9 Reporting in Brachytherapy: Dose and Volume Specification

202 Reporting in Brachytherapy

Fig 6.27: Geometry for measurement of the dimensions of the pear-shape volume (Treated Volume, “point A” volume, reference volume) in a typical treatment of cervix carcinoma. -Plane a is the “oblique frontal plane” that contains the intrauterine device. It is obtained by rotation of the frontal plane around a transverse axis. -Plane b is the “oblique sagittal plane” that contains the intrauterine device. It is obtained by rotation of the sagittal plane around the AP axis. The height (dh) and the width (dw) of the pear-shape volume are measured in plane a as the maximal sizes parallel and perpendicular to the uterine applicator, respectively. The thickness (dt) of the pear-shape volume is measured in plane b as the maximal size perpendicular to the uterine applicator. The dimensions dh, dw and dt refer to the brachytherapy application only (left part of the figure). When combination of brachytherapy and external beam therapy is taken into account (right part of the figure), these dimensions are symbolized by dH, dW and dT, respectively. (modified from ICRU Report 38 (43) In definitive treatment the total dose at point A and/or in the PTV2 (tumour extension at the time of brachytherapy) currently ranges between 75 and 90 Gy (and above). This includes limited disease (e.g. stage IB1, stage IIB proximal) and the wide range of extensive disease (stage IB2 to IIIB/IVA). Therefore, one should consider selecting as reference volume, the volume encompassed by a higher dose level e.g. the 75 or 85 Gy isodose. Selection of 60 Gy (definitive treatment for very early disease; combination treatment with surgery for limited disease) and 75/85 Gy (definitive treatment for limited and extensive disease) to define the reference volume does not imply that these doses are recommended as optimal therapeutic doses. It is only for purpose of comparison that the concept of reference volume was introduced in ICRU Report 38 and is recommended here again. (Fig 6.28).