1. Robert Taylor MD, PhD, Alexander Whitley MD, PhD, Craig Baden MD, Javier Lopez-Araujo MD, Sui Shen PhD, O. Lee Burnett MD, Jennifer De Los Santos MD and Robert Kim MD Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham AL

2. Background/Purpose High Dose Rate Brachytherapy is being used with interstitial implants but the vaginal mucosal dose has not been well described Especially important in interstial implants which are highly patient specific and result in significant variations in vaginal mucosal dose Other work has failed to find a correlation between D2cc of the vagina and toxicity Our hypothesis was that the upper and lower vagina have different tolerance and by being considered separately a relationship might more readily be found. We had a high fistula rate (32%) in our patient population and our primary goal was to determine a dosimetric relationship.

3. Materials and Methods 19 pt who received HDR interstial brachytherapy between 2010 and 2012 were reviewed. Our institution started treating insterstitial with HDR in 2010 The Distal Vagina (defined as lower 3.5 cm) and Proximal Vagina were contoured separately with a 0.5 cm brush. Toxicity was recorded and D2cc/D0.1cc were calculated for combined external beam and brachytherapy α/β ratio assumed to be 3 for normal tissue

4. Figure 1 Patient with D2cc > 100 Gy contained mainly in the Upper Vagina with no fistula (EQD2 D2cc Upper Vagina = 181 Gy / Lower Vagina = 62 Gy) Figure 2 Patient with D2cc > 100 Gy extending to the Lower Vagina resulting in fistula (EQD2 D2cc Upper Vagina = 151 Gy / Lower Vagina = 144 Gy) D2cc Lower Vagina = 62 Gy D2cc Upper Vagina = 181 Gy EQD2 100 Gy Isodose D2cc Lower Vagina = 62 Gy D2cc Upper Vagina = 181 Gy EQD2 100 Gy Isodose

5. Results Median FU 18 months Grade 2-4 rectal toxicity and vaginal fistula were statistically significantly related to the distal vaginal EQD2 dose of ≥ 100 Gy p=0.022 and p=0.021 by Kaplan-Meier respectively No relationship was found to other dosimetric indices including total vagina D2cc or proximal vagina D2cc Local Control = 89% with DM = 16%

6. Conclusions Local Control using interstial HDR is excellent Contouring the distal and proximal vagina as two separate structures may better predict for fistula formation. Distal vaginal EQD2 D2cc of ≥100 Gy was the best predictor of vaginal fistula as well as Grade 2-4 rectovaginal toxicity