Urethral dosimetry and toxicity with high-dose-rate interstitial brachytherapy for vaginal cancer

PURPOSE: The tolerance and complication rates of the urethra are unknown for the interstitial high-dose-rate brachytherapy (HDR-BT) for vaginal cancer. METHODS AND MATERIALS: Patients with vaginal cancer near/involving the urethra who were treated with HDR-BT between 2008 and 2011 were included. Patients received mean external beam dose of 48.0 Gy followed by mean HDR-BT dose of 4.5 Gy/fraction for five fractions. With CT-based planning, the urethra was contoured from the bladder neck to the meatus. Doses were converted to the biologically equivalent dose in 2 Gy/fraction (EQD(2)). RESULTS: A total of 16 patients were included, and the EQD(2) D-90 was 74.9 Gy. The urethral volume was 1.31 cm(3), and the EQD(2) to 0.1 and 1 cm(3) were 76.2 and 48.9 Gy, respectively. Two of the 6 patients with urethral involvement developed urethral necrosis. The D-90 for these 2 patients was 76.8 Gy, and the urethral doses to 0.1 and 1 cm(3) were 95.1 and 45.8, respectively. Those who developed severe urethral toxicity had a trend to urethral EQD(2) (95.1 Gy vs. 73.4 Gy, p = 0.1) and significantly higher dose per fraction of HDR-BT to 0.1 cm(3) of the urethra (5.7 Gy vs. 3.7 Gy, p = 0.02) when compared with those who did not develop severe urethral toxicity. CONCLUSIONS: This study is among the first to assess urethral dosimetry for patients treated with HDR-BT for vaginal cancer. Patients who received five fractions of higher than 5 Gy/fraction to 0.1 cm(3) of urethra (estimated EQD(2) of 85 Gy) are at increased risk of severe urethral toxicity. (C) 2013 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.