Sigmoid dose using 3D imaging in cervical-cancer brachytherapy

Background and Purpose: To evaluate the proximity, variance, predictors of dose, and complications to the sigmoid in cervical-cancer brachytherapy using 3D planning. Materials and methods: Over 36 months, 50 patients were treated for cervical cancer with either low-dose-rate (LDR) or high-dose-rate (HDR) brachytherapy. The distance from the central tandem to the sigmoid, the D0.1cc and the D2cc to the sigmoid, rectum and bladder doses, and toxicity were analyzed. Results: The median sigmoid EQD2 D0.1cc and D2cc were 84 Gy and 68.3 Gy for HDR versus 71.1 Gy and 65.9 Gy for LDR (p = 0.02 and 0.98, respectively). Twenty percent of the HDR fractions required manipulation of the superior dwell positions to decrease the sigmoid dose. The median distance from the sigmoid to the tandem was 1.7 cm (range [rg], 0.1-6.16 cm) for HDR and 2.7 cm (rg, 1.17-4.52 cm) for LDR; from the sigmoid to the 100% isodose region the median distances were -0.1 cm (rg, -1.4 to 2.5 cm) and 0.44 cm (rg. -0.73-5.2 cm), respectively. The proximity of the sigmoid to the tandem is significantly related to sigmoid dose (p < 0.0001). Within-patient (among-fraction) variation in sigmoid-to-tandem distance during HDR was substantial (coefficient of variation =40%). No grade 3-4 sigmoid toxicity was seen after a median 31-month follow-up period. Conclusions: 3D imaging in cervical-cancer brachytherapy shows the sigmoid in close proximity to the tandem. The sigmoid-to-tandem distance varies substantially between fractions, indicating the importance of sigmoid dose-volume evaluation with each fraction.