Often reported as a replacement for the maximum
absorbed dose.
39. DOSE VOLUME SPECIFICATION FOR TREATMENT PLAN CONTD.
• Median Absorbed Dose (D50%) – Absorbed dose in 50% of the volume. Rationale for reporting
the median absorbed dose is to report an absorbed dose that is largely representative of the
absorbed dose in the PTV.
• D95% - Absorbed dose in 95% of the volume. May be reported in addition to D98% .
• D50% & D2% of CTV are similar to these metrics for the PTV.
• D98% of the CTV & PTV diverge.
• Clinician can report other DV if deemed clinically significant.
40. DOSE VOLUME SPECIFICATION FOR TREATMENT PLAN CONTD.
• Previous ICRU Recommendation –
Absorbed Dose in the PTV be confined within 95% - 107% of the prescribed absorbed dose
(ICRU 62).
• In IMRT these constraints should not be followed if avoidance of normal tissue is more
important than target dose homogeneity.
• ICRU 83 – Extent of high & low dose regions are specified using Dose – Volume metrics like D2%
& D98% respectively.
• In IMRT small regions of low or high dose can develop when avoidance of sensitive structure is
of prime importance.
41. ORGAN AT RISK (OAR)
• Serial – like structure – Spinal cord, Esophagus
• Parallel – like structure – Liver, Lung
• Serial – Parallel - like structure - Kidney
42. DOSE METRICS FOR OAR
Parallel – Like Structure
• Damage to a sizeable volume can be tolerated without a complication being developed.
• Whole organ should be contoured.
• More than one dose volume specification be reported.
• Mean absorbed dose is an useful measure.
• VD should be reported where D is the absorbed dose to a certain percentage of total volume
which if exceeded causes serious complications (eg :- V20 of lung).
• Median dose should not be used because of inhomogeneous dose to the OAR.
43. DOSE METRICS FOR OAR CONTD.
Serial – Like – Structure
• Destruction of a single functional unit is sufficient to cause dysfunction of entire organ.
• Dmax. is important.
• Not easy to establish minimum dimension for the maximum absorbed dose region.
• Report recommends to report D2%.
• Contouring of only portion exposed to high dose will result in higher estimate of D2%.
• For incomplete delineation, anatomic description of the delineated region should be
mentioned.
44. DOSE METRICS FOR OAR CONTD.
Serial – Parallel Like structure
• Features of both serial & parallel organ.
• D2%, DMean & VD should be reported.
• Other specifications deemed clinically relevant by radiation oncologists may be mentioned.
45. REPORTING OF TREATMENT FIELD DELIVERY
• In IMRT daily absorbed dose to different treatment volumes are different.
• Biological effect varies accordingly.
• Recommended that all fields are delivered on all days.
• If not, exact nature of treatment delivery be clearly reported.
46. REPORTING OF SOFTWARE VERSION
• Level 2 Reporting includes –
1) Make, model & software version of the treatment planning system.
2) Information on optimizer software.
3) Information on treatment delivery software.
47. DOSE HOMOGENEITY & DOSE CONFORMITY
• Dose Homogeneity :- Characterizes the uniformity of the absorbed
dose distribution within the target volume.
• Dose Conformity :- Characterizes the degree to which the high dose
region conforms to the target volume, usually the PTV.
48. DETERMINATION OF HOMOGENEITY INDEX (HI)
• HI – Ratio of maximum absorbed dose to the prescription dose. Usually used in radiosurgery.
Formula only indicates the magnitude of overdosing & not the underdosing.
• HI – Ratio of fraction of the CTV with an absorbed dose > 95% and fraction receiving < 107%
of the prescribed dose. Does not indicate the magnitude of underdosage or overdosage.
• HI - D2% - D98% / D50%. Often used for 3D – CRT & IMRT plan evaluation.
49. DOSE CONFORMITY
• Conformity can be achieved between the high dose volume & the PTV in 3D – CRT & IMRT.
• Concave dose distribution is a hallmark of IMRT.
• Conformity Index – VRI / TV where TV – Treated Volume, VRI – Reference isodose volume.
CI – must be between 1 – 2, CI of 0.9 – 1 & 2 – 2.5 means minor violation, CI of < 0.9 &
> 2.5 means major violation.
• Increasing availability & use of DVH formats for dose reporting, make these indices less
relevant in IMRT.
51. CLINICAL & BIOLOGICAL EVALUATION METRICS
• Report recommends developing biologically based quantities to provide additional
quantitative tools for radiation oncology.
1) Tumor Control Probability (TCP)
2) Normal Tissue Complication Probability (NTCP)
3) Equivalent Uniform Dose (EUD)
54. GROSS TUMOR VOLUME ( GTV )
• GTV is the gross demonstrable extent & location of the tumor.
• GTV consists of
1) Primary Tumor GTV or GTV – T
2) Nodal GTV or GTV – N
3) Gross Metastatic Disease or GTV – M
• When Primary tumor & Metastatic node cannot be separated – a single GTV encompassing
both may be delineated.
• In case of post – operative radiation, after assumed complete surgical resection (R0 or R1)
there is no GTV to define.
• GTV can occur for non – malignant lesions treated with radiation (Glomus Tumor in Carotid
body, Arterio – venous malformation & Pituitary Adenoma).
55. IMPORTANCE OF GTV DELINEATION
• Required for staging (TNM system)
• An adequate absorbed dose must be delivered to the whole GTV for local tumor
control.
• Evaluation of regression of GTV needed for redefining the CTV & PTV.
• Changes in the GTV during treatment might be predictive of treatment outcome.
56. REPORTING OF GTV
• Location & Tumor extent according to TNM/AJCC cancer staging system & the WHO
International Code for disease in Oncology (ICD-O).
• Method used to delineate the GTV should be mentioned.
• Use of Functional imaging like PET, Functional MRI reveal some key biological factors
(metabolic status, hypoxia, cellular proliferation) that are likely to impact treatment outcome.
• Functional information used to define sub – GTVs that are to receive additional dose.
• Any change occurring in the GTV during treatment can be quantified with anatomic and/or
functional imaging techniques. Modified GTV might be used to adjust the absorbed dose
distribution.
57. REPORTING OF GTV CONTD.
• Reporting should include
a) Component of GTV (primary, metastatic node etc.)
b) Timing of GTV delineation
c) Diagnostic model used to delineate the GTV.
• GTV-T (clin, 0 Gy): tumor GTV evaluated clinically before the start of the radiotherapy.
• GTV-T (MRI-T2, 30 Gy): tumor GTV evaluated with a T2-weighted MRI scan after an absorbed
dose of 30 Gy of external beam irradiation.
58. COMPARISON AMONG VARIOUS
MODALITIES FOR THE
DEFINITION OF THE PRIMARY
HEAD-AND-NECK TUMOR GTV.
DELINIATION OF GTV WITH CECT, MRI & PET
AT VARIOUS STAGE OF TREATMENT
59. COMPARISON AMONG VARIOUS
MODALITIES FOR THE DEFINITION
OF THE PRIMARY RECTAL TUMOR
GTV.
DELINIATION OF GTV WITH CECT, MRI,
FDG PET & F – MISONIDAZOLE PET AT
VARIOUS STAGE OF TREATMENT
60. CLINICAL TARGET VOLUME (CTV)
• CTV is a volume of tissue containing a demonstrable GTV & / or subclinical malignant disease with a
certain probability of occurrence considered relevant for therapy.
• Includes microscopic tumor spread at the boundary of the primary tumor.
• Includes possible infiltration of lymph nodes.
• Includes potential metastatic involvement of other organs despite their normal appearance on clinical
& radiological appearance.
• No CTV is associated with benign tumor GTV.
• Post R0 or R1 resection, only CTV can be delineated.
61. SELECTION OF CTV
• Depends on knowledge of surrounding anatomy :
a) Muscle fascia, Bony cortex are natural barrier to tissue infiltration. Eg:- In Head & Neck
cancer, para – laryngeal, para - pharyngeal spaces bounded by natural barriers – limits
tumor spread.
b) Fatty spaces are easy conduits for tumor spread.
• Depends on the biological & clinical behavior of the tumor. Eg:- Sarcoma – less propensity for
lymph nodal spread – Post op CTV includes post op tumor bed using the principle of fascial
confinement.
62. DELINEATION OF CTV
• 3 D delineation of CTV for both the primary tumor GTV & nodal GTV should be guided by
published recommendation.
• Aim is to translate regions at risk for microscopic dissemination into boundaries easily
identifiable on planning CT / MRI.
• Each malignant tumor GTV associated with a CTV.
• Several contiguous GTVs – associated with same CTV. Eg:- multiple contiguous lymph nodes in
level II & III.
• For sub GTVs delineation – whole anatomic GTV is surrounded by a common CTV.
63. CLINICAL TARGET
VOLUME
Macroscopic section (left) and
microscopic view (right) of a
breast carcinoma after surgical
removal. Tumor-cell projections
into the surrounding fatty tissue
are marked with arrows.
64. INTERNAL TARGET VOLUME (ITV)
• Concept introduced in ICRU 62.
• ITV = CTV + Margin taking into account uncertainties in size, shape & position of the CTV within
the patient.
• Margin is called Internal Margin (IM).
• ITV is important only when uncertainty in the CTV location dominates Set – Up uncertainties
and/or when they are independent.
• It is an optional tool helping to delineate the PTV.
65. PLANNING TARGET VOLUME (PTV)
• Concept was first introduced in ICRU 50.
• Geometrical concept for treatment planning & evaluation.
• Margin takes into account both the internal and the set up uncertainties.
• Recommended tool to shape absorbed dose distribution to ensure that the prescribed dose is
delivered to all parts of CTV with a clinically acceptable probability despite geometrical
uncertainty.
• Used for absorbed dose prescription & reporting.
66. FACTORS AFFECTING PTV MARGIN
• Internal Variation :
a) Variation in size & shape of the CTV.
b)Variation in anatomic site.
c) Protocol Variation (eg : bowel preparation)
d) Patient specific differences.
• External Variation :
a) Patient positioning.
b) Mechanical uncertainties (eg : sagging of gantry, couch & collimators)
c) Dosimetric uncertainties (penetration of beam)
d) Transfer error from CT & simulator to the treatment unit.
e) Human factor
67. Illustration of the impact of rectal
volume [increase due to either gas
(middle) or stool (right)] on
prostate CTV displacement.
68. STEPS TO REDUCE UNCERTAINTIES
• Use of patient immobilization.
• Application of Quality Assurance program.
• Skill & experience of the radiation therapy technologists.
• Use of daily image guidance
• Following same treatment protocol for all patients (eg : bladder or bowel protocol).
70. ORGAN AT RISK
• Definition : OAR or critical normal structure are tissues that if irradiated could suffer significant
morbidity & thus might influence the treatment planning and/or the absorbed dose
prescription.
• Normal tissue considered as OAR – depends on location of CTV and/or prescribed absorbed
dose.
• Concept of tissue organization is useful for determining Dose – Volume constraints.
• In IMRT dose distribution is heterogeneous larger volume of normal tissue
irradiated optimization process require enhanced consideration of biological
response of normal tissue.
71. CONTOURING OF ORGAN AT RISK
• Serial – like organ : Volume irradiated has less impact clinically. Contouring of entire organ may
not be needed. But guidelines must be followed for comparisons between centers.
• Tubular type organs : Preferred to delineate wall or surface rather than the entire volume.
• Parallel – like organ : Volume assessment is crucial & complete organ delineation is required.
72. DVH of Rectum & Rectal
wall are different, but
difference is minimal for
their respective PRV
73. Dose – Volume comparison of
whole parotid & superficial lobe of
parotid. The mean absorbed dose
reached 26.5 Gy & 21.8 Gy
respectively for the whole &
superficial lobe of parotid.
74. PLANNING ORGAN AT RISK VOLUME
• Needed for uncertainties & variations in the position of the OAR during treatment.
• More relevant for serial – like structure.
• PRV margin should not be compromised even for PTV – PRV overlap.
• Normal tissue sparing done by priority rules planning system.
• For reporting, PRV be described including the size of the margin around the OAR.
76. TREATED VOLUME (TV)
• Definition : The TV is the volume of tissue enclosed within a specific isodose envelope, with
the absorbed dose specified by the radiation oncology team as appropriate to achieve tumor
eradication or palliation, within the bounds of acceptable complications.
• Concept introduced by ICRU 62.
• D98% could be selected to determine Treated Volume.
• Evaluation is important to find the cause of local recurrence (inside or outside TV).
77. REMAINING VOLUME AT RISK (RVR)
• In IMRT all normal tissue that could potentially be irradiated should be outlined.
• RVR = Imaged volume within the patient – any delineated OAR & CTV (s).
• If not contoured there may be areas of unsuspected regions of high absorbed dose.
• Useful in estimating the risk of late effects such as carcinogenesis.
78. PLANNING AIMS, PRESCRIPTION & TECHNICAL DATA
• Planning Aim : Initial specifications of the desired absorbed dose to various delineated
volumes of interest.
• Planning Prescription : Complete set of finally accepted values after complex beam delivery
optimization (modification of initial planning aims).
• Accepted Treatment Plan : Final Plan Prescription + Required Technical Data.
79. PLANNING AIM
• Planning Aims are dosimetric goals defined for various volumes. Eg : PTV, PRV etc.
• Multiple dose – volume constraints for each volume lead to more precision in planning aims.
• For study purpose biological metrics (TCP, NTCP, EUD) may be used as additional constraints.
• Process of optimization involves prioritization of one constraint over another and/or one
volume over other.
• Initial planning aims might be physically unachievable (steep dose gradient, overlap of
volumes).
• Initial set of constraints & objectives can evolve to achieve an acceptable plan.
80. • Color legend is: the CTV in orange, the
PTV in red, the PRV rectum in green,
the PRV bladder in dark blue, and the
PRV femoral heads in light blue.
• PTV overlaps with the PRV rectum and
the PRV bladder.
• Resolution of these conflicts is
achieved by selecting various sets of
priorities for the PTV and the PRV
and/or by using different dose-
constraints on the overlapping region
between the PTV and the PRVs.
81. SPECIAL SITUATIONS
1. Planned absorbed dose in the buildup region and in a PTV extending outside the body
contour.
2. Overlapping volumes and conflicting absorbed dose objectives.
3. Unexpected high absorbed dose to part of the remaining volume at risk (RVR).
82. PTV EXTENDING TO THE BUILD UP REGION OR PTV IN AIR
Problems of Optimization
1. Difficulty in achieving desired dose in the build up region or in PTV extending in the air.
2. Attempts to increase dose in these areas leads to inhomogeneous or high absorbed dose
elsewhere.
3. Flash will not be created by bounding PTV expansion by skin contour.
83. SOLUTIONS FOR OPTIMIZATIONS
Solutions for Optimization :
• Under dosage of CTV near the skin clinically unacceptable Use BOLUS
• Under dosage of CTV near the skin clinically acceptable
• Flash Region – Intensity values from the periphery is extrapolated to the regions of the PTV in
the air.
Division of PTV in to number of
Subvolumes
Accept under dosage in one of the
Subvolumes
Increasing the acceptable absorbed
dose range for the whole PTV
84. (A) Beam’s eye view (BEV) of a conventional tangential field (dashed outline). The
blue contour shows the PTV extending outside the breast to secure flash.
(B) IMRT optimization is performed on the part of the PTV a few millimeter inside
the skin surface to avoid (unwanted) absorbed-dose compensation in the build-
up region by the optimizer. No intensity is assigned to beamlets projecting
outside the BEV of the breast into the PTV; thus flash is not secured.
(C) Creation of flash by extending the same intensity values from the breast
periphery to the regions of the PTV outside the breast BEV.
85. OVERLAPPING VOLUMES
• Overlap between different PTVs, between PTV & PRV or between different PRVs common in
IMRT.
• Conflict occurs in Planning Aims.
• Optimization solution –
a) Sub-volumes are defined within the PTV Different absorbed dose
objectives are set for each subvolumes.
b) Absorbed dose objectives are relaxed for one or more of the contoured volumes
that exhibit overlap.
87. REMAINING VOLUME AT RISK
• Absorbed dose objectives are set for PTVs & PRVs.
• For plan optimization, additional dose may be dumped in RVR.
PROBLEMS
• Proper dose prescription for the RVR.
HIGH ABSORBED DOSE IN
RVR
GENTLE ABSORBED DOSE
GRADIENT BETWEEN PTV & RVR
88. TREATMENT PLAN
TREATMENT PLAN
• Physician approval of a given plan implies approval of the technical aspects of the treatment.
• Laws require plan approval from additional personnel (eg :- medical physicist)
PRESCRIPTION – Complete set of finally optimized
planning aim
TECHNICAL DATA – Data to execute the treatment.
Eg:- No.of beams, aperture shape, Monitor Units etc.
90. PHOTON INTERACTIONS AND ENERGY-DEPOSITION
PROCESSES
• Interaction of photon with head unit produces charge particle contamination.
• Have limited penetration.
• Dosimetry to be done beyond the range of charged particle contamination.
• Energy imparted by charged particles released from first interactions in the patient of photons from the
incident beam constitutes the primary absorbed dose.
• Drastic change in absorbed dose occurs for – a) small change in small field, b) low density material & c)
higher energy beams.
• Photons scattered in patient constitutes patient – or phantom scatter dose.
91. The phantom is a slab phantom comprising adipose
tissue (A), muscle (M), bone (B), and lung (L).
Greater dose perturbation for smaller field size & low
density material.
92. DOSE CALCULATION ALGORITHM
Correction based approach
• The absorbed dose in a water phantom from a
rectangular beam incident normally on the
surface of the phantom is first measured.
• Parameterized into absorbed-dose
distributions as functions of the distance from
the source to the surface of the phantom, field
size, depth, and lateral position.
• Correction needed for beam modifying
devices, beam not incident normally on a flat
surface & for tissues not well simulated by
water.
Model based approach
• Measurements of the absorbed dose distributions
for a variety of situations.
• Data used to develop parameters for a model that
describes the attenuation of incident photons & the
production of secondary electrons
93. PRINCIPLES OF MODEL BASED ALGORITHM
• Direct computation of the absorbed dose per energy fluence in the patient.
• Convolution or Superposition methods – transport kernels are generated by MONTE CARLO
Simulation.
• MONTE CARLO SIMULATION – Direct simulation of the particle transport.
• In MONTE CARLO simulation dose is calculated in water equivalent material which mimics the
soft tissue.
• For dose calculation in bone, the correction is made by scalar multiplication using a ratio of
average mass collision stopping power of water to bone.
94. CALCULATIONOF ABSORBED DOSE PER MONITOR UNIT
• Monitor Unit – used in place of Exposure time (used for 60 Co radiation).
• Monitor Unit – Integrated (ionization reading) from a linear accelerator’s coxial parallel plate
ionization chamber.
• The monitor ionization chamber used for photon beams is normally sealed so that it is immune
to the effects of temperature and pressure.
• The absorbed dose per monitor unit is normally calibrated with respect to the absorbed dose
measured in a phantom under standard irradiation conditions.
• Planning process determines the number of Monitor Units to be delivered in each field.
95. CALCULATIONOF ABSORBED DOSE PER MONITOR UNIT CONTD.
• Segmented MLC IMRT Delivery – Monitor Unit for each segment Relative output of the
field + Contribution from indirect sources + back scatter from collimator system.
• Dynamic MLC IMRT Delivery - Monitor Unit delivered refer to the total ionization signal
accumulated during the dynamic treatment requiring careful synchrony of the leaves so that
the leaf pattern is completed exactly when the total monitor units have been delivered.
• Binary Collimator – Fluence delivered is nearly proportional to the time for which leaf is
retracted. Correction is done for indirect photons from a neighboring open leaf & for the
transit time.
96. Energy Fluence per MU Absorbed Dose per unit
energy fluence for field
size A & reference depth r
Correction for Back Scatter
97. cal – Beam calibration geometry
Acal - Calibration field size (10 cm x 10 cm square field)
rcal - Reference depth (10 cm)
[ 1+ b(Acal)] – back scatter correction
98. QUALITY ASSURANCES OF IMRT
• QA of IMRT delivery systems :
i) Conventional MLC delivery system
ii) Binary MLC delivery system
• Patient specific QA
99. QUALITY ASSURANCES OF CONVENTIONAL MLC DELIVERY
SYSTEM
(1)Small-field penumbra measurement/modelling
(2) Small-field output factors
(3) Leaf-gap offset factor to correct discrepancies between the light field and radiation field
(4) MLC leakage/transmission factors
(5) Leaf-sequencer accuracy
100. SMALL FIELD PENUMBRA MEASUREMENT /
MODELLING
• Multiple small field is characteristic of IMRT.
• Penumbra modelling inaccuracies of each small field significantly affects final absorbed dose
distribution.
• Coarse ionization chambers produces erroneous results.
• Penumbra profiling recommended using radio-chromic films or other detectors with high
spatial resolution.
101. SMALL FIELD OUTPUT FACTOR
• Output factor decreases sharply for small fields (3 cm x 3 cm)
a) Obscuration of primary source
b) Lack of lateral electronic equilibrium
c) Less Head & Phantom scatter
d) Increased collimator back scatter
• Essential to compare calculated dose for small fields with measured data with small field
detector.
• Restriction of lower limit of field size needed for IMRT.
102. LEAF – GAP OFFSET
• Curved end leaves – Mismatch between the field defined by light & actual radiation field.
• Leaf Gap Offset is the distance that the leaf has to move away from its light field image to
match the radiation field.
• Depending on offset there may hot or cold spot between adjacent strips.
105. LEAF LEAKAGE & TRANSMISSION
• Leaf Leakage
a) Inter – leaf
b) Intra – leaf
c) Leakage between opposed leaf ends
• Dosimetric Leaf Gap – Dosimetric measure of the effective gap that results from transmission through
curved leaf ends of a pair of abutting leaves.
• Dosimetric Leaf Offset – Amount that the leaf would need to retract to add the same fluence as is
transmitted through the rounded leaf end.
• These factors need to be considered during treatment planning & should ideally not change as a
function of time.
106. LEAF SEQUENCER ACCURACY
• Leaf Sequencing Algorithm – used to convert the treatment planning- system-derived intensity
maps into a deliverable set of MLC leaf sequences.
• Physicist can adjust the number of intensity level & MLC step size for different patient &
different disease size.
• Smaller step size & multiple intensity level result in many small field size.
• Places greater importance on MLC positioning accuracy, monitor linearity and resolution, and
accelerator stability, as well as on potential limitations in absorbed-dose modeling for these
small segments.
107. QUALITY ASSURANCE FOR BINARY MLC
• Test for transition time of Binary Leaf.
• Measurement of influence of neighboring open leaves on the intensity through a particular
open leaf [Fluence output Factor].
• Synchrony of the leaves & gantry.
• Synchrony of the couch & gantry.
108. PATIENT SPECIFIC QUALITY ASSURANCE
• Measurements of intensity from individual beams.
• Measurements of absorbed dose in phantoms.
• Independent absorbed dose calculation.
• In – Vivo Dosimetry.
• Recommendations for accuracy of absorbed dose delivery.
109. MEASUREMENTS OF INTENSITY FROM
INDIVIDUAL BEAMS
Individual beams are directed on to a phantom
Film dosimeter or an array of suitable detectors are placed on block tray or on couch at the
isocenter
Irradiation patterns of each beam generated by planning system
Compared with measured data
If acceptable, treatment can be done
110. LIMITATIONS
• Subtle errors in the intensity pattern of individual field are difficult to detect.
• Absorbed dose in the planar dosimeter does not reflect the 3D absorbed dose distribution in
patient.
• Solution – Comparison of measured planar absorbed dose with calculated planning absorbed
distribution for the same beam & measurement depth using a 2D overlay comparison or using
the gamma value.
111. MEASUREMENTS OF ABSORBED DOSE IN PHANTOMS
• Absorbed dose distributions from all beams can be measured in phantom.
• No single dosimetry system can measure all the absorbed – dose information needed for
patient specific QA.
• Dosimetric system exploits the characteristics of point dosimeter (ionization chamber) for low
dose gradient region & planar dosimeters (film dosimeter) for measuring spatial position of
irradiated area (steep dose gradient area) .
112. ANTHROPOMORPHIC PHANTOM
• Advantage – Similar in size & shape of the patients to be treated.
• About 1/3rd of IMRT QA audited, failed to meet accuracy criteria.
• Reasons –
i) Spacing between TLD chips might be larger than desired.
ii) Difficulty in film preparation for anthropomorphic phantom.
• Cuboid or cylindrical phantom may be more convenient.
114. MEASUREMENTS OF ABSORBED DOSE IN PHANTOMS
• Calibration of a reference field is highly reliable for External Beam Radiotherapy.
• Reference Field Condition – Dose measured at center of 10 x 10 cm unmodulated open field in
water at 10 cm depth with a Source to Surface of water distance of 100 cm.
• IMRT absorbed dose distribution is dissimilar from those in reference field condition.
• Reason – Dose in reference field condition is measured in large field & IMRT consists of large
number of small field.
115. INDEPENDENT ABSORBED-DOSE
CALCULATIONS
• Independent Methods used for absorbed dose distribution in patient.
• Often these algorithms are less accurate than those used for actual dose calculation.
• Can detect large errors.
• For inferior quality algorithm, there may be false – positive or false – negative results for subtle
errors.
• Accuracy of the absorbed dose calculation should at least be as accurate as the Treatment –
Planning System.
116. IN – VIVO DOSIMETRY
• Detectors used – TLD, Diodes etc.
• Absorbed dose measured throughout the treatment time – dosimetry system should be
independent of time dependence & dose rate.
• Detectors placed on the skin near critical structure.
• Limitations :-
a) Measure data at one or few points.
b) May not represent the dose at target volume.
c) Steep dose gradient make the readings more uncertain.
117. RECOMMENDATIONS FOR ACCURACY OF
ABSORBED-DOSE DELIVERY
• Comparison of dose distribution of a voxel between calculated & measured absorbed dose.
• Two criteria used –
a) Absorbed dose difference ( ) used in region of low dose gradient.
b) Distance to Agreement or DTA ( ) for high dose gradient.
• Software determines the gamma value which incorporates both of them.
• Binary pass – fail test & degree of failure is not indicated.
119. rm - rc D(rm) – D(rc) / Dprescription
rm – position of measured absorbed dose
rc – position of calculated absorbed dose
D(rm) & D(rc) – dose at rm & rc respectively
For acceptance of plan gamma value should be =< 1
121. CASE 1
• 59 year old male, smoker presented with hoarseness of voice.
• ECOG – 1.
• FOL – Exophytic lesion extending from right ary – epiglottic fold to right false cord. Right
Hemilarynx fixed.
• Biopsy – Squamous cell carcinoma.
• MRI neck – Infiltration of pre – epiglottic space. No cervical lymphadenopathy.
• Stage – cT3N0M0, Stage III, Carcinoma Supraglottic larynx.
123. PATIENT POSITIONING & IMAGE ACQUISITION
• Immobilised with thermoplastic mask in supine position.
• Contrast – Enhanced CT scan with a slice thickness of 2.7 mm from frontal sinus to sterno –
clavicular joint acquired.
• FDG – PET CT scan done in same position with mask & flat table top.
• Fusion of PET CT with planning CT scan.
124. TARGET VOLUME DELINEATION
• Gross Tumor Volume – Automatically delineated from FDG PET scan, GTV – T (FDG PET, 0 Gy).
• Clinical Target Volume –
1) Primary Tumor
a) CTV – T1 encompassing the entire mucosa of the larynx.
b) CTV – T2 – 5 mm extension of GTV – T.
2) Lymph Node – B/L Level II – IV cervical lymph nodes selected as CTV – N.
• Planning Target Volume – 4 mm expansion around all CTVs. Only 1 mm expansion towards skin.
125. ORGAN AT RISK & PLANNING ORGAN AT RISK VOLUME
• OAR delineated –
a) Spinal Cord (down to T1 vertebrae)
b) Brainstem
c) Both Parotid Glands
• 4 mm margin was added around spinal cord for PRV.
• For brainstem & parotid glands, PRVs were equal to OARs.
127. PLANNING AIMS
• SIB – IMRT technique was used.
• PTV-N: 55.5 Gy in 30 fractions of 1.85 Gy in 6 weeks.
• PTV-T1: 55.5 Gy in 30 fractions of 1.85 Gy in 6 weeks.
• PTV-T2: 69 Gy in 30 fractions of 2.3 Gy in 6 weeks.
128. TREATMENT PLANNING SYSTEM & TREATMENT UNIT
• Treated using Hi – Art unit (Tomotherapy)
• Collimator width of 2.5 cm, a pitch of 0.215, and a modulation factor (ratio of the maximum
opening time for any leaf divided by the mean opening time for all leaves used) of 2 selected.
• Absorbed-dose calculations were performed using convolution/ superposition and collapsed-
cone algorithms with heterogeneity corrections.
132. QUALITY ASSURANCE
• Performed on a dedicated phantom before start of treatment.
• All measurements were within 5% of expected.
• Before each fraction patient underwent a daily MVCT for daily positioning.
133. CASE 2
• 70 year old male, ex – smoker, ECOG – 1.
• CT scan thorax – Tumor located in apex of lower lobe, right lung & an enlarged infra – carinal node.
• Fiber – optic bronchoscopy – Endoluminal mass in right lower lobar bronchus.
• HPE – Squamous Cell Carcinoma Grade – II.
• PET CT – FDG avid right lung mass & subcarinal node. No distant metastasis.
• Stage cT3N2M0.
• Planned for concurrent chemoradiation.
135. • Imaging consisting of FDG – PET & CT scan on an integrated PET – CT scanner.
• Patient lying on a flat couch in supine position, arms alongside body, with a neck support &
knee – rest.
• Contrast – Enhanced CT scan was obtained.
• All images transferred to planning system through local network.
136. TARGET VOLUME DELINEATION
• Gross Tumor Volume – 2 steps
1) Auto – delineation of FDG – avid primary tumor & lymph node [GTV – T (FDG – PET, 0
Gy) & GTV – N (FDG – PET, 0 Gy)]. PET scan was acquired during quiet breathing & thus
included Internal Motion.
2) Delineation done by using both modalities. Auto – delineated volumes were modified
using CT information where needed.
• Clinical Target Volume – 5 mm expansion of GTV T + N, with restriction on the expansion at
anatomic boundaries.
• Planning Target Volume – 5 mm isotropic margin was automatically added to the CTV T + N.
137. ORGAN AT RISK & PLANNING ORGAN AT RISK VOLUME
• Organ at Risk –
a) Spinal cord
b) Right & Left Lungs
c) Esophagus
d) Heart
e) Liver
f) Both kidneys
• PRV spine – 5 mm expansion of spinal cord.
• PRV esophagus – 3 mm expansion of esophagus.
May fall in exit dose trajectories of non –
coplanar beams
139. TREATMENT PLANNING SYSTEM & TREATMENT UNIT
• Absorbed dose computation done on Convolution / Superposition algorithm.
• Step & shoot Delivery on Elekta Sli – Plus linear accelerator.
• Software allows automatic creation of beam direction
• Optimization stopped after achieving planning aim or stopped when planner failed to achieve
planning aim after several cycles of optimization.
143. QUALITY ASSURANCE
• Performed automatically by another absorbed dose computation algorithm.
• During 1st week daily orthogonal portal imaging done to correct set – up error.
• Images from 1st week used to correct systematic set – up error.
• Less than daily imaging accepted if set – up error remained repeatedly below 5mm.
144. CASE 3
• 72 year old man with no urinary symptoms and a PSA level of 7.1 ng/ml.
• Digital Rectal Examination – Demonstrated enlarged prostate gland.
• Trans – Rectal Ultrasound – Showed 47 cc. prostate & a suspicious hypo – echoic nodule (10mm x 6mm x 16mm)
in left median peripheral zone.
• Double sextant core biopsy revealed Adenocarcinoma. 9/13 cores were positive. Gleason Score – 3 + 4 = 7 at left
base & 3 + 3 = 6 at all other sites.
• Stage – T1cN0M0
• Patient opted for radical radiotherapy.
145. PATIENT POSITIONING & IMAGE ACQUISITION
• 3 Gold seeds implanted in prostate under ultrasonic guidance to serve as fiducial markers for
radiographic localization.
• Pelvis & upper limb immobilized with vacuum cushion in supine position.
• Non – Contrast Planning CT scan obtained with 2.5 mm slice thickness from L5 vertebrae to 3
cm below lesser trochanter.
146. TARGET VOLUME DELINEATION
• Gross Tumor Volume – Not visible on planning CT images & hence not contoured.
• Clinical Target Volume – CTV – T defined as the entire prostate gland.
• Planning Target Volume – PTV – T defined by adding anisotropic margin to the CTV. Margin was
7 mm posteriorly & 10 mm in all other direction.
147. ORGAN AT RISK & PLANNING ORGAN AT RISK VOLUME
• Organ at Risk –
a) Bladder wall
b) Rectal wall
c) Left & right femoral head & neck
• No margin was added to OAR to create the PRV.
149. TREATMENT PLANNING SYSTEM & TREATMENT UNIT
• Planned with the Pinnacle treatment-planning system (Philips Medical Systems), and treated
with IMRT using a Synergy - S unit (Elekta AB).
• Planning and treatment were completed with 6 MV x rays using seven coplanar fields spaced
at approximately equal gantry angles.
• Absorbed-dose calculations performed using the standard convolution/superposition
algorithm and included CT-based heterogeneity corrections.
153. QUALITY ASSURANCE
• Individualized patient QA performed before start of treatment.
• Dosimetric Accuracy measured using 3rd party software.
• Using Mega – Voltage portal imaging absorbed dose distribution for all fields acquired &
compared with the planned absorbed dose distribution.
• Before each fraction orthogonal portal images obtained to match the implanted fiducials.
Notes de l'éditeur
Though this approach appears highly simplified , but it was well suited for the technology of 70s & 80s where a conventional simulator is used to design beam portals based on bony & soft tissue landmarks.
The optimization algorithm often uses a simpler model (usually because of speed issues) of energy deposition than is utilized for the final absorbed-dose calculation and monitor-unit calculation. Secondly, the final absorbed-dose calculation takes into account limitations in the MLC delivery that are not accounted for in the absorbed dose
calculation used by the optimizer. Examples of MLC limitations include the minimum opening distance that must be maintained between two opposing leaves (so that the leaves do not touch and cause mechanical damage) and the minimum allowed monitor units that can be accurately delivered by a segment.
