Estimation of dose-volume parameters of female urethra as organ-at-risk during interstitial brachytherapy in gynecological malignancies

Purpose: Interstitial brachytherapy (ISBT) is often used as post-external beam radiotherapy (EBRT) to treat locally advanced gynecological malignancies. Female urethra is in close proximity to the target during ISBT. However, it has not been evaluated as an organ-at-risk (OAR). Overlapping symptoms caused by radiation-induced bladder toxicity vs. urethral toxicity make it difficult to identify and report urethral toxicities separately. This was a retrospective study to estimate dose-volume parameters of female urethra during high-dose-rate ISBT. Material and methods: Data of 24 patients with gynecological malignancies treated by ISBT were selected. Urethra and periurethral regions were retrospectively contoured. Mean volume, D(max), D(mean), D(2cc), D(1cc), D(0.5cc), D(0.2cc), and D(0.1cc) were documented. Unpaired t-test was used for comparison of means. Results: 20/24 Ca. cervix, 1/24 Ca. vagina, and 3/24 Ca. vaginal vault received 6-6.5 Gy in 4 ISBT fractions. Mean urethral length was 3.54 ±0.55 cm. Mean doses received by urethra per BT fraction were D(max) = 4.23 ±1.32 Gy, D(mean) = 2.71 ±1.01 Gy, D(0.2cc) = 3.31 ±1.07, and D(0.1cc) = 3.54 ±1.09 Gy. Comparison of total BT 2 Gy equivalent dose (EQD(2)) with 4 fractions for urethra between patients with (9/24) and without anterior vaginal wall (15/24) involvement included D(mean) = 18.79 ±7.49 Gy vs. 11.14 ±6.15 Gy*, D(1cc) = 10.90 ±10.03 Gy vs. 4.54 ±3.93 Gy*, D(0.5cc) = 19.50 ±8.69 Gy vs. 11.97 ±6.54 Gy*, D(0.2cc) = 23.78 ±8.94 Gy vs. 15.51 ±7.39 Gy*, and D(0.1cc) = 25.88 ±9.37 Gy vs. 17.39 ±8.03 Gy*, respectively (*p < 0.05). Conclusions: Female urethra receives significant doses during ISBT for gynecological malignancies, especially when the anterior vaginal wall is within the target volume. Reporting doses to urethra would enable to develop clinical correlation and dose-volume constraints for urethra as organ-at-risk in future.