Effects of Bladder Distension on Organs at Risk in 3D Image-Based Planning of Intracavitary Brachytherapy for Cervical Cancer

Purpose: To investigate the effects of bladder distension on organs at risk (OARs) in the image-based planning of intracavitary brachytherapy for cervical cancer. Methods and Materials: Thirteen patients with cancer of the cervix were treated with high-dose radiation brachytherapy (800 cGy/fraction for 3 fractions). For the three-dimensional (3D) analysis, pelvic CT scans were obtained from patients with indwelling catheters in place (defined as empty bladder) and from patients who received 180-cc injections of sterile water in their bladders (defined as full bladder). To compare the International Commission on Radiation Units and Measurements (ICRU) point doses with 3D-volume doses, the volume dose was defined by using two different criteria, D_2cc (the minimum dose value in a 2.0-cm³ volume receiving the highest dose) and D_50% (the dose received by 50% of the volume of the OAR) for OARs. Results: The bladder D_2cc was located more cranially in the bladder base and was distributed in multiple spots in 46% of patients. The rectal D_2cc was located in the area of the ICRU point as a single "hot spot." For patients with a full bladder, the mean bladder D_2cc increased from 634 to 799 cGy (28.8%, p = 0.002). However, the bowel D_2cc decreased from 475 to 261 cGy (45.0%, p < 0.001). There were no substantial differences in rectal and sigmoid D_2cc values. However, the mean D_50% values of both the bladder and the bowel decreased from 108 to 80 cGy (23.7%, p < 0.001) and from 282 to 221 cGy (19.7%, p = 0.004) with a full bladder, respectively. Conclusions: An increase in bladder volume resulted in a significant reduction in bowel D_2cc values at the expense of an increase in bladder D_2cc values. Treatment with a distended bladder is preferable to protect the bowel.