Target Volume Definition

1. Target Volume Definition
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. Developments in Radiotherapy
• Advances in imaging have been integrated with technological
developments in radiotherapy delivery so that 3D planning of volumes has
replaced 2D field arrangements.
• Major developments in tumor localisation have included the publication of
ICRU reports on target definition (Report 62 in 1999 and Report 71 in
2004) and the possibility of registration of different imaging modalities
including CT, MRI and PET. In treatment delivery,
• multi-leaf collimation has enabled treatments to be shaped to tumors and
intensity-modulated dose plans have provided solutions to previous
planning dilemmas.
• Accuracy of treatment delivery has been ensured by development of portal
imaging and daily image-guided and adaptive radiotherapy techniques.

3. Target volume definition
• ICRU target volume
definitions showing GTV,
CTV, PTV, treated and
irradiated volume. (ICRU
report 50).

4. Target Volume Definition

5. A Schematic Representation Of Tumor
Volume And Target Volume

6. References
• ICRU Report 50 – Prescribing,
Recording and Reporting Photon
Beam Therapy (1993)
• ICRU Report 62 – Prescribing,
Recording and Reporting Photon
Beam Therapy (Supplement to ICRU
Report 50) (1999)
• ICRU Report 71 – Prescribing,
Recording and Reporting Electron
Beam Therapy (2004)

7. Gross Tumor Volume
• The GTV is the gross palpable or visible/demonstrable extent
and location of the malignant growth.

8. Gross Tumor Volume (GTV)
• The gross tumor volume (GTV) is the demonstrable extent and
location of the malignant growth.
• This extent can be determined by palpation or direct
visualization, or indirectly through imaging techniques.
• GTV cannot be defined if the tumor has been surgically
removed.
– An outline of the tumor bed can be substituted by examining
preoperative and postoperative images.

9. Gross Tumor Volume
• Gross tumor volume (GTV) is the primary tumor or other
tumor mass shown by clinical examination, at
examination under anaesthetic or by imaging.
• Tumor size, site and shape may appear to vary depending
on the imaging technique used and an optimal imaging
method for each particular tumor site must therefore also
be specified.

10. Gross Tumor Volume
• A GTV may consist of primary tumor (GTV-T) and/or
metastatic lymphadenopathy (GTV-N) or distant
metastases (GTV-M).
• GTV always contains the highest tumor cell density.
• If Surgery is performed before radiotherapy, it removes the
gross tumor volume (GTV) so that GTV is absent and a
clinical target volume (CTV) must be used for planning.

11. Clinical Target Volume (CTV)
• The GTV is generally surrounded by a region of normal
tissue, which may be invaded by subclinical microscopic
extensions of the tumor.
• Additional volumes may exist with presumed subclinical
spread, such as to regional lymph nodes.
• These volumes are designated clinical target volumes
(CTV).
• The CTV is an anatomical concept, representing the
volume of known or suspected tumor.

12. Clinical Target Volume (CTV)
• A region to account for uncertainties in microscopic tumor
spread .
• A tissue volume that must be eliminated.
• This volume must be treated adequately in order to
achieve the aim of radical therapy.

13. Clinical Target Volume
• Clinical target volume (CTV) contains the GTV when
present and/or subclinical microscopic disease that has to
be eradicated to cure the tumor.
• This volume has to be treated adequately in order to achive
the aim of the therapy: cure or palliation.

14. Clinical Target Volume
• A CTV containing a primary tumor may lie in continuity
with a nodal GTV/CTV to create a CTV-TN (e.g.
tonsillar tumor and ipsilateral cervical nodes).
• When a potentially involved adjacent lymph node which
may require elective irradiation lies at a distance from the
primary tumor, separate CTV-T and CTV-N are used (as
shown in the next slide), e.g. an anal tumor and the
inguinal nodes.

15. Clinical Target Volume (CTV)
• Delineation is based on purely anatomical-topographic and
biological considerations, before choice of treatment modality
and technique.
• anatomical-clinical concept
• local invasive capacity of tumor regional lymph nodes
• CTV I: GTV + surrounding volume of local subclinical
involvement (often a margin of about 1 cm)
• CTV II, CTV III etc.: additional volumes with presumed
subclinical spread (e.g., regional lymph nodes)

16. Clinical Target Volume
ICRU illustrations to show (a) GTV-T plus GTV-N in continuity
(CTV-TN) and (b) CTV-T and CTV-N at a distance (ICRU, 2004).

17. Clinical Target Volume
• CTV can be denoted by the dose level prescribed, as for
example, CTV-T50 for a particular CTV given 50 Gy.
• For treatment of breast cancer, three CTVs may be used for an
individual patient:
– CTV-T50 (50 Gy is prescribed to the whole breast);
– CTV-T66 (66 Gy to the tumor bed);
– and CTV-N50 (50 Gy to regional lymph nodes).

18. Clinical Target Volume
• Variation in CTV delineation by the clinician (‘doctor’s
delineation error’) is the greatest geometrical uncertainty
in the whole treatment process.
• Delineation of CTV assumes that there are no tumor
cells outside this volume.
• The CTV must receive adequate dose to achieve the
therapeutic aim.

19. Planning Target Volume
• When the patient moves or internal organs change in size
and shape during a fraction of treatment or between
fractions (intra- or inter-fractionally), the position of the
CTV may also move.
• Therefore, to ensure a homogeneous dose to the CTV
throughout a fractionated course of irradiation, margins
must be added around the CTV.

20. Planning Target Volume
• These margins allow for physiological organ motion
(internal margin) and variations in patient positioning and
alignment of treatment beams (set-up margin), creating a
geometric planning target volume.
• The planning target volume (PTV) is used in treatment
planning to select appropriate beams to ensure that the
prescribed dose is actually delivered to the CTV.

21. Planning Target Volume (PTV)
• The volume that includes CTV with an Internal Margin
(IM) as well as a set-up margin (SM) for patient movement
and set-up uncertainties is called the planning target volume
(PTV).
• To delineate the PTV, the IM and SM are not added linearly
but are combined rather subjectively.
• The margin around CTV in any direction must be large
enough to compensate for internal movements as well as
patient-motion and set-up uncertainties.

22. Internal Margin (IM)
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.

23. Internal Margin
The motion occurs when the CTV position changes on
a day-to-day level and is mainly associated with organs
that are part of or adjacent to the digestive or breath
system. Changes in the patient’s condition, such as
weight gain/loss, can also affect the relative position of
the CTV.

24. Set-Up Margin (SM)
The uncertainties depends on different factors:
• variations in patient positioning
• mechanical uncertainties of the equipment
• dosimetric uncertainties (light-radiation field
agreement)
• transfer set-up errors
• human related uncertainties

25. Set-Up Margin (SM)
The margin that must be added to account specifically
for uncertainties (inacuracies and lack of
reproducibility) in patient positioning and aligment of
the therapeutic beams during treatment planning and
through all treatment sessions.

26. Planning Target Volume (PTV)
The PTV is a geometrical concept, and it is defined to
selcet 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.

27. Internal Target Volume (ITV)
• ICRU Report 62 recommends that an internal
margin (IM) be added to the CTV to
compensate for internal physiological
movements and variation in size, shape, and
position of the CTV during therapy in relation
to an internal reference point.
• The volume that includes CTV with these
margins is called the internal target volume
(ITV).

28. Organ Motion/Internal Margin
• Variations in organ motion may be;
– small (e.g. brain),
– larger and predictable (e.g. respiration or cardiac pulsation), or
– unpredictable (e.g. rectal and bladder filling).

29. Organ Motion/Internal Margin
• When treating lung tumors, the displacement of the CTV
caused by respiration can be dealt with in several ways:
– by increasing the CTV-PTV margin eccentrically to include all
CTV positions during a respiratory cycle;
– by using suspended respiration with a technique such as the
active breathing control (ABC) device;
– or by delivery of radiation using gating or respiratory correlated
CT scanning and treatment.

30. Organ Motion/Internal Margin
• Uncertainties from organ motion can also be reduced by
using fiducial markers. Radio-opaque markers are inserted
and imaged at localisation using CT or MRI, and at
treatment verification, using portal films, electronic portal
imaging devices (EPIDs) or online cone beam CT image-
guided radiotherapy (IGRT).
• The internal margin therefore allows for inter- and intra-
fractional variations in organ position and shape which
cannot be eliminated.

31. Set-Up Margin (SM)
• To account specifically for uncertainties in patient positioning
and alignment of the therapeutic beams during treatment
planning and through all treatment sessions, a Set-up Margin
(SM) for each beam is needed.
• Uncertainties to be compensated for may vary with different
anatomical directions, and thus the size of such margins
depends on the selection of beam geometries.

32. Sources Of Set-Up Uncertainty
• Uncertainties depend on different types of factors, such as:
– variations in patient positioning,
– mechanical uncertainties of the equipment (e.g., sagging of gantry,
collimators, and couch),
– dosimetric uncertainties,
– transfer set-up errors from CT and simulator to the treatment unit,
– human factors.

33. Set-Up Variations/Set-up Margin
• During a fractionated course of radiotherapy, variations in
patient position and in alignment of beams will occur both
intra- and inter-fractionally, and a margin for set-up error must
be incorporated into the CTV-PTV margin.
• Errors may be systematic or random.

34. Set Up Systematic Errors
• Systematic errors may result from;
– incorrect data transfer from planning to dose delivery, or
– inaccurate placing of devices such as compensators, shields, etc.
• Such systematic errors can be corrected.

35. Set Up Random Errors
• Random errors in set-up may be operator dependent,
or result from changes in patient anatomy from day to
day which are impossible to correct.

36. Set Up Random Errors
• Accuracy of set-up may be improved with better
immobilization, attention to staff training and/or implanted
opaque fiducial markers, such as gold seeds, whose position
can be determined in three dimensions at planning, and
checked during treatment using portal imaging or IGRT.
Translational errors can thereby be reduced to 1 mm and
rotational errors to 1°.

37. Treated Volume
• Additional margins must be provided around the target
volume to allow for limitations of the treatment technique.
• The minimum target dose should be represented by an
isodose surface which adequately covers the PTV to provide
that margin.
• The volume enclosed by this isodose surface is called the
treated volume.
• The treated volume is, in general, larger than the planning
target volume and depends on a particular treatment
technique.

38. Treated Volume
• This is the volume of tissue that is planned to receive a
specified dose and is enclosed by the isodose surface
corresponding to that dose level, e.g. 95 per cent.
• I.e. the TV is the volume enclosed by an isodose surface,
selected and specified by the radiation oncologist as being
appropriate to achive the purpose of treatment (e.g., tumor
eradication, palliation).

39. Treated Volume-Reasons For Identifying
• The shape and size of the Treated Volume relative to the
PTV is an important optimization parameter
• Recurrence within the 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.

40. Conformity Index
• This is the ratio of PTV to the treated volume, and indicates how
well the PTV is covered by the treatment while minimizing dose
to normal tissues.

41. Irradiated Volume
• This is the volume of tissue that is irradiated to a dose
considered significant in terms of normal tissue tolerance,
and is dependent on the treatment technique used.
• The size of the irradiated volume relative to the treated
volume (and integral dose) may increase with increasing
numbers of beams, but both volumes can be reduced by
beam shaping and conformal therapy.

42. Irradiated Volume
• The volume of tissue receiving a significant dose (e.g., ≥50% of
the specified target dose) is called the irradiated volume.
• The irradiated volume is larger than the treated volume and
depends on the treatment technique used.

43. Irradiated Volume
The irradiated volume is that tissue volume which receives a
dose that is considered significant in relation to normal tissue
tolerance.

44. Irradiated Volume
• Depend on treatment technique used, comparison of Treated
Volume and Irradiated Volume for different beam
arrangements can be used as a part of optimization procedure
• If the volume is reported, the significant dose level must be
expressed either in absolute values or relative to the specified
dose to the PTV
• Tissue volume which receives a dose that is considered
significant in relation to normal tissue tolerance

45. Reminder
• Palpable tumor (1cm3) = 109cells !!!
• Large mass (1kg) = 1012 cells - need three orders of magnitude
more cell kill
• Microscopic tumor, micrometastasis = around 106 cell -
need less dose

46. Strategies For Margins
• Margins are most important for clinical radiotherapy - they
depend on:
– organ motion - internal margin
– patient set-up and beam alignment - external margin
• Margins can be non-uniform but should be three dimensional
• A reasonable way of thinking would be: “Choose margins so
that the target is in the treated field at least 95% of the time”

47. Irradiated Volume
• The volume of tissue receiving a significant dose (e.g., 50% of
the specified target dose) is called the irradiated volume.
• The irradiated volume is larger than the treated volume and
depends on the treatment technique used.

48. Scenario A
• A margin is added around the GTV to take into account
potential “subclinical” invasion.
– The GTV and this margin define the CTV.
• An IM is added for the variations in position and/or shape and
size of the CTV.
• A SM is added to take into account all the
variations/uncertainties in patient-beam positioning.
• CTV + IM + SM define the PTV on which the selection of
beam size and arrangement is based.

49. Scenario A

50. Scenario B
• The simple (linear) addition of all factors of geometric
uncertainty (case A) often leads to an excessively large PTV
– Incompatible with the tolerance of the surrounding normal tissues.
• Instead of adding linearly the IM and SM, a compromise has to
be sought, and a smaller PTV has to be accepted.

51. Scenario B

52. Scenario C
• In the majority of clinical situations, a “global” safety margin is adapted.
– In some cases, the presence of ORs dramatically reduces the width of the
acceptable safety margin.
• Since the incidence of subclinical invasion may decrease with distance
from the GTV, a reduction of the margin for subclinical invasion may
still be compatible with chance for cure, albeit at a lower probability rate.

53. Scenario C

54. Example Of Treatment Volumes

55. Treatment Volumes
GTV
CTV
ITV
PTV

56. Organs At Risk
• These are critical normal tissues whose radiation sensitivity
may significantly influence treatment planning and/or
prescribed dose.

57. OAR
• An OAR is a critical structure located very close to the
target for which the dose of radiation must be severely
constrained.
• This is because overdosing with radiation within the
critical structure may lead to medical complications.
• OAR is also termed as “sensitive structure” or “critical
structure“ in the literature.

58. Organs At Risk
• Any movements of the organs at risk (OAR) or uncertainties of set-
up may be accounted for with a margin similar to the principles for
PTV, to create a planning organ at risk volume (PRV).
• The size of the margin may vary in different directions. Where a
PTV and PRV are close or overlap, a clinical decision about
relative risks of tumor relapse or normal tissue damage must be
made.
• Shielding of parts of normal organs is possible with the use of
multi-leaf collimation (MLC).
• Dose–volume histograms (DVHs) are used to calculate normal
tissue dose distributions.

59. Organs at risk
• Any movements of the organs at risk (OAR) or uncertainties of
set-up may be accounted for with a margin similar to the
principles for PTV, to create a planning organ at risk volume
(PRV).
• The size of the margin may vary in different directions.

60. Planning Organ At Risk Volume
• The organ(s) at risk (OR) needs adequate protection just as
CTV needs adequate treatment.
• Once the OR is identified, margins need to be added to
compensate for its movements, internal as well as setup.
• Thus, in analogy to the PTV, one needs to outline planning
organ at risk volume (PRV) to protect OR effectively.

61. Organs At Risk
• Where a PTV and PRV are close or overlap, a clinical
decision about relative risks of tumor relapse or normal tissue
damage must be made.
• Shielding of parts of normal organs is possible with the use
of multi-leaf collimation (MLC). Dose–volume histograms
(DVHs) are used to calculate normal tissue dose
distributions.

62. OAR May Be Divided Into Three
Different Classes
• Class I organ: Radiation lesions are fatal or result in severe
morbidity
• Class II organ: Radiation lesions results in moderate to mild
morbidity
• Class III organ: Radiation lesions are mild, transient, and
reversible, or result in no significant morbidity provisional

63. Planning Organ at Risk Volume (PRV)
• As with the PTV, any movements of the Organ(s) at Risk
during treatment, as well as uncertainties in the set-up during
the whole treatment course, must be considered.
– In particular, Internal and Set-up margins can be identified.
• This leads, in analogy with the PTV, to the concept of a
Planning Organ at Risk Volume (PRV).
• Note that a PTV and a PRV may overlap.

64. Functional Sub Units
• The FSU-concept suggests that, for the purpose of evaluation of
the volume-fractionation-response, the tissues of an Organ at
Risk can be considered to be functionally organized as either
“serial,” “parallel,” or “serial-parallel” structures.

65. Different Tissue Types
• Serial organs (e.g. spine) • Parallel organs (e.g. lung)

66. Different Tissue Types
• Serial organs (e.g. spine) • Parallel organs (e.g. lung)
Effect of radiation on the organ is different

67. Organ Types
• Serial organs - e.g. spinal cord • Parallel organ - e.g. lung
What difference in response would
you expect?
High
dose
region
Serial
organ
High
dose
region
Parallel
organ

68. Functional Sub Units

69. • Examples of tissue organization structures in the parallel-
serial model:
a) a serial string of subunits (e.g., the spinal cord)
b) a parallel string of subunits (e.g., the lungs)
c) a serial-parallel string of subunits (e.g., the heart)
d) a combination of parallel and serial structures (e.g., a nephron)
Functional Sub Units

70. In practice not always
that clear cut
❖ICRU report 62
❖Need to understand
anatomy and
physiology
❖A clinical decision
Functional Sub Units

71. Radiobiology: Normal Tissues
• Sparing of normal tissues is essential for good therapeutic outcome
• The radiobiology of normal tissues may be even more complex as
the one of tumors:
– different organs respond differently
– there is a response of a cell organization not just of a single cell
– repair of damage is in general more important

72. Different Tissue Types
• Serial organs (e.g. spine) • Parallel organs (e.g. lung)
Effect of radiation on the organ is different

73. Volume Effects
• The more normal tissue is irradiated in parallel organs
– the greater the pain for the patient
– the more chance that a whole organ fails
• Rule of thumb - the greater the volume the smaller
the dose should be
• In serial organs even a small volume irradiated
beyond a threshold can lead to whole organ failure
(e.g. spinal cord)

74. Planning Organ At Risk Volume (PRV)
Process of
Outlining PTV
and PRV

75. In Many Organs, Dose And Volume
Effects Are Linked
Dose
(Gy)
Rectal
volume(%)
>65 40
>70 30
>75 5
This is just an
example for the
probability of
complications
increasing with both
dose and volume.

76. Example Of Treatment Volumes

77. GTV

78. GTV

79. GTV

80. GTV

81. CTV

82. CTV

83. CTV

84. CTV

85. PTV

86. PTV

87. Treatment Volumes
GTV
CTV
ITV
PTV

88. Treatment Volumes