(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: In high-dose-rate brachytherapy (HDR-BT) for cervical cancer, cumulative dose is obtained by simple DVH addition of the D2cc corresponding to different fractions. This method assumes that the organs-at-risk receive maximum dose always in the same location. To check this hypothesis, we investigate inter-fractional changes in the anatomical locations of D2cc hotspots in bladder and rectum using deformable image registration (DIR).
Methods: 41 cervical cancer HDR-BT fractions from 10 patients were used for the study. Pairs of fractions were DIR registered using our previously validated semi-automatic DIR algorithm to compare the position of V2cc from different fractions. The position of the 2cc volume (V2cc) with the highest dose (D2cc) was then determined in the reference image from the intersection of the contour and the individual D2cc isodose line in each fraction. The Dice similarity coefficient (DSC) between the fractions’ V2cc is calculated. We examine the relationship between the V2cc location change and the difference between the cumulative D2cc using DIR (D2ccT-DIR) and by simple DVH-addition (D2ccT-SA).
Results: DSC values ranged from 0.17-0.9 for bladder and 0.13-0.92 for rectum. DSC was larger than 0.75 in just 32% of cases for bladder and 47% for rectum. Even with a DSC of 0.9, variations larger than 10% between the D2ccT-DIR and the D2ccT-SA were found. In contrast, large V2cc changes (DSC=0.16) with small D2cc differences (0.14%) were found.
Conclusion: A large range of hotspot overlap was observed for both bladder and rectum. The impact of changes in hotspots position on the total D2cc is difficult to predict a priori. DIR should be used for dose accumulation in order to properly calculate hotspots from the total treatment.
Funding Support, Disclosures, and Conflict of Interest: Work funded by the CTRMI of UC San Diego
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