7. Radiobiological Considerations

Historically, variation in dose is kept to a minimum inside each CTV and planning target volume (PTV) in external-beam treatments, with the aim of achieving a homogeneous dose distribution with the dose varying between 95 % and 107 % of the prescribed dose (ICRU, 1993b; 2000). However, with new techniques [such as intensity-modulated radiation therapy (IMRT)], some dose inhomogeneity (dose painting) may be specifically planned (ICRU, 2010). For intracavitary brachytherapy (ICBT), the dose may be prescribed to a point but more typically to a CTV with a volume of 10 cm to 200 cm (Pötter et al., 2006). Dose to a CTV may be prescribed as a D98 or other dose–volume value or to other clinical volumes as discussed in Sections 5 and 6. Even in the smaller CTVs, the dose distribution is heterogeneous with relatively low doses near the margin of the CTV and two to three times greater dose rates and doses delivered immediately adjacent to the radioactive sources. The average dose and dose rate within the CTV is consequently much higher than the dose and dose rate at the periphery. In external beam therapy, because of dose–volume–effect relationships, these high doses would not be tolerated by normal tissues, given the larger volumes commonly exposed. A study by De Brabandere et al. (2008) shows that the GTV receives on average 146 % (108 % to 273 %) of the dose to the periphery of the CTV. Outside of the CTV, there is a steep decrease in dose rate and dose.