Impact of Healthy Volunteer MR-Linac Imaging on Clinical Implementation of Stereotactic MR-Guided Online Adaptive Radiotherapy

Abstract

Stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) combines real-time MR imaging with RT delivery, allowing for online adaptive planning and continuous motion management. However, uncertainty remains in optimal patient positioning, image quality standards, and contouring guidelines for SMART. To determine the optimal combination of MR sequence parameters, coil and patient positioning, and treatment immobilization, we conducted a healthy volunteer (HV) study utilizing a 0.35T MR-Linac (MRL). We hypothesized that with optimization of these parameters, image quality and SMART workflows for HVs would be acceptable across a range of clinically relevant organs, facilitating subsequent patient imaging. The HV study was approved by the IRB. Adults of all races and genders were included. MR simulation and simulated SMART were performed in HVs on an MRL. For each scan, the following parameters were assessed on a scale from 1-5 (5 = extremely high, 1 = extremely poor): visibility of target, visibility of nearby normal organs, and overall image quality. Adaptive recontouring time was recorded. With IRB approval, we retrospectively reviewed recontouring times for the first fraction for the first 18 patients treated clinically with SMART. HV and clinical times were compared with a t-test (alpha = 0.05). 46 HVs consented to the study. 18 underwent simulated treatments that included simulation of all aspects of SMART clinical workflow, except RT delivery, and targeted the following organs: adrenal, prostate, pancreas, lung, kidney, and brain. Average visibility of target and nearby organs were 4.58 and 4.62, respectively, and average overall image quality was 4.62 with a range of 4-5 for all measures. The average recontouring time was 15 minutes (range 4-35) in the HVs, comparable to an average of 16.1 minutes (range 7-33) in patients (p = 0.886). The clinical cases targeted adrenal, prostate, pancreas, lung, and pelvic and abdominal nodes. HV scanning on an MRL resulted in acceptable image quality and target visibility for a range of organs. Contouring time, the longest step in our workflow, was well-simulated in HVs, with similar times achieved clinically. The organs targeted in HVs correlated well with the organs treated clinically. The HV study provided a non-clinical environment to optimize all aspects of SMART, except for RT delivery. Further study is needed to determine the optimal number of HV scans required to prepare for clinical implementation of SMART.