Volume effects, regional responses and risk models

Volume specifications in radiotherapy (Figure 15.1) have been defined over time in reports 50, 62, 83 and 89 of the International Commission on Radiation Units and Measurements (23–25). Even the smallest target volume defined, i.e. the gross tumor volume (GTV), contains normal tissue elements such as blood vessels and connective tissue components. To account for various sources of uncertainty, the GTV is expanded to ensure adequate treatment. This expanded volume inevitably encompasses a significant volume of normal tissues. Moreover, meeting minimum dose objectives on the constructed target volume results in a treated volume (TV) receiving a clinically effective isodose. The difference between this treated volume and the target volume is exclusively composed of normal tissue components, exposure of which may induce adverse events. In addition to this volume of normal tissue receiving a high dose, the lower doses in the surrounding tissue have been shown to critically contribute to normal tissue damage. The clinical consequences of the exposure of normal tissues may impact patient quality of life and limit the dose that can be given to the target volume. Figure 15.1 Volume definitions in radiotherapy according to ICRU report 62 (<xref ref-type="bibr" rid="TNF-C015_CIT00024">24</xref>). GTV, gross tumor volume (detectable tumor volume); CTV, clinical target volume (GTV plus volumes with expected subclinical spread; PTV, planning target volume (CTV plus safety margin for movements or deformations of CTV, technical uncertainties, etc.; TV, treatment volume receiving the prescribed dose. A set of concentric circles which are labelled from inner to outer as G T V, C T V, and P T V, enclosed within T V. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003278337/46954fb0-e6f0-4004-a6ff-abb1818c3363/content/fig15_1_C.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>.