9 Reporting in Brachytherapy: Dose and Volume Specification

196 Reporting in Brachytherapy

8.2 Dose distribution in brachytherapy (radiotherapy) for cervix cancer Brachytherapy for cervix cancer is in fact a particular case of intraluminal brachytherapy. Due to the inverse square law, the dose decreases steeply as a function of the distance from the sources and this dose gradient is steepest close to the sources. However, specifically for cervix treatment, combination of an intrauterine source and intravaginal sources results in (para)frontal “pear shape” isodose surfaces (volumes), used to extend dose distributions laterally and in (para)sagittal “banana shape” isodose surfaces (volumes), used to spare rectum and bladder in the AP direction (Fig 6.23). This “pear and banana shape” volume critically depends on the source arrangement.

Fig 6.23 Comparison of absorbed dose patterns of external and intracavitary therapy. Fig 23A shows the dose distribution for an irradiation of the pelvis with a 60 Co teletherapy machine and four fields – the so called “box technique”. The border of the target area is indicated by the heavy dotted line (from ICRU Report 29, [42]). Fig 23B shows a typical dose distribution in an intracavitary application (from IAEA [40]), Fig 6.23C compares the resulting dose profile (from ICRU Report 38 [43]).