Dosimetric effects of bladder volume changes in MR-guided radiotherapy for cervical cancer

Bladder volume variations during radiotherapy can significantly influence dose distribution to both target volumes and surrounding organs-at-risk (OARs). This study aims to assess the dosimetric impact of variable bladder volume on the clinical target volume (CTV) and OARs in cervical cancer patients undergoing MR-guided radiotherapy.

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.

Regarding the dosimetric effects on the CTV, in the postoperative radiotherapy group, D₉₈ and D₉₅ of the CTV decreased as the bladder filled. In contrast, for patients undergoing radical radiotherapy, the mean dose of the CTV increased from 5223.55 cGy to 5273.93 cGy as the bladder filled. 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 5347.68 cGy to 5581.63 cGy. 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. 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.

This study evaluated the dosimetric and volumetric impact of bladder filling on the Clinical Target Volume (CTV) and Organs at Risk (OAR) using daily magnetic resonance (MR) images from the MR-linac. The findings indicate that variations in bladder filling significantly affect dose distribution to both the CTV and OAR.