4. ICRU REPORT - 50
PRESCRIBING, RECORDING, AND
REPORTING PHOTON BEAM THERAPY
When delivering a radiotherapy tretament, parameters
such as volume and dose have to be specified for
different purposes: prescription, recording, and reporting.
The aims are –
To have a consistent treatment policy and improve it in the
light of experience
To be able to compare the results of treatment with those of
departmental colleagues
Other radiation oncologists should be able to benefit from the
department’s experience
The results to be meaningfully compared with those of other
centers, without having access to the complete data
6. GROSS TUMOUR VOLUME (GTV)
The GTV is the gross demonstrable extent of the
malignant growth as determined by palpation or
imaging study.
PURPOSE :
For proper staging (TNM)
To define area requiring adequate dose delivery for
treatment
To define the extent of CTV and PTV
To have a record to assess tumour response and
predict treatment outcome based on the variation.
ICRU-50
8. CLINICAL TARGET VOLUME (CTV)
The CTV is the tissue volume that contains GTV
and/or subclinical microscopic malignant disease. It
includes suspected but unproven malignant tissue.
The density of malignant cells is considered to be
maximum near the GTV, decreasing in density
towards the margin of the CTV with
microextensions along natural avenues of spread
(lymph node, perivascular, pernineural extensions
and distant sites).
The GTV and CTV are anatomical and clinical
concept that must be defined before a choice of
treatment modality and technique is made.
ICRU-50
10. PLANNING TARGET VOLUME (PTV)
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.
AFFECTED BY : size and shape of the GTV & CTV, the
effects of internal motions of organs and the tumour as well as
the treatment technique (beam orientation and patient fixation,
daily setup errors) used.
The PTV can be considered as a 3-D envelope in which the
tumour and any microscopic extensions reside. The GTV and
PTV can move within this envelope, but not through it.
ICRU-50
12. TREATED VOLUME
It is the volume enclosed by an isodose surface that
is selected and specified by the radiation oncologist
as being appropriate to achieve the purpose of
treatment (palliation or cure).
Usually taken as the volume enclosed by the 95%
isodose curve.
ICRU-50
13. IRRADIATED VOLUME
It is the volume that receives a dose considered
significant in relation to normal tissue tolerance
Usually taken as the volume enclosed by the 50%
isodose curve.
ICRU-50
14. ORGANS AT RISK
Organs at Risk are defined as critical normal
tissues, such as the spinal cord, whose radiation
sensitivity may significantly influence treatment
planning and/or prescribed dose.
ICRU-50
15. DOSE REPORTING
PURPOSE : to have uniformity between centres
and have an understandable treatment chart.
Acceptable dose heterogeneity :+7% to - 5% of the
prescribed dose.
Doses reported are :
Minimum dose to PTV
Maximum dose to PTV
Mean dose to PTV
Modal dose
Median dose
Dose at ICRU reference point
ICRU-50
16. Hotspot- an area outside PTV recieving > 100% of
prescribed dose and more than 15mm in diameter
(for small critical organs even point dose is a hot
spot)
ICRU-50
17. LEVELS OF DOSE EVALUATION FOR
REPORTING:
Level 1 –BASIC TECHNIQUE - minimum standards, 2-
D reporting (using depth dose tables)
Level 2 – ADVANCED TECHNIQUE -prescribing and
reporting state-of-the-art techniques (using
computational dosimetry and 3D imaging)
Level 3 – DEVELOPMENTAL TECHNIQUE -optional
research-and-development reporting (using techniques
for which reporting criteria are not yet established)
ICRU-50
18. ICRU REFERENCE POINT
It is a point with in the PTV to which the dose is
prescribed or calculated
The ICRU reference point must be selected based on
the following criteria :
It should be representative of the PTV
It should be clinically relevant and can be defined in a clear
and unambigious way
Be located where the dose can be accurately determined
Be located in a region where there are no steep dose
gradients
These recommendations will be fulfilled if the ICRU
reference point is located :
Always at the centre of PTV
When possible at the intersection of the treatment beam axes.
ICRU-50
20. REFERENCE POINTS
Internal reference points are anatomical landmarks
e.g., bony structures or gas filled cavities
External reference points are palpable or visible
points located on or near the surface of the body or
on the surface of the immobilisation devices that fit
closely to the exterior of the body
As external reference points one may also use skin
markings or alignment tattoos
ICRU-50
21. RADICAL VERSUS PALLIATIVE TREATMENT
ICRU defines that if adequate dose cannot be
delivered to the GTV, the whole aim of therapy
should be termed as palliative.
ICRU-50
26. ICRU-62
PTV has been separated into two components: an
internal margin and set-up margin.
Classisfied organs at risk depending on response to
radiation.
Defined planning organ at risk volume (PRV)
Report dose to the OAR/PRV
Introduced conformity index
Gives recommendations on graphics
ICRU-62
27. INTERNAL TARGET VOLUME (ITV)
Defined by adding an internal margin to the CTV. [internal
uncertainties only]
INTERNAL MARGIN - A margin that must be added to the
CTV to compensate for expected physiologic movements and
the variations in size, shape and position of the CTV during
therapy in relation to the Internal Reference Point and its
corresponding Coordinate System. Motion is associated with
adjacent respiratory and digestive organs.
Changes in the patient’s condition, such as weight gain/loss,
can also affect the relative position of the CTV.
K.M.Langen, D.T.L. Johnes. Organ motion and its management. International Journal of Radiation
Oncology Biology, Physics
Vo. 50, No.1, pp. 265-278, 2001
ICRU-62
28. SETUP MARGIN
The uncertainties depend on different factors:
Variations in patient positioning
mechanical uncertainties of the equipment
dosimetric uncertainties (light-radiation field agreement)
transfer set-up errors
Beam related parameters
human related uncertainties
They can be on the CT scanner, simulator or
machine.
The net effort of combining an Internal Margin and
a set up margin and adding it to the CTV results in
the PTV previously defined in ICRU-50. PTV = ITV
+ SM
ICRU-62
30. CLASSIFICATION OF ORGANS AT
RISK
Classified as :
Serial – whole organ is a continuous unit and damage at
one point will cause complete damage of the organ
(spinal cord, digestive system). So even point dose is
significant
Parallel – organ consists of several functional units and
if one part is damaged, the rest of the organ makes up
for the loss (lung, bladder). Dose delivered to a given
volume or average/mean dose is considered
Serial-parallel – kidney (glomerulus- parallel, tubules-
serial), heart (myocardium- parallel, coronary arteries-
serial).
ICRU-62
31. PLANNING ORGAN AT RISK VOLUME
PRV to OAR is analogous to the PTV for the CTV.
Aim is to account for movements of the OAR due to
movements, changes in size and shape and setup
uncertainities.
PTV and PRV may overlap, then it is the
responsibility of the radiation oncologist to decide
depending on the importance of the treatment
versus risk of critical organ damage.
ICRU-62
32. SYSTEMATIC AND RANDOM
ERRORS
Systematic errors – treatment preparation errors
(influence all fractions) like full rectum
Random errors – treatment execution errors
(influence only the single fraction) like positioning
ICRU-62
33. CONFORMTY INDEX
Conformity index (CI) = TV/PTV
CI iso
= TV covered by % Isodose/Planning Target
Volume
CI = > optimized closed to 1.0
ICRU-62
34. Now it is the digital era and with the advent and use
of IMRT, new guidelines need to be set for
prescription, reporting and recording of dose.
The ICRU-83 deals with these new guidelines
ICRU-62
38. PARADIGM SHIFT
In 3D conformal therapy, ICRU-50 recommended
Uniform dose to PTV (-5% to +7%)
Report dose @ isocenter
IMRT represents ≈40% of the radiation treatment
with a paradigm shift
Non uniform dose (dose painting)
Large dosimetric variations
Isocenter dose is meaningless
Radiobiological consequence of large
heterogeneous dose is uncertain (ie 180cGy/day
versus 250cGy/day)
ICRU-83
39. 3-DCRT VERSUS IMRT
Simple
Forward planning
Give what you wish to target
volume
Uniform fluence
Uniform dose
Low MU
Analogue dose
Dose defined to volume but
specified at isocenter
Complex
Inverse planning/optimized
planning
Requires dose-volume
constraints and cost
function
Non-uniform fluence
High gradient dose
High MU
Digital dose
Isocenter dose undefined
and meaningless
3-DCRT IMRT
ICRU-83
40. In IMRT, the delineation of the target and normal
structures has to be very precise due to the high
conformality achieved by IMRT as compared to 3-
DCRT. This benefit is especially seen in concave
volumes.
The tolerance for the MLC leaf position is smaller
when using the MLC for IMRT than for 3-DCRT.
ICRU-83
44. Key distinctions between
traditional and IMRT
planning :
1. Use of mathematical
objective functions and
incorporation of user
defined dose-volume
constraints
2. Employment of an
iterative computer-
based IMRT algorithm
to seek the optimal
solution.
ICRU-83
45. Plan improvisation can be done by changing the
relative importance given to PTV and other
structures
ICRU-83
48. If there are 5 beam directions and the field area is
discretized into 40x40 beamlets each, then the
search space consists of 5x40x40 = 8000
dimensions. It there are 10 allowed beam
intensities/weight for each beamlet, then the
number of possible intensity patterns is 108000.
Optimzation can be done by both beamlet based
optimization and aperture based optimization.
Contrary to the typical systems using absorbed
dose only for optimization, EUD (equivalent uniform
dose) can be used to set constraints
ICRU-83
50. There are 3 levels of reporting as discussed in the
previous report (ICRU-50)
Level 3 – includes biological factors which are still
under evaluation and not in routine clinical use
(TCP, NTCP, EUD)
Dose-volume based calculations (Level 2 reporting)
are routinely used now rather than ICRU reference
point reporting
Using DVH, we can find the presence (but not the
location) of minima and maxima in the contoured
volumes.
The location of these minima & maxima can be
seen in sections-wise viewing the isodose
curves/colour wash.
ICRU-83
53. Using IMRT the PTV dose distribution can be less
homogenous than conventional but are more
conformal and can produce much higher absorbed-
dose gradients.
Doses to PTV, CTV, PRV and RVR should be
reported in parameters discussed below
Now convolution/superposition methods are in use
for the iterative process. Soon Monte-Carlo
calculations will come in routine use.
54. The parameters used for reporting are : Dmean , D98%
(near-minimum) , D95%, D2% (near-maximum) , DV,
VD.
Dmean is much like D50%.
For PTV and CTV, report the Dmean , D98% , D2% .
55. The parameters used for reporting are : Dmean , D98%
(near-minimum) , D95%, D2% (near-maximum) , DV,
VD.
Dmean is much like D50%.
For PTV and CTV, report the Dmean , D98% , D2% .
For PRV, parameters used for reporting are :
Serial organs – Dmax/D0% rather D2%
Parallel organs – VD (volume receiving atleast the
absorbed dose D), Dmean
Serial-parallel organs – all three parameters should be
reported
Since most organs do not have such a specific
distinction into serial or parallel, best way to do reporting
is to report all the three parameters – Dmax, VD , Dmean
ICRU-83
57. DOSE HOMOGENEITY AND CONFORMITY
INDEX
They are independent specifications of the
absorbed dose-distribution.
Dose homogeneity characterises the uniformity of
the absorbed dose within the target volume.
A perfectly homogenous dose to the PTV would be
characterized by :
a spike (a delta function) in the differential DVH
a vertical drop of the cumulative DVH line for the PTV.
It is usually a near Gaussian shape, tightly
distributed around the mean.
Another useful measure for such a distribution is
the standard deviation.
So mean and SD should be included in all
treatment planning systems for reporting HI.
ICRU-83
58. Several definition of HI have been made. But the
following definition for homogeneity is suggested in
this report:
HI= D2% - D98% / D50%
An HI of zero indicates that the absorbe-dose
distribution is almost homogenous. D50% is
suggested as the normalization value. Reporting of
D50% is strongly recommended in Level 2 reporting
ICRU-83
59. Dose conformity characterises the degree to which
the high dose region conforms to the target volume,
that is the PTV.
However, due to the presence and use of DVH, the
use of HI and CI is likely to be limited
For IMRT, the +7% to -5% specification may be a
constraint as the organ sparing is more significant
than target homogeneity.
ICRU-83
61. REPORTING OF CONFIDENCE INTERVALS
Although difficult, confidence intervals should be
reported whenever possible.
Confidence intervals are presently considered to be
part of Level 2 or Level 3 reporting whenever
possible.
TPS must make an effort to involve reporting of
uncertainities both in terms of systematic (Type B)
and random (Type A) errors.
ICRU-83
62. 85% confidence interval = 1.5 standard deviations
is a useful accuracy constraint for many
applications.
PTV is such designed that is receives lower dose at
its boundaries than interior, so DVH of PTV might
show heterogeneity of CTV which is not there and
DVH of CTV may show more homogeneity than is
actually the case. Hence both CTV and PTV should
be shown on the same graph as these define the
envelope of possible values of the DVH of the CTV
depending on movements (intrinsic or setup
related)
ICRU-83
65. GROSS TUMOUR VOLUME
Definiton as before
Specifications needed :
TNM Staging, ICD code
Mention site (GTV-T, GTV-
N, GTV-M)
Mention imaging modality
(CT/MRI/PET)
Record any changes
during treatment to plan
adaptive treatment
ICRU-83
66. CLINICAL TARGET VOLUME
Definition as before
Typically a probability of occult disease higher than
5-10% is assumed to require treatment.
There might be no CTV in case of a benign tumour,
only a GTV.
ICRU-83
67. INTERNAL TARGET VOLUME
Definition as before
Better delineation is possible by using 4-D imaging
ICRU-83
68. PLANNING TARGET VOLUME
Definition same as before
Compromising the PTV if it overlapped the OAR is
no longer recommended as in the previous ICRU
reports
In such a situations, the PTV is subdivided into as
subvolumes.
ICRU-83
69. In the case of absorbed-dose
compromise in the
overlapping region between
the PTV and the PRV,
reporting theabsorbed dose
in the sub-PTV, PTVSV-1,
(left DVH) can incorrectly
represent the absorbed
dose to the underlying CTV
ICRU-83
71. ORGANS AT RISK
Definition same
Classified in the same way as in ICRU into serial,
parallel, serial-parallel
ICRU-83
72. PLANNING ORGAN AT RISK VOLUME
Definition same
In case of overlap with PTV, the PTV and PRV can
be subdivided into sub-volumes and given
individual absorbed dose constraints.
ICRU-83
74. REMAINING VOLUME AT RISK
Ideally when delineating the OAR, especially for
IMRT, all normal tissues that could potentially be
irradiated should be outlined.
RVR is operationally defined by the difference
between the volume enclosed by the external
contour of the patient and that of the CTVs and
OARs on the slice.
Rationale :
To detect any unsuspected regions of high absorbed
dose outside the PTV and PRV
Useful in estimating the risk of late effects such as
carcinogenesis.
ICRU-83
76. PLANNING AIMS
Planning aims are dosimetric goals used to develop
treatment plans.
The use of multiple dose-volume constraints (Dmean
, D98% , D95%, D2% , DV, VD) for each defined volume
leads to more precision in the planning aims and is
therefore recommended.
Biological metrics (e.g., TCP, NTCP, EUD) might
be used as additional constraints for a desireable
absorbed-dose distribution. Though this approach
is still investigational.
ICRU-83
77. SPECIAL SITUATION ILLUSTRATING
THE USE OF PLANNING AIMS
Some difficult situations for planning aims:
Planned absorbed dose in the buildup region and in a PTV
extending outside the body contour
Overlapping volumes and conflicting absorbed-dose
objectives
Unexpected high absorbed dose to part of the RVR
ICRU-83
78. DOSE PLANNING IN BUILDUP REGION AND IN A PTV
EXTENDING OUT OF BODY CONTOUR
PTV may extend into buildup region or even into air
(in-air PTV) if sufficient fluence in the surrounding
air is needed to prevent the CTV from extending
outside the beam edge by movement/setup error.
In tangential breast irradiation, the region of the
beam that has been deliberately planned to bypass
the skin surface is called as “flash region”
Problems are that : sufficient dose might not be
obtained in the “flash region” and build up area or
there may be inhomogeneity in the PTV dose.
ICRU-83
80. OVERLAPPING VOLUMES AND CONFLICTING
PLANNING AIMS
As discussed previously :
Define sub volumes with separate dose constraints
for each
Or relax the absorbed-dose objectives for planning
Assigning importance to the constraints during
iteration.
ICRU-83
81. Subdivision of the planning volume and setting
different constraints for each subvolume
ICRU-83
82. UNEXPECTED HIGH DOSE TO THE RVR
Such a situation can be avoided by prescribing
constraints for the RVR considering it as analogous
to PRV.
ICRU-83
83. TECHNICAL DATA
Examples of technical data detailing :
Number, direction of beams, no of beam segments
and their intensity distribution.
Aperture shapes or multileaf collimator settings
Positioning and immobilization parameters for the
patient on the couch.
It is recommended that the electronic recording of
data is standardized e.g., the DICOM-RT protocol
ICRU-83
85. SUMMARY ICRU-83 RECOMMENDATION
Use of PET imaging should be explored for CTV & PTV
definition along with CT and MRI
Definition of sub volume with overlaps between PTV and
PRV
RVR (Remaining volume at risk) should be defined.
Optimization should be fully explored
Maximum dose is defined as D2%
Median dose, D50% is closed to prescription dose and
very close to reference dose
Minimum dose is defined as D98%
DVH is a measure of delivered dose, and be used in
clinical practices rather than reference point reporting.
ICRU-83
86. CONTD…
It also evaluates biological models (TCP, NTCP, EUD)
with the hope that in future it can be a part of level 2
reporting
Uncertainty & confidence interval should be included
Quality assurance for IMRT optimization and
uniqueness of the IMRT solution should be thoroughly
investigated
Commissioning of accelerator should include thorough
investigation of MLC leaf, gap and transmission
Treatment planning system should incorporate advance
algorithm for inhomogeneity correction & MUST be used
for dose calculation
ICRU-83
87. It is recommended to record and retain the dose
absorbed-dose distribution related parameters for
atleast for the life of the patient plus a minimum of 5
years and in clinical trials as long as scientifically
needed.
ICRU-83
89. FOR EVERY BODY
THAT IS
AFFLICTED BY THE
CURSE OF CANCER,
A THOUSAND
MINDS ARE
UNNERVED BY THE
FEAR OF CANCER. I
FOLD MY HANDS
IN PRAYER FOR ALL
OF THEM…
