Introduction/Background To investigate the dosimetric effects of bladder volume on organs at risk (OARs) during the treatment of endometrial cancer.

Methodology Forty-eight patients with locally advanced endometrial cancer received high-dose-rate brachytherapy in the vaginal vault between 2020 and 2023. All patients underwent 3D planning CT scans with an indwelling catheter in place. As a treatment, 8 Gy in 2 fractions were delivered to 5 mm depth of the vaginal mucosa using 3–3.5 cm diameter cylinders. The bladder, rectum and small bowel were delineated as OARs. For each OAR, the volume dose was defined by use of the mean dose value in a 2.0-cc volume receiving the highest dose (D2cc). The dose-volume histograms were retrospectively generated for the D2cc of each OAR. Pearson correlations and regression analyses were conducted.

Results In all patients, the adjuvant radiotherapy dose was 45 Gy in 25 fractions. The mean bladder volume was 87 cc [11–404]. The bladder and rectal equivalent dose in 2- Gray (Gy) fractions (EQD2) D2cc were respectively 53 Gy [46–67] and 55 Gy [49–68]. The Bladder volume appreciably impacted the dosimetry of the bladder and small bowel, but the dosimetry of the rectum was unaffected. The bowel's EQD2 D2cc decreased significantly with greater bladder inflation (p < 0.01). The cylinder-to-bowel distance was increased proportionally with the increase of the bladder volume. There was also a significant correlation between bladder volume and (EQD2) D2cc of the bladder (p=0.0005). The higher the bladder was inflated, the higher the bladder dose became.

Conclusion The dose to the empty bladder is lower than when the bladder is full. Simultaneously, the doses to the bowels increase proportionally in the empty state of the bladder. Protection of the bowels, which are more radiosensitive, suggests treating the patients in the full state of the bladder.

Disclosures No financial interest.