Dose-Volume Relationship of Urinary Bladder to Organs At Risk in Image-Guided Brachytherapy for Cancer of the Uterine Cervix

Abstract

Background
External beam radiation therapy concurrent with chemotherapy followed by intracavitary brachytherapy is the standard of care in the treatment of locally advanced cervical cancer. Treatment success is not only measured in terms of tumor control but also in the complication rates that are kept to a minimum. Significant genitourinary and gastrointestinal complications have been noted during long term follow-up, hence there is a need to find ways to improve outcomes. This study aims to determine the optimal volume of the urinary bladder that will result to the least radiation dose to the rectum, sigmoid colon, small intestines and the bladder itself using two-dimensional (2D Point A-based) and Image guided brachytherapy (IGBT) plans.

Methods
24 patients with histologic diagnosis of cervical cancer stage IB2 –IIIB were treated using a combination of external beam radiation therapy (EBRT) of 45 to 50.4 Gy and high dose rate (HDR) brachytherapy for four fractions. Patients were treated with different bladder volumes per fraction. The urinary bladder was instilled with 0-20cc (empty), 50-70cc, 100-120cc and 150-170cc of normal saline via Foley catheter prior to CT simulation and brachytherapy. A Point A-based and three-dimensional (3-D) plans were done using the Oncentra planning system. The dose volume histogram (DVH) of the urinary bladder, rectum, sigmoid colon and small intestines using the D2cc parameter were obtained.

Results
Using 3D-IGBT, a dose reduction trend was noted in the small intestines by increasing bladder volume from a mean dose of 2.18 Gy (empty bladder) to 1.17 Gy (150-170cc). However, this decrease did not lead to a statistically significant difference (p= 0.068). Varying bladder volumes did not influence the doses received by the rectum and sigmoid. Increasing bladder volume resulted to increased in radiation dose received by the bladder from a mean dose of 5.32 Gy (empty) to 5.95 Gy (150-170cc) which was not significant. For Point A based plan this increased dose to the bladder from mean dose of 6.42 Gy (empty) to 7.63 Gy (150-170cc) was observed to be statistically significant (p = 0.0495).

Conclusion
A trend of better sparing of the small intestines was observed with increasing bladder volume (50-150cc) but this was not seen on the rectum and sigmoid. The dose received by the urinary bladder is directly proportional with its volume.