Background. Quantifying the interfraction dose variations in the organs at risk (OAR) in HDR intracavitary brachytherapy (HDR ICBT). Methods. Rectum and bladder were contoured in 44 patients of cervical carcinoma on CT after each fraction of HDR ICBT (9？Gy/2 fractions). Interfraction dose variations (VARact) were calculated. Rigid image registration of consecutive fraction images allowed quantification of the hypothetical variation in dose (VARhypo) arising exclusively due to changes in applicator placement and geometry. VARhypo was regressed against the VARact to find out to what extent the applicator variation could explain the VARact in the OAR. The rest of the variation was assumed to be due to organ deformation. Results. The VARact in the dose to 2？cc of bladder and rectum were 1.46 and 1.16？Gy, respectively. Increased dose was seen in 16 and 23 patients in the subsequent fraction for bladder and rectum, respectively. Doses to OAR would have exceeded constraints in 16% patients if second fraction was not imaged. VARhypo explained 19% and 47% of the VARact observed for the bladder and rectum respectively. Conclusions. Significant interfraction variations in OAR doses can occur in HDR ICBT. Organ deformations are mostly responsible for this variation. 1. Background Intracavitary brachytherapy (ICBT) forms an integral part of management of cervical carcinoma. While high dose rate ICBT (HDR ICBT) has become popular due to its logistical advantages over low dose rate ICBT (LDR ICBT), it also necessitates dose fractionation in order to reduce normal tissue complications [1]. This results in inadvertent changes in the position/geometry of the applicator. In addition there are interfraction deformations in organs at risk (OAR) due to movement, shape changes, and variable filling of these hollow organs. These in turn result in organ dose variations, which have important implications in dose reporting. In the past several authors had described the interfraction variation in applicator geometry as well as that of the organ point doses using data from orthogonal radiographs [2–9]. Recent advances in technology have allowed the use of volumetric imaging in gynecological brachytherapy planning [10, 11]. As a result greater information is available on the volumetric doses received by these OAR themselves. Data from some of the recent series have highlighted the problem of interfraction dose variation in HDR brachytherapy using volumetric imaging modalities [12–14]. Interfraction variations in the doses to these OAR may arise as a consequence of changes in two
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