Late G2 vagina toxicity in post-operative endometrial carcinoma is associated with a 68 Gy dose equivalent to 2 Gy per fraction(α/β=3Gy) at 2 cm3 of vagina

Purpose To evaluate if the dose equivalent to 2 Gy per fraction (EQD2)(α/β=3Gy) at 0.1 cm3, 1 cm3, and 2 cm3 of vagina in vaginal-cuff-brachytherapy (VBT) (high-dose-rate [HDR] 192Ir-source) ± external-beam-irradiation (EBRT) is associated with toxicity in post-operative endometrial carcinoma (P-EC). Material and methods From June 2014 till November 2015, 67 consecutive P-EC patients underwent VBT ± EBRT; 44 patients received EBRT (median, 45 Gy; range, 44-50.4) + VBT (7 Gy), and 23 exclusive-VBT (6 Gy x 3 fractions). The upper 2.5 cm of vagina was delineated on computed tomography (CT). The active-length source was 2.5 cm, and the brachytherapy dose was prescribed at 5 mm from the applicator. D90, V100, and EQD2(α/β=3Gy) at 0.1 cm3, 1 cm3, and 2 cm3 of the most exposed part of the vagina were calculated. Vaginal toxicity assessment was completed with a LENT-SOMA-objective-criteria. Statistics were done with the use of χ2 and Student's-t test. Results The mean follow-up was 23.2 months (7.6-46.8). Median D90 was 7.8 Gy(α/β=3Gy). Late toxicity: 8 G1 and 9 G2. Median EQD2(α/β=3Gy) in vagina was 88.6 Gy (62.8-177.6) for 0.1 cm3, 72.4 Gy (57.1-130.4) for 1 cm3, and 69 Gy (53-113.4) for 2 cm3. Exclusive VBT vs. EBRT+VBT showed no differences in vaginal toxicity. There was no relationship between EQD2(α/β=3Gy) at 0.1 cm3 and 1 cm3 of vagina with G1-G2 toxicity (p = 0.62 and p = 0.58, respectively). G2 toxicity was related to EQD2(α/β=3Gy) at 2 cm3 (p = 0.03). EQD2(α/β=3Gy) > 68 Gy caused G2 late toxicity in 20.5% patients. All patients presenting G2 toxicity received > 68 Gy EQD2(α/β=3Gy). Conclusions More than 68 Gy EQD2(α/β=3Gy) at 2 cm3 was related to G2 toxicity in P-EC-VBT. Further studies including larger number of patients are needed to confirm these results. Patients receiving these doses should be informed of the risk of toxicity, with individualized treatment planning and follow-up to reduce G2 toxicity.