Dosimetric Effects of Bladder Volume Changes in MR-Guided Radiotherapy for Cervical Cancer

Abstract

Purpose This study aims to assess the dosimetric impact of variable bladder volume on CTV and OAR in cervical cancer patients undergoing radiotherapy. Method A total of 27 cervical cancer patients were included in this study: 12 received radical radiotherapy, and 15 underwent postoperative radiotherapy. All patients were treated with the Elekta Unity MR-linac system. The dose requirement was 95–100% of the prescribed dose to the PTV(45 Gy/25 sessions/5 weeks). Daily images were acquired at the time of treatment using the MR-linac. For this study, MR images from the first three treatments of each patient were selected to contour the CTV and OAR (bladder, small bowel, rectum, right and left lateral femoral heads), and the treatment plan was recalculated using the Monaco TPS. The dosimetric effects of bladder volume changes on the CTV and OAR were analyzed by SPSS. Result Regarding the dosimetric effects on the CTV, in the postoperative radiotherapy group, D₉₈ and D₉₅ of the CTV decreased as the bladder filled, with D₉₅ decreasing from 5175.5 cGy to 5000.7 cGy. In contrast, for patients undergoing radical radiotherapy, the mean dose of the CTV increased from 5143.4 cGy to 5368.1 cGy as the bladder filled (ρ = 0.306, P = 0.035). For the dosimetric effects on the bladder, in the postoperative radiotherapy group, V₃₀ and V₂₀ of the bladder decreased as the bladder filled. In the radical radiotherapy group, the minimum dose of the bladder decreased with increasing bladder volume, but the maximum dose increased from 5103.3 cGy to 5794.7 cGy (ρ = 0.451, P = 0.006). For the rectum and small bowel, in the postoperative radiotherapy group, changes in bladder volume did not significantly affect the dose of the small bowel and rectum (p > 0.05). However, in the radical radiotherapy group, the minimum and mean doses to the rectum and the D₂ of the small bowel decreased with bladder filling. Conclusion This study assessed the dosimetric and volumetric effects of bladder filling on the CTV and OAR using daily MR images from the MR-linac. Bladder filling influences the dose distribution to CTV and OAR.