Evaluation of radiotherapy treatment planning

1. Evaluation of Radiotherapy
Treatment Planning
Prof Amin E AAmin
Dean of the Higher Institute of Optics Technology
Prof of Medical Physics
Radiation Oncology Department
Faculty of Medicine
Ain Shams University

2. Aim of Radiation Therapy
❖Deliver a precisely measured dose of irradiation to targets
❖Cause minimal damage to surrounding healthy tissue

3. Goals of Treatment Planning
❖Prescription dose covers and conforms to target volume
➢Normal tissues are not excessively irradiated
❖PTV receives uniform dose
❖Doses to OARs do not exceed tolerance values

4. The following tools are used in the evaluation of the planned
dose distribution:
➢ Isodose curves
➢ Orthogonal planes and isodose surfaces
➢ Dose distribution statistics
➢ Differential Dose Volume Histogram
➢ Cumulative Dose Volume Histogram
Tools For Dose Distribution Evaluation

5. Isodose Distribution
Slice-by-slice evaluation
❖Review dose distributions in all slices
❖Whether a selected isodose level adequately
covers the target volume or not
➢One has to select as high a isodose level as possible
❖Identification of hot spots / cold spots
➢Location of these spots

7. Isodose Distribution
Slice-by-slice evaluation

8. Orthogonal planes
Orthogonal planes
are important
tools that used in
the evaluation of
the planned dose
distribution

9. Isodose surfaces
Isodose
surfaces are
also are
important tools
that used in the
evaluation of
the planned
dose
distribution

10. Visual assessment of dose distributions
➢ The most direct and informative representation of a
treatment plan available
➢ however….
➢ 3-D dose distributions are large and cumbersome and
difficult to analyse quantitatively
➢ User interactivity is essential to extract the most
information from dose distributions (slice
selection/multi slice display, dose banding, dose
querying etc).

11. Disadvantages Of 3-d Dose Distributions
➢Huge amount of information to assess
➢Difficult to quantify visually
➢Difficult to understand relationship between dose and
anatomy in 3-d
➢Dose is itself only a surrogate for clinical outcome (Michael
Goitein).

12. Dose statistics provide quantitative information on the volume
of the target or critical structure, and on the dose received by
that volume. These include:
➢ Minimum dose to the volume
➢ Maximum dose to the volume
➢ Mean dose to the volume
➢ Dose received by at least 90-95% of the volume
➢ Volume irradiated to at least 90-95% of the prescribed dose.
Dose statistics for volumes

13. Dose statistics for volumes
❖Minimum dose
➢strong correlation between target minimum dose and clinical
outcome
➢high percentage of the dose maximum
❖Maximum dose
➢useful tool for critical structures
➢typically tolerance dose
❖Mean dose
➢indicator of dose uniformity within the target volume
➢should be very close to maximum dose

14. Goals of treatment planning
• To ensure 100% PTV is covered by 95% of the prescription
dose
• In other words, underdose to any part of PTV shall not exceed
5% of prescription dose
• To ensure overdose to any part of PTV shall not exceed 7%
of prescription dose

15. Maximum Dose ( Dmax )
• It is the maximum dose to the PTV and the Organ at Risk.
• The maximum dose to normal tissue is important for limiting
and for evaluating the side-effects of treatment.
• Dose is reported as maximum only when a volume of tissue of
diameter more than 15mm is involved (smaller volumes are
considered for smaller organs like eye, optic nerve, larynx).
• When the maximum dose outside PTV exceeds the prescribed
dose, then a “Hot Spot” can be identified.

16. Maximum Dose ( Dmax )
The maximum dose at any part of PTV shall not exceed 7% of
prescription dose

17. Global Dose Maximum
• what is its value?
–not more than 107% for 3-D CRT
–can be higher for IMRT, but within 115%
• where is it located?
–it should be within CTV
–preferably within GTV

18. Hot Spots
• It represents a volume outside the PTV which receives a dose
larger than 100% of the specified dose.
• A Hot Spot is considered significant only if the minimum
diameter exceeds 15mm (in smaller organs like eye, optical
nerve, larynx etc. a diameter smaller than 15mm is also
considered significant).

19. Minimum Dose ( Dmin )
➢It is the smallest dose in a defined volume.
➢In contrast to maximum adsorbed dose, no volume limit is
recommended when reporting minimum dose.
➢Underdose to any part of PTV shall not exceed 5% of
prescription dose

20. Hot And Cold Spots
• Three questions to ask about all hot and cold spots
–volume?
–magnitude?
–location?
• Is there a consensus to any of these questions in any
tumor site?

21. Hot Spots: Recommendations
• volume: <15-20% of the PTV
• magnitude: overdosing exceeds 15% of the prescription
dose
– <15% volume at the 110% dose level
– <1% volume at the 115% level
• location: within the CTV (preferably GTV); not acceptable
on the periphery of PTV

22. Cold spots: Recommendations
• volume: <1% of PTV
• magnitude: underdosing exceeds 5% of the prescription dose
• location: periphery of PTV; never acceptable within the CTV

23. Dose Volume Histogram (DVH)
❖Most important evaluation tool for
3-D planning
❖Graphical summarization of 3-D
dose distribution
❖Represents a frequency distribution
of dose values within a defined
volume that may be the PTV itself
or a specific organ in the vicinity of
the PTV

24. Dose volume histograms (DVH)
Volume(%)
Dose (%)
DVHs reduce 3-d dose distributions within a defined volume of
interest to simple 1-d curves.
For example...
Plan 1 Plan 2
Comparative DVH’s
Right globe

25. Provide a succinct and quantitative method of representing 3D
dose within selected VOI’s - however…
➢ DVH’s should only be used in conjunction with careful
visual analysis of 3-d dose distributions
➢ In particular, care should be taken when analysing large
volumes using DVH’s
➢ DVH’s should always be assessed in conjunction with
dose-volume statistics.
Dose volume histograms

26. DVH
• There are two types of DVH
• Cumulative (integral)
• most used
• Differential (True or direct)
• dose and volume axes can be absolute or
relative
Cumulative
Differential

27. Cumulative DVH
The computer calculates the
volume that receives at least
the given dose and plots this
volume (or percentage
volume) versus dose.

28. Cumulative DVH
Basically the area under
curve is the volume of tissue
getting a dose and the smaller
this area, the better for an
OAR.
The tail of the curve should
not ideally taper too much to
the right as this will mean a
smaller volume getting a
higher dose.

29. Cumulative DVH
For PTV Dose should cover a large volume as well (>95%)

30. The Ideal DVH
• Tumor:
• High dose to all
• Homogenous dose
• Critical organ
• Low dose to most of the
structure
100%
dose
100%
dose

31. Differential DVHs
A good tool for assessing PTV dose uniformity

32. Limitations of DVH
➢ no spatial information
➢ where the hot / cold spot occurred
➢ whether it occurred in one or several disconnected regions
➢ DVHs are insensitive to small ‘hot’ and ‘cold’ spots
➢ The shape of a DVH alone can be misleading
➢ DVHs cannot be the sole criterion for
evaluating/ disclosing the best plan
➢ DVHs can only be calculated for defined VOIs

33. Limitations of DVH
• DVH analysis was
insufficient to
compare
complicated and
advanced planning
techniques.

34. Limitations of DVH
❖As the rectum DVHs in the this
figure, it was difficult to
distinguish whether plan1
(continuous green line) or plan2
(dashed red line) were superior.
❖For low dose volume (V0 to
V20), plan2 was more favorable
than plan1. However, this
relationship reversed for high
dose volume (V20 to V50).

35. Goals of Treatment Planning
❖Prescription dose covers and conforms to target volume
➢Normal tissues are not excessively irradiated
❖PTV receives uniform dose
❖Doses to OARs do not exceed tolerance values

36. Homogeneity Indices

37. Homogeneity Indices (HI)
It is a measure of dose uniformity within PTV

38. Inhomogeneity within Target
Volumes
➢ Inhomogeneity within target volume kept to ± 10% of the
prescribed dose.
➢ ICRU 83 report is used for describing IMRT has described
D98%, D50%, and D2%. (Dmax, Dmedian and Dmin)

39. Adsorbed Dose Distribution
•The dose given to the tumor should be as homogenous as
possible.
•In cases of heterogeneity of doses, the outcome of the
treatment cannot be related to the dose. Also, the comparison
between different patient series becomes difficult.
•However, even if a perfectly homogenous dose distribution is
desirable, some heterogeneity is accepted due to technical
reasons.

40. ❑The ICRU previously recommended that the absorbed dose in the
PTV be confined within 95 % to 107 % of the prescribed absorbed
dose(ICRU, 1999).
❑With IMRT, these constraints can be unnecessarily confining if the
avoidance of normal tissue is more important thantarget- dose
homogeneity.
❑In ICRU Report, it is recommended that the extent of high- and
low-dose regions is specified using dose–volume quantities such
as D2 % and D98 % for regions of high and low absorbed dose,
respectively.
Dose Volume Specifications

41. Homogeneity Index (HI)
• Certain definitions of HI were described, for example;
𝐻𝐼 =
𝐷 𝑀𝑎𝑥
𝐷 𝑀𝑖𝑛
• Where Dmax, Dmin; are maximum dnd minimum doses
respectively.
• This equations is the most commonly used formula in the
literature.

42. Homogeneity Index (HI)
• In most literatures Dmax is expressed in terms of D2, or
D5 and Dmin is expressed in terms of D98, or D95. So
HI is expressed as;
𝐇𝐈 =
𝐃 𝟐
𝐃 𝟗𝟖
𝐇𝐈 =
𝐃 𝟓
𝐃 𝟗𝟓

43. Homogeneity Index (HI)
• HI is expressed as the ratio D2/D98 orD5/D95
–D2is the maximum dose received by at least 2% of the PTV
–D98is the minimum dose received by at least 98% of the PTV
–D5is the maximum dose received by at least 5% of the PTV
–D95is the minimum dose received by at least 95% of the PTV

44. Homogeneity Index (HI)
• This formula is recommended byICRU.
• The lower (closer to one) the index, the better is the
dose homogeneity
𝐇𝐈 =
𝐃 𝟐
𝐃 𝟗𝟖

45. Homogeneity Index (HI)
• For a typical 3-D CRT plan, it is around 1.07
• For IMRT it should be ≤ 1.15

46. Homogeneity Index (HI)
• Another definitions of HI were described as;
𝐻𝐼 =
𝐷 𝑀𝑎𝑥 − 𝐷 𝑀𝑖𝑛
𝐷 𝑃
• Where Dmax, Dmin, Dp; are maximum, minimum, and prescribed
dose respectively

47. Homogeneity Index (HI)
ICRU defined HI as;
𝐻𝐼 =
𝐷 𝑀𝑎𝑥 − 𝐷 𝑀𝑖𝑛
𝐷 𝑚𝑒𝑑
𝐻𝐼 =
𝐷2 − 𝐷98
𝐷50
Dmed is (Dmedian), is the median dose i.e. Dose received
by 50% of PTV (D50)

48. Homogeneity Index (HI)
• The reason of not using the actual minimum and maximum doses in
calculating HI is their sensitivity to the dose-calculation parameters,
such as grid size and grid placement.
• Therefore, the true minimum or maximum dose is typically not
reliable.

49. HIRTOG
Therapy Oncology Group (RTOG) described HIRTOG as,
𝑯𝑰 𝑹𝑻𝑶𝑮 =
𝑰 𝑴𝒂𝒙
𝑹𝑰
where, Imax = maximum isodose in the target,
and RI = reference isodose.

50. HIRTOG
• If the HI was ≤2, treatment was considered to comply
with the protocol.
• If this index was between 2 to 2.5, it was considered as
minor violation.
• If the index exceeded 2.5, the violation of the protocol
was considered to be major.

51. Goals of Treatment Planning
❖Prescription dose covers and conforms to target
volume
➢Normal tissues are not excessively irradiated
❖PTV receives uniform dose
❖Doses to OARs do not exceed tolerance values

52. Target Coverage Indices

53. Target Coverage Index TCI
❖TCI accounts for the exact coverage
of PTV in a treatment plan at a given
prescription dose.
❖TCI is defined as the ratio of the
target volume receiving at least the
prescription dose, Vtp, to the total
target volume, Vt.
❖Typically, the coverage index should
be at least 95%.
𝑻𝑪𝑰 =
𝑽 𝒕𝑷
𝑽 𝒕

54. Prescription isodose to target
volume (PITV) ratio
• The PITV ratio, obtained by dividing
surface volume surrounded by
prescription isodose level (inside and
outside the PTV), Vp divided by target
volume Vt.
• The PITV ratio is a conformity measure,
and a value of 1.0 indicates that the
volume of the prescription isodose
surface equals that of the PTV.
𝑷𝑰𝑽 =
𝑽 𝑷
𝑽 𝒕

55. Prescription isodose to target volume
(PITV) ratio
• A PITV ratio of 1.0 does not
necessarily imply that both
volumes are similar. To ensure
adequate PTV coverage, this
measure should always be used
in conjunction with a PTV-
DVH.
miss Body
PTV
RI

56. Goals of Treatment Planning
❖Prescription dose covers and conforms to target
volume
➢Normal tissues are not excessively irradiated
❖PTV receives uniform dose
❖Doses to OARs do not exceed tolerance values

57. Dose Conformity

58. Conformity Index (CI)
• The conformity index (CI) is
defined as the ratio of the total
volume receiving at least the
prescription dose, Vp, to the target
volume receiving at least the
prescription dose, Vtp .
• The value of CI is always greater
than unity. A value that is closer to
unity represents a better target
conformity of radiation dose in the
treatment plan.
𝑪𝑰 =
𝑽 𝒑
𝑽 𝒕𝒑

59. Conformity Index (CI)
• CI is generally used to indicate the portion of a prescription
dose that is delivered inside the PTV.
• CI of 1 indicates that 100% of a prescription dose is delivered
to the PTV, and no dose is delivered to any adjacent.
• The CI is less than 1 for most clinical cases.
• Higher CI values indicate poorer dose conformity to the PTV.

60. Conformity number (CN)
• Dose conformity evaluates the dose fit of the PTV
relative to the volume covered by the prescription dose.
• Ideally the prescribed dose should fit tightly to the target
volume, therefore, reducing the side effects occurred by
treating surrounding tissues and organs.

61. Conformity number (CN)
• The CN simultaneously takes into account irradiation of the target
volume and irradiation of healthy tissues.
• A CN value closer to 1 indicates that the dose distribution fits
more tightly to the target volume preserving healthy tissue.
𝑪𝑵 = 𝑻𝑪𝑰 𝑿 𝑪𝑰
=
𝑽 𝒕𝑷
𝑽 𝒑
𝑿
𝑽 𝒕𝒑
𝑽 𝒕
where Vp is the total volume
receiving the prescription,
Vt is the target volume, and
Vtp is the target volume
covered by the prescription.

62. Goals of Treatment Planning
❖Prescription dose covers and conforms to target volume
➢Normal tissues are not excessively irradiated
❖PTV receives uniform dose
❖Doses to OARs do not exceed tolerance values

63. Dose Gradient

64. Gradient index (GI)
• The gradient index (GI) is defined as the ratio of the
volume covered by at least a given percentage of the
prescription dose (VG) to the volume covered by the
full prescription dose (VP).
𝑮𝑰 =
𝑽 𝑮
𝑽 𝒑

65. Gradient index (GI)
• In most of dosimetric studies, the given
percentage is set at 50% of the
prescription dose.
• The value of GI is greater than unity. A
value that is closer to unity represents a
faster dose fall-off in normal tissue.
𝑮𝑰 =
𝑽 𝟓𝟎
𝑽 𝒑

66. Gradient Measure (GM)
• Gradient measure is the difference between the equivalent
sphere radius of the prescription and half prescription
isodoses.
𝑮𝑴 =
𝑹 𝟓𝟎
𝑹 𝒑

67. Critical Organ Scoring Index

68. Critical Organ Scoring Index (COSI)
❖Critical Organ Scoring Index (COSI) takes into account both the
target coverage and the critical organ irradiation.
❖The main advantage of this index is its ability to account of the
irradiation of all OARs simultaneously in one index.

69. Critical Organ Scoring Index
(COSI)
The COSI can be expressed as;
Where;
"V(OAR)>tol” is the fraction of the volume of OAR that
receives more than a predefined tolerance dose,
TC is the volumetric target coverage, which is defined as
the fractional volume of the PTV covered by the prescribed
isodose.
𝐶𝑂𝑆𝐼 = 1 − ෍
𝑖=1
𝑛
𝑤𝑖
𝑉𝑖(𝑂𝐴𝑅)>𝑡𝑜𝑙
𝑇𝐶

70. Critical Organ Scoring Index (COSI)
• The target coverage (TC) is defined as the ratio of the target volume
receiving at least the prescription dose (VT,PI) to the total target volume
(VT). Typically; the coverage index should be at least 95%.
• i.e. TC is expressed as:
TC = VT,PI / VT

71. Critical Organ Scoring Index (COSI)
• The above expression of COSI is used if the plan has more
than one OAR.
• If the plan has only one OAR, COSI is expressed as
𝐶𝑂𝑆𝐼 = 1 −
𝑉(𝑂𝐴𝑅)>𝑡𝑜𝑙
𝑇𝐶

72. Modified Critical Organ Scoring Index
(mCOSI)
❖Modified Critical Organ Scoring Index (mCOSI) is the other
index that takes into account both the target coverage and the
critical organ irradiation.
❖As in COSI, mCOSI has the advantage of the ability to
account of the irradiation of all OARs simultaneously in one
index.

73. Modified Critical Organ Scoring Index
(mCOSI)
• The modified COSI (mCOSI) which is expressed as:
𝑚𝐶𝑂𝑆𝐼 = ෍
𝑖=1
𝑛
𝑤𝑖
𝐶𝑂𝑆𝐼10 + 𝐶𝑂𝑆𝐼20 + … + 𝐶𝑂𝑆𝐼80
8

74. Modified Critical Organ Scoring Index
(mCOSI)
• Although the COSI focuses only on OARs receiving high
dose region volumes, the mCOSI considers both high dose
and low dose regions.

75. Modified Critical Organ Scoring Index
(mCOSI)
• The above expression of mCOSI is used if the plan has more
than one OAR.
• If the plan has only one OAR, mCOSI is expressed as
𝑚𝐶𝑂𝑆𝐼 =
𝐶𝑂𝑆𝐼10 + 𝐶𝑂𝑆𝐼20 + … + 𝐶𝑂𝑆𝐼80
8

76. Quality factor (QF)
• The QF of a plan can be analytically expressed as:
• In the above equation, Xi represents all of the PTV
indices used for evaluating a plan, including the HIRTOG,
HI, TCI, PITV, CI, CN, GI, and GM.
• The values of the weighting factor (Wi) can be adjusted
between 0 and 1 for all relatively weighted indices for a
user-defined number of indices (N).
𝑸𝑭 = 𝟐. 𝟕𝟏𝟖 𝒆𝒙𝒑 − ෍
𝒊
𝒏
𝑾𝒊 𝑿𝒊

77. Quality Factor

78.  QF is the best tool for physical evaluation of treatment plans
because it is sensitive to the any variation in dose
homogeneity, conformity and gradient.
QF

79. Total Number of Monitor Units MU’s
❖The total number of MU’s is an important factor in
selecting the optimal treatment plan.
❖The higher the total number of MU’s is the higher
the probability of secondary cancer.
❖In addition to that increasing the total number of
MUs increases the treatment delivery time i.e. less
comfortable and less accurate treatment.

80. Tumor Control Probability (TCP)
Normal Tissue Complication Probability (NTCP)
Complication Free Tumor Control Probability (P+)
Biological Evaluation Indices

81. Tumor Control Probability (TCP)

82. Normal Tissue Complication Probability
(NTCP)

83. Complication Free Tumor Control
Probability (P+)

84. Biological Evaluation
• TCP, NTCP and P+ can be calculated using Eclipse biological
evaluation software which applies Poisson statistics.

85. Biological Effective dose BED
• For two dose fractionation schedules, each point of physical
dose distribution has been converted into BED for the target
and OARs using the following equation;

87. Proposed Model
Dose-Volume Constrains
Dose Distribution
DVH
Physical Indices
HI
TCI
PITV
CI
CN
GI
GM
MU
COSI
mCOSI
QF
Biological Indices
TCP
NTCP
P+
BED
BED-Volume Histogram
BED Distribution