Plan evaluation in Radiotherapy

1. PLAN EVALUATION
IN RADIOTHERAPY
Kiran Kumar BR

2. Plan evaluation methods
 To define the physical parameters such as
DVH used for plan evaluation.
 To illustrate the use of biological parameters
for plan evaluation.

4.  The following tools are used in the evaluation of
the planned dose distri-bution:
 BEV-Isodose curves;
 REV
 Colour wash
 Dose distribution statistics;
 Differential DVHs;
 Cumulative DVHs.

5.  Typically, the initial beam arrangement has been
selected based primarily on clinical experience using
BEV displays.

6. Isodose curves
 Evaluate treatment plans along a single plane or over several planes in the patient.
 The isodose covering the periphery of the target is compared with the isodose at
the isocentre.
 If the ratio is within a desired range (e.g. 95– 100%), the plan may be acceptable
provided that critical organ doses are not exceeded.
 This approach is ideal if the number of transverse slices is small.

9.  “Room-view” or room’s-eye-view (REV), in which the
planner can simulate any arbitrary viewing location
within the treatment room.
 The REV display is used to display “dose clouds” along
with rendered PTVs and OARs.
 Hot or cold spots that occur in the volumes of interest
are clearly seen.

13.  “skin view,” in which the beam aperture projection
can be clearly seen on the skin of the (virtual)
patient .

15. Dose statistics
 Provide quantitative information on the volume of the target or critical
structure and on the dose received by that volume. From the matrix of
doses to each volume element within an organ, key statistics can be
calculated. These include:
 The minimum dose to the volume;
 The maximum dose to the volume;
 The mean dose to the volume;
 The dose received by at least 95% of the volume;
 The volume irradiated to at least 95% of the prescribed dose.

16. Dose–volume histograms
 DVHs summarize the information contained in the 3-D dose distribution
and are extremely powerful tools for quantitative evaluation of
treatment plans.
 Direct (or differential) DVHs;
 Cumulative (or integral) DVHs.

18.  +7% and –5% of the prescribed dose,with doses to critical structures held
below tolerance levels.

21. DEVELOPMENT OF THE CONCEPT OF
CONFORMITY INDEX
 CI can be defined as an absolute value resulting
from the relationship between tumor volume or
a fraction of this volume and the volume
delineated by an isodose or a fraction of this
volume.
 The conformity index (CI) was developed as an
extension of section-by-section dosimetric
analysis and DVH.

22.  It can also be defined by the ratio of an isodose with
another isodose (prescription isodose, reference
isodose, minimum isodose, maximum isodose).

23. Conformity index=TVRI/TV
 TVRI is the target volume covered by the reference isodose
 TV is the target volume.
 This conformity index ranges from 0 to 1.
 The quality of irradiation of the target volume can be correctly
determined with this index, but it does not provide sufficient
information about the overall treatment plan.

25. Where VRI is reference isodose volume, and TV is target volume.

26.  CI = 1 ideal conformation.
 CI >1 irradiated volume > target volume
 CI < 1 target volume is only partially irradiated
 CI values have been defined to determine the quality of conformation
(RTOG).
 1 < CI < 2 comply with the treatment plan
 2 < CI < 2.5 or 0.9 < CI < 1minor violation
 2.5 CI < 0.9 major violation.

27.  Drawback: It can never take into account the degree of spatial
intersection of two volumes or their shapes.
 Eg :In some cases, it may be equal to 1 while these two volumes are
situated away from each other and present entirely different shapes

29. Global conformity index (target volumes
and healthy tissues)
 The previous two indices (Eqs. Conformity index=TVRI/TV and HTCI =TVRI/VRI)
provide indissociable, complementary information (irradiation of the target volume
and irradiation of healthy tissues).
 To compensate for the defects of these two indices, van’t Riet et al. proposedan index
called conformation number (CN).

30.  This number is defined as follows:
CN = TVRI/TV X TVRI/VRI
 where CN conformation number, TVRI target volume covered by the reference
isodose, TV target volume, and VRI volume of the reference isodose.
 The first fraction of this equation defines the quality of coverage of the target;
the second fraction defines the volume of healthy tissue receiving a dose
greater than or equal to the prescribed reference dose.

32.  0 <CN<1 where 1 is the ideal value.
 A value close to 0 indicates either total absence of
conformation, i.e., the target volume is not
irradiated or a very large volume of irradiation
compared to the target volume.
 where the index tends toward 0, because the VRI
is much greater than the TVRI

34.  In 1993, the Radiation Therapy Oncology Group (RTOG) proposed
guidelines for routine evaluation of stereotactic radiotherapy (SRT) plans
based on several parameters and
 HI was described as,
HI RTOG = I max /RI
where, I max = maximum isodose in the target, and RI =
reference isodose.
 HI ≤2- treatment was considered to comply with the protocol.
 2 <HI<2.5- minor violation.
 HI> 2.5- major violation, but might nevertheless
considered acceptable.

35.  Certain other definitions were later described,
 HI = D 5 /D 95
D5 = minimum dose in 5% of the Planning
Target Volume (PTV), indicating the "maximum
dose"
D 95 = minimum dose in 95% of the PTV,
indicating the "minimum dose".
 The lower (closer to one) the index, the better is
the dose homogeneity.

36.  A more descriptive formula is
HI = D 2 -D 98 /D p ×100
D 2 = minimum dose to 2% of the target volume
indicating the "maximum dose”
D 98 = minimum dose to the 98% of the target
volume , indicating the "minimum dose“
D p = prescribed dose.
 This is the most commonly used formula in the
literature.

37. HI basically indicates the ratio between
the maximum and minimum dose in the
target volume
 Lower value indicates a more
homogenous dose distribution within this
volume.

39. Limitation of homogeneity index
 The multiple indices proposed in the literature
 Difficulty in their interpretation.
 Limited information regarding possible correlation between clinical data.
 No studies till date which suggest that the plans with a better HI are associated with a
better clinical outcome as compared to those plans with inferior HI.

40. Cautions while using HI
 HI depend upon the particular formula used.
 Clearly specify the target volume.
 Tumors - heterogeneous group of cell population may benefit , delivering higher dose to
the areas where there is increased density of malignant cells or there are pockets
containing resistant cells.
 Non-homogenous dose with higher central dose may improve local control in such cases.

41.  It is not always justified to try for achieving the ideal value of HI at any cost.
 Try to achieve homogeneity within the target volume at any cost, the system tends to
dump the extra dose (hot spot) outside the target to improve the HI, which may prove
detrimental.
 The importance of dose homogeneity in biologically optimized IMRT plans is
controversial.

42.  There is thus no consensus about the acceptable limit of HI.
 A value of less than 2.0 (RTOG)

45. Biological models
 Tumor control probability (TCP)
 Normal tissue complication probability (NTCP)

46. THERAPEUTIC RATIO
 It is ratio of NTT/ TLD.
 The more the curve B is to the right
of curve A the more is therapeutic
ratio
 The optimum choice of radiation
dose delivery technique is one that
maximizes the TCP &
simultaneously minimizes the
NTCP

47. THERAPEUTIC INDEX

48.  Tumor Control Probability
 TCP plotted as a function of dose has a classic sigmoid shape.
 Moiseenko et al.
 Uniform dose distribution

49.  Non uniform

50.  Tumor Control Probability
 The probability of tumor control is directly
proportional to dose and inversely to the
volume/number of tumour cells.

53. Normal Tissue Complication
Probability
 The empiric model -Lyman and Wolbarst
 Functional models -concepts of serial and parallel tissue organization
andfunctional subunits (FSUs)

54.  The Lyman NTCP model - expressed in terms of an error
function of dose (D) and volume (v) as follows:

55. with v equal to the partial volume (V/Vref) and the
tolerance dose volume dependence given by the
following power-law relationship:

56.  D50(1) is the tolerance dose for 50% complications for uniform whole-organ
irradiation
 D50(v) is the 50% tolerance dose for uniform partial-organ irradiation to the
fractional volume v.
 m and n are found by fitting tolerance doses for uniform whole and uniform
partial-organ irradiation
 m characterizes the gradient (slope) of the dose–response function at D50
 n characterizes the effect of volume.

57.  Nonuniform organ irradiation.
 The interpolation method - Lyman and Wolbarst,modifies the DVH to one in
which the whole organ receives an effective uniform dose, Deff, that is less than
or equal to the maximum organ dose.
 The effective volume method, proposed by Kutcher and Burman,modifies the
DVH to one in which a fraction of the organ, veff, receives the maximum organ
dose.
 The Lyman model coupled with the Kutcher–Burman DVH reduction scheme
(now called the Lyman–Kutcher–Burman model) is the most widely used
NTCP model.

58.  Niemierko and Goitein—the critical element
model, used for serial-like organs, and the critical
volume model, for parallel-like organs. These are
similar in form to that of Lyman and Wolbarst,include
additional terms to better account for the
radiosensitivity of the FSUs.

59. SERIAL ORGANS

62. Thank You