TH‐E‐BRA‐08: MR Guided Radiotherapy for Cervix Cancer Treatment; Retrospective Feasibility Study

Purpose: To evaluate the efficacy of on‐line MR guided radiotherapy for cervix cancer patients. MR guidance was simulated in order to optimize the fractional dose to the on‐line targets. Methods: 33 cervical cancer patients underwent planning and weekly pelvic MRI scans during radiotherapy. In the previous retrospective adaptive planning study using 3 mm PTV margin, 5 over 33 patient cases were identified and enrolled in this study in which the coverage of GTV/CTVs was not acceptable with single IMRT adaptation with bone matching. MR guidance was simulated in order to maximize online high risk CTV (HRCTV) volume to be within 95% of the prescription dose (95p). Fractional dose after the image guidance was calculated, and was deformed back to the reference (planning) image for dose accumulation. Accumulated dose of the proposed technique was compared with that of current standard image guidance technique, bone matching in terms of the target coverage (cervix, GTV, HRCTV, lower uterus, parametria, and upper vagina) and OAR sparing (bladder, bowl, rectum, and sigmoid). Target coverage was considered acceptable if 95p dose or more was delivered to 98% of the target volume. OAR sparing was evaluated with accumulated V45 and D2cc. Results: On line MR based soft tissue guidance proposed in this study achieved the acceptance of target coverage to 97% from 53% (bone matching). Dose delivery to HRCTV and lower uterus was significantly improved (p<0.001, paired t‐test). The mean D2cc and V45 were reduced in bladder, rectum and sigmoid compared to bone matching. Conclusions: The retrospective study revealed that the on‐line MR based soft‐tissue image guidance is very effective for cervix cancer treatment. The technique significantly and successfully improved target coverage for the most difficult patient group identified from the previous study. Statistically significant improvement in OAR sparing was also noted. Anna Lundin and Henrik Rehbinder are both employees and shareholders of RaySearch Laboratories AB. All other authors have no conflicts to report.