Applicator-guided proton therapy versus multichannel brachytherapy for vaginal vault irradiation

To dosimetrically compare applicator-guided intensity-modulated proton therapy (IMPT) and multichannel brachytherapy (MC-BRT) for vaginal vault irradiation (VVI) with special focus on dose to organs at risk (OARs) and normal tissues. Ten patients with uterine confined endometrial cancer who received adjuvant vaginal cuff brachytherapy were included in this study. For each patient an additional IMPT treatment plan was created using the same computed tomography dataset and contours segmented for MC-BRT plans. Clinical target volume (CTV) was defined as the proximal 3.5 cm of the vagina including the entire thickness of vaginal wall. Planning target volume for IMPT plans was generated from the CTV with an addition isotropic 3 mm margin. OARs included rectum, bladder, sigmoid, small bowel and femoral heads. The prescribed dose was 21 Gy in 3 fractions. For simplicity, all doses were expressed in Gy and a constant relative biological effectiveness of 1.1 was used for IMPT plans. Plan comparison was performed using dose-volume histogram and treatment planning parameters. A significant improvement of the D98% coverage for CTV was reached by the applicator-guided IMPT plans (p < 0.01). IMPT also provided a dose reduction in all OARs except for femoral heads due to the lateral beam direction, especially significant reduction of V5Gy, D2cc, D0.1 cc, Dmean, V95% values for the rectum and Dmean, D0.1 cc to bladder, sigmoid, small bowel. Additionally, IMPT plans showed a significant reduction of integral dose to normal tissue with respect to MC-BRT (221.5 cGy.L vs. 653.6 cGy.L, p < 0.01). Applicator-guided IMPT has the potential for improving plan quality in VVI while maintaining the high conformity afforded by the state-of-the-art intracavitary brachytherapy.