The document discusses International Commission on Radiation Units and Measurements (ICRU) reports related to defining target volumes in radiation oncology. It summarizes key definitions from ICRU reports 29, 50, 62, and 83 including gross tumor volume, clinical target volume, planning target volume, treated volume, irradiated volume, and organs at risk. Parameters for accurately delineating volumes, accounting for uncertainties, and standardizing dose reporting are described to ensure radiation treatments are precise and can be compared.

1. TARGET VOLUMES IN RADIATION
ONCOLOGY
PRESENTER: DR. ASHISH NIGAM
MODERATOR: DR. N. P. PATEL
DEPT. OF RADIATION ONCOLOGY, PGIMS, ROHTAK
DATE OF PRESENTATION: 12/03/2019

2. INTRODUCTION
 The aim of radiation therapy is to deliver precisely prescribed dose of radiation
to a defined tumour volume with minimal damage to the healthy surrounding
tissue, resulting in eradication of tumour, high quality of life and prolongation of
survival.
 In 1978, the International Commission on Radiation Units and Measurements
(ICRU) recognized the need for a general dose-specification system that could
be adopted universally.
 Its objective is to develop concepts, definitions and recommendations for the use
of quantities and their units for ionizing radiation and its interaction with matter,
in particular with respect to the biological effects induced by radiation.

3. ICRU REPORTS RELATED TO TUMOR
VOLUME
 ICRU Report-29 (1978) Dose specification for reporting external
beam therapy in photons and electrons.
 ICRU Report-50 (1993) Prescribing, Recording, and Reporting
photon beam therapy.
• Supersedes and updates Report 29.
 ICRU Report-62 (1999) Supplement to ICRU Report No: 50 (ICRU
50 still valid)
 ICRU Report- 83 (2010) Prescribing, Recording, and Reporting
photon beam IMRT(Intensity Modulated Radiation Therapy).

5. ICRU REPORT 29
 The ICRU first addressed the issue of consistent volume and dose specification in
radiation therapy with the publication of ICRU Report 29 in 1978.
 Even though published in the 2D era, it attempted to address spatial uncertainties
by pointing out that the size and shape of a target volume may change during the
course of a treatment and that one should take into account the following
parameters when describing the target volume-
1. Expected movements (e.g., caused by breathing) of those tissues that contain the
target volume relative to anatomic reference points (e.g., skin markings, suprasternal
notch),
2. Expected variation in shape and size of the target volume during a course of
treatment (e.g., urinary bladder, stomach),
3. Inaccuracies or variations in treatment setup during the course of treatment.

6. What all was defined by ICRU 29
 Target Volume
 Treatment Volume
 Irradiated Volume
 Organs at Risk
 Hot Spot

7. DEFINITIONS
 Target volume : Volume containing those tissues that are to be irradiated
to a specified absorbed dose according to a specified time- dose pattern.
 Treated volume :volume enclosed by the isodose surface representing
the minimal target dose(lowest absorbed dose in the target area).
 Irradiated volume : volume that receives a dose considered significant
in relation to normal tissue tolerance (e.g., 50% isodose surface)
Limitations of ICRU 29: No attempt was made to define and separate the
margins for the different types of uncertainties.

8. Organs at risk (OAR)
Specially radiosensitive organs in or near the target volume whose
presence influences treatment planning and/or prescribed dose.

9. HOT SPOTS
 It represents a volume outside the PTV which receives a dose larger
than 100% of the specified dose.

10. ICRU REPORT 50
Volumes defined prior to treatment planning : -
 Gross Tumour Volume (GTV)
 Clinical Target Volume (CTV)
Volumes defined during the treatment planning : -
 Planning target Volume (PTV)
 Organs at Risk (OAR)
 Treated Volume (TV)
 Irradiated Volume (IrV)

11. Gross Tumour Volume
 The gross tumour volume (GTV) is the gross demonstrable extent
and location of the tumor.
 It may consist of primary tumor, metastatic lymphadenopathy, or
other metastases.
 Delineation of GTV is possible if the tumor is visible, palpable, or
demonstrable through imaging.
 GTV cannot be defined if the tumor has been surgically removed,
although an outline of the tumor bed may be substituted by
examining preoperative and postoperative images.

12. Reasons to describe GTV accurately
 Staging of the tumor according to the TNM.
 To define area requiring adequate dose delivery for treatment.
 Regression of GTV used as predictive of tumor response.

13. GTV
GTV
GTV
GTV

14. Clinical Target Volume
 The clinical target volume (CTV) consists of the demonstrated
tumour(s) if present and any other tissue with presumed tumor.
 It represents therefore the true extent and location of the tumour.
 Delineation of CTV assumes that there are no tumour cells outside
this volume.
 The CTV must receive adequate dose to achieve the therapeutic aim.

15. CTV
CTV
CTV

17. 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 variations
and inaccuracies in order to ensure that the prescribed dose is
actually absorbed in the CTV.
 The PTV can be considered as a 3-D envelope in which the tumour
and any microscopic extensions reside. The GTV and CTV can
move within this envelope, but not through it.
 It is used for dose planning and for specification of dose.

18. PLANNING TARGET VOLUME (PTV) AFFECTED BY :
 Size and shape of the GTV & CTV.
 Effects of internal motions of organs and the tumor.
 Treatment technique (patient fixation and daily setup errors).

19. PTV
PTV

21. TREATED VOLUME
 Additional margins must be provided around the target volume to
allow for limitations of the treatment technique.
 Thus, the minimum target dose should be represented by an isodose
surface that adequately covers the PTV to provide that margin. The
volume enclosed by this isodose surface is called the treated volume.
 Usually taken as the volume enclosed by the 95% isodose curve.
 The treated volume is larger than the planning target volume.

22. Reasons for identification of Treated Volume are :
1. The shape and size of the Treated Volume relative to the PTV is an
important optimization parameter.
2. Recurrence within a Treated Volume but outside the PTV may be
considered to be a “true”, “in-field” recurrence due to inadequate dose
and not a “marginal” recurrence due to inadequate volume.

23. Irradiated Volume
 The volume of tissue receiving a significant dose in relation to
normal tissue tolerance (e.g., ≥50% of the specified target dose) is
called the irradiated volume.
 The irradiated volume is larger than the treated volume.

25. ICRU REPORT 62:
 Gives more detailed recommendations on the different margins that
must be considered to account for anatomical & geometrical
variations & uncertainties.
 PTV has been separated into two components: an internal margin
and set-up margin.
 Classified 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

27. 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.

28. INTERNAL TARGET VOLUME (ITV)
 It is the margin given around the CTV to compensate for all
variations in the site, size and shapes of organs and tissues contained
in or adjacent to CTV. These may result from respiration, different
fillings of the bladder and rectum, swallowing, heart beat,
movements of bowel etc.

29. SET-UP MARGIN ( SM )
 It is the margin that must be added to account specifically for
uncertainties (inaccuracies and lack of reproducibility) in patient
positioning and alignment of the therapeutic beams during treatment
planning and through all treatment sessions.
 These uncertainties depend on factors like :
• Variations in patient positioning
• Mechanical uncertainties of the equipment (sagging of gantry,
collimators, and couch)
• Dosimetric uncertainties
• Transfer set-up errors from CT & simulator to the treatment unit
• Human factors

30. ITV = CTV + IM
PTV = ITV + SM

31.  Accuracy- how close the values are to the goal
 Precision- how close the values are to each other or how well they can be
reproducible
• Systematic errors
(Inaccuracy)
• Random errors
(Imprecision)
Set up Errors

32. Systematic error
 It is a treatment preparation error and is introduced into the
chain during the process of-
 Patient Positioning
 Simulation
 Target delineation
 Distortion between skin marks and patient anatomy
 If uncorrected, affects all treatment fractions uniformly.
Shift the entire dose distribution away from the clinical target
volume

33. Random error
 It is a treatment execution error.
 It is unpredictable and varies with each fraction.
 They can originate from:
 Unpredictable target motion.
 Inevitable fluctuations in daily setup.
Random errors blur the dose distribution around the Clinical
target volume.

34. Systematic error is more important, If uncorrected
it would be propagated throughout the treatment
course and lead to deleterious effect on local
control.

35. CLASSIFICATION OF ORGANS AT
RISK
 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).

37. PLANNING ORGAN AT RISK
VOLUME(PRV)
 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 uncertainties.
 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.

40. CONFORMITY INDEX ( CI )
 It is defined as the quotient of the Treated Volume and the volume of
PTV.
 Conformity index (CI) = TV/PTV
 It can be employed when the PTV is fully enclosed by the Treated
Volume.
 It can be used as a part of the optimization procedure.
 Dose conformity characterizes the degree to which the high-dose
region conforms to the target volume, usually the PTV.

41. GRAPHICS
 These are used to delineate the different volumes and the other
landmarks. These are in different colors for an easy and uniform
interpretation. The convention recommended and used in ICRU 62
are:
GTV - Dark Red
CTV – Light Red
ITV – Dark Blue
PTV – Light Blue
OR – Dark Green
PRV – Light Green
Landmarks - Black

42. 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.

43. MINIMUM DOSE ( Dmin )
 It is the lowest absorbed dose in the target area.
 In contrast to maximum adsorbed dose, no volume limit is
recommended when reporting minimum dose.

44. ICRU REFERENCE POINT
It has to be selected according to the following general criteria :
 the 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 dose should be accurately
determined.
 the point should be in a region where there is no steep dose gradient.
 The recommendations will be fulfilled if the ICRU reference point is
located :always at the centre ( or in the central part ) of PTV, and
when possible, at the intersection of the beam axes.

45.  For a single beam, the target absorbed dose should be specified on
the central axis of the beam placed within the PTV.
 For parallel opposed, equally weighted beams, the point of target
dose specification should be on the central axis midway between the
beam entrances.
 For parallel opposed, unequally weighted beams, the target dose
should be specified on the central axis placed within the PTV.
 For any other arrangement of two or more intersecting beams, the
point of target dose specification should be at the intersection of the
central axes of the beams placed within the PTV.

46. ICRU REFERENCE DOSE
 It is the dose at the ICRU Reference Point and should always be
reported.

47. 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).
 When the maximum dose outside PTV exceeds the prescribed dose,
then a “Hot Spot” can be identified.

48. ICRU REPORT 83

49. AIM OF THIS REPORT
 To provide the information necessary to standardize techniques and
procedures of IMRT, and to harmonize the prescribing, recording
and reporting of IMRT.
 The applicable concepts and recommendations of other ICRU
reports concerning radiation therapy (in particular reports 50 and 62)
are aimed to be adopted, and extended where required.

50. DEFINITION OF VOLUMES
 Concept using GTV, CTV, PTV and ITV is maintained.
 The report recommends clear annotations to further specify volumes,
e.g.:
GTV-T = Primary tumor GTV
GTV-N = Regional node GTV
GTV-M = Distant metastatic GTV
GTV-T(clin,0Gy) = Tumor GTV evaluated clinically before the start of
therapy
CTV-T+N(MRI-T2,30Gy) = CTV (tumor plus regional lymph nodes)
evaluated using T2-weighted MRI after treatment with an absorbed dose
up to 30 Gy.

51.  Remaining Volume at Risk (RVR): Introduced by ICRU
Report 83. The 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 slices that have
been imaged.
 Volume which have been missed in OAR and PRV.

52.  In case of overlap of the PTV with an OAR, no compromise
to the margins for expanding the CTV to the PTV.
 Separate planning aims for each sub-volume should be used.
 The reporting should however be done for the whole PTV.

53. Schematic description of
the PTV subvolume:
delineated in case of
overlap between the PTV
and the PRV. The dose
reporting should, however,
be done for the whole
PTV.

55. CONCLUSION
 Proper identification and delineation of GTV is the most important
factor in treatment.
 Other volumes like CTV, PTV, ITV should also be properly
delineated.
 The errors like set-up error and human errors should be kept to a
minimum.
 Dose prescription, fractionation and calculation should be done in
the same way by all the different centers throughout the world for
the proper exchange of information and reporting.

56.  An internationally standardized system of dose specification (e.g.,
ICRU Report 50 and 62) must be followed in reporting dosages in
the patient’s chart as well as in the literature.
 A treatment plan must show, at a minimum, PTV and organs at risk
with suitable margins. Other volumes such as the GTV, CTV, and
ITV are useful in evaluating a treatment plan.

58. ABSORBED 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.
 The heterogeneity should be foreseen while prescribing a treatment,
and, in the best technical and clinical conditions should be kept
within +7% and -5% of prescribed dose.