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. 2007 Jun 8;7(1):104–116. doi: 10.1102/1470-7330.2007.0013

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© 2007 International Cancer Imaging Society

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Schematic illustration of the relationship between GTV-T/N(s) and CTV-T/N(s) in different clinical situations^[6]. (Reproduced by kind permission from the ICRU). Scenario A. A margin is added around the gross tumour volume (GTV) to take into account potential ‘subclinical’ invasion. The GTV and this margin define the clinical target volume (CTV). In external beam therapy, to ensure that all parts of the CTV receive the prescribed dose, additional margins for geometric variations and uncertainties must be considered. An internal margin (IM) is added for the variations in position and/or shape and size of the CTV. This defines the internal target volume. A set-up margin is added to take into account all the variations/uncertainties in patient-beam positioning. CTV+IM+SM define the planning target volume (PTV) on which the selection of beam size and arrangement is based. Scenario B. The simple (linear) addition of all factors of geometric uncertainty, as indicated in scenario A, often leads to an excessively large PTV, which would be incompatible with the tolerance of the surrounding normal tissues. In such instances, instead of adding linearly the internal margin and the set up margin, compromise combinations are used, e.g., formalism). This quantitative evaluation is only possible if all uncertainties and their σ are available, i.e., in a few sophisticated protocols. Scenarios C and D. In the majority of clinical situations, a ‘global’ safety margin is adopted. In some cases, the presence of an organ at risk dramatically reduces the width of the acceptable safety margin (e.g., presence of the spinal cord, optical nerve, etc.). In other situations (scenario C), larger safety margins may be accepted. Since the incidence of subclinical invasion may decrease with distance from the GTV (see Fig. 2.4), a reduction of the margin for subclinical invasion may still be compatible with chance for cure, albeit at a lower probability rate. It is important to stress that the thickness of the different safety margins may vary with the angle at which one looks at the PTV (e.g., bony structures or fibrotic tissue may prevent, at least temporarily, malignant cell dissemination). (Note that if an adequate dose cannot be given to the whole GTV, the whole aim of therapy shifts from radical to palliative).

Inline graphic — Schematic illustration of the relationship between GTV-T/N(s) and CTV-T/N(s) in different clinical situations^[6]. (Reproduced by kind permission from the ICRU). Scenario A. A margin is added around the gross tumour volume (GTV) to take into account potential ‘subclinical’ invasion. The GTV and this margin define the clinical target volume (CTV). In external beam therapy, to ensure that all parts of the CTV receive the prescribed dose, additional margins for geometric variations and uncertainties must be considered. An internal margin (IM) is added for the variations in position and/or shape and size of the CTV. This defines the internal target volume. A set-up margin is added to take into account all the variations/uncertainties in patient-beam positioning. CTV+IM+SM define the planning target volume (PTV) on which the selection of beam size and arrangement is based. Scenario B. The simple (linear) addition of all factors of geometric uncertainty, as indicated in scenario A, often leads to an excessively large PTV, which would be incompatible with the tolerance of the surrounding normal tissues. In such instances, instead of adding linearly the internal margin and the set up margin, compromise combinations are used, e.g., formalism). This quantitative evaluation is only possible if all uncertainties and their σ are available, i.e., in a few sophisticated protocols. Scenarios C and D. In the majority of clinical situations, a ‘global’ safety margin is adopted. In some cases, the presence of an organ at risk dramatically reduces the width of the acceptable safety margin (e.g., presence of the spinal cord, optical nerve, etc.). In other situations (scenario C), larger safety margins may be accepted. Since the incidence of subclinical invasion may decrease with distance from the GTV (see Fig. 2.4), a reduction of the margin for subclinical invasion may still be compatible with chance for cure, albeit at a lower probability rate. It is important to stress that the thickness of the different safety margins may vary with the angle at which one looks at the PTV (e.g., bony structures or fibrotic tissue may prevent, at least temporarily, malignant cell dissemination). (Note that if an adequate dose cannot be given to the whole GTV, the whole aim of therapy shifts from radical to palliative).