Introduction
This topic explains how radiotherapy methods are grouped by the way the dose is spread through space (the target and the surrounding normal tissue) and over time (fraction size, number of fractions, overall time and dose rate), and how the site, depth and extent of the lesion in the body determine which options are chosen. An examiner expects you to name each class, give one example and one limitation, and justify the choice for a given localisation.
Classification by dose distribution in space
The aim is a high absorbed dose in the tumour and as low a dose as possible in normal tissue. The spatial pattern is judged on isodose curves, the dose-volume histogram and the dose homogeneity index.
- Homogeneous (uniform) irradiation: the dose is nearly equal throughout the target, within the ICRU limits of about minus 5 to plus 7 percent of the prescribed dose. Examples: whole-brain or whole-breast fields, total skin electron treatment.
- Heterogeneous (non-uniform) irradiation: dose gradients are deliberately created, with a higher dose in part of the target and a lower dose in the rest. Examples: a boost to the tumour bed after breast-conserving surgery, intensity-modulated radiotherapy.
- Conformal and focal irradiation: the high-dose volume follows the shape of the target, with steep dose falloff at its edges. This needs precise imaging, planning and immobilisation.
- Point or line source irradiation (brachytherapy): the dose falls with the inverse square of the distance from the source, so the distribution is strongly heterogeneous and the normal tissue beyond the implant receives a small dose.