In-Room MRI for Adaptive Radiotherapy for Cervical Cancer Using an Integrated MR-Guided Radiation Therapy System

Abstract

The superior soft-tissue visualization enabled with online MRI provides an opportunity for high-precision image guidance, response assessment and novel adaptive radiotherapy (ART) processes. At our institution, we developed a novel platform integrating a 1.5 T MR scanner-on-rails with a 6 MV linear accelerator capable of daily in-room MRI. The aim of this work was to develop a hybrid CBCT-MRI image-guided radiotherapy (IGRT) workflow and describe our preliminary experience with implementing in-room MRI for offline ART for intact-cervix cancer. Process mapping, hazard analysis and mock procedures were used to design a hybrid IGRT workflow consisting of CBCT-guided treatment delivery, immediately followed by post-delivery in-room MRI. Patients were transferred to the MR in the treated position via an air-cushion hover-board and then coils were placed for imaging. Treatment planning using VMAT was performed as per EMBRACE II (45 Gy/25 fractions). The primary CT was acquired with full-bladder, and simulation MRI with both full and empty-bladder were included to define the internal target volume (ITV). Daily CBCT guidance used the primary CT as the reference and in-room MRI consisted of a fast T2w axial scan covering the pelvis and primary target. All images were imported to the planning system, co-registered to the plan via the treatment position alignment and reviewed offline by a multi-disciplinary team for geometric assessment of organ motion and target coverage. Two patients with intact-cervix cancer were treated to date with the CBCT-MRI workflow. In-room MRI were acquired for 17/25 and 8/25 fractions. Facility down-time for servicing was the primary reason for missed MRI. The mean time between CBCT and MRI was 16 mins (range = 10–27 mins). The mean time between treatment delivery completion and MRI was 10 mins (6–18 mins). During offline review, large inter-fraction variations in bladder, rectum and target positioning were noted for the first patient. Systematic motion of the uterine fundus beyond the PTV was noted on CBCT, but complete visualization of the GTV, vagina and uterus required MRI. An adaptive replan with modified ITV was created for the final 7 fractions for this patient, which effectively maintained target coverage by the PTV. Comparing the CBCT and MRI, large intra-fraction variations in the bladder and primary target were observed. The mean change in bladder volume was 60 cc (1–161 cc) for the first patient and 29 cc (1–83 cc) for the second. We have successfully demonstrated a hybrid IGRT workflow with in-room MRI for offline ART for cervical cancer. Monitoring daily variations with MRI empowers a comprehensive adaptive paradigm that will become our new standard-of-care for intact-cervix cancer treated with VMAT. Further efforts are underway to optimize IGRT workflows with pre-delivery MRI, as well as to develop state-of-the-art dose accumulation and online ART processes.