Assessing the Need for Dose Calculations to Organs at Risk Using MRI Planning in Vaginal Brachytherapy

Abstract

With the overall low morbidity of adjuvant vaginal brachytherapy (VB) and its relatively fixed geometry, the need for individual calculation of doses to organs at risk (OARs) such as the bladder and rectum has been questioned, and ICRU bladder and rectal point dose calculations have been previously recommended. Using T1 and T2-weighted MRI simulation scans during VB, we evaluated patient anatomic variability and doses to OARs. Nineteen MRI simulation scans from 9 endometrial cancer patients undergoing adjuvant VB were obtained. In this retrospective study, all cases were planned to treat the proximal 4cm of vagina to a total dose of 21 Gy in 3 fractions prescribed to 5mm. The urethra, bladder, rectum, and sigmoid were contoured and the minimum dose in Gy to the most exposed 0.1cc, 1cc, and 2cc of the organ (D0.1cc, D1cc, D2cc) and the ICRU 38 bladder and rectal points were calculated. Additionally, patient anatomic variability in relation to OARs was assessed. The mean D2cc to the bladder, sigmoid, and rectum per 7Gy treatment fraction prescribed to 5mm were 5.6 Gy (3.3 - 7.32 Gy), 2.4 Gy (0.6 - 5.9 Gy), and 6.0 Gy (4.1 - 7.1 Gy), respectively. The mean D0.1cc to the urethra was 5.2 Gy (2.0 - 7.7 Gy). MRI scans revealed that significant variability can exist in the dose to the sigmoid colon, especially when positioned superiorly at the apex of the vaginal cylinder. Due to the anatomy of the internal urethral sphincter, the urethra deviates from the bowel at the location of the vaginal vault. However, even when treating the proximal 4cm of the vagina, the D0.1cc urethra exceeds 75% and 100% of the prescription dose in 36.8% and 15.8% of cases, respectively. The bladder and rectal ICRU points underestimated D0.1cc to their respective structures by more than 10% of the prescription dose in 68.4% and 78.9% of cases, respectively. An initial MRI simulation scan may guide VB treatment regarding vaginal length and prescription (surface or 5mm) relative to the location of critical structures. In cases when bowel is located above the vaginal apex, or the urethral point dose approaches the prescription dose, a more prolonged treatment course and prescription to the vaginal surface can be considered. Based on this study, it appears that the ICRU bladder and rectum points underestimate the dose to 0.1cc of the organ by greater than 10% of the prescription dose in most cases.