Room: Stars at Night Ballroom 2-3
Purpose: MR-guided online adaptive radiotherapy (MRgoART) relies on a two-step deformation: simulation CT (CTsim) to simulation MR (MRsim) to MR scan-of-the-day (MRdaily). We have quantified the accuracy of electron density deformation in MRgoART.
Methods: 35 patients (breast, pancreas, liver, lung, rectum, esophagus, gastric) were included. Patients were treated on a clinical MR-guided radiotherapy (MRgRT) system, with CTsim and MRsim acquired for initial plan creation. Each patient was retrospectively simulated for MRgoART with inputs of CTsim, MRsim with planning contours, and MRdaily. To quantify the CTsim-MRsim-MRdaily deformation accuracy, the mutual information (MI) between the CTsim deformed to the MRdaily frame-of-reference (FOR) and the MRdaily was quantified. The mean absolute error (MAE) of the daily deformed electron density was also computed with respect to the organ-of-interest from the CTsim.
Results: 105 MRgoART fractions were analyzed. The MI was 1.00Â±0.13 for thoracic sites (lung, esophagus), 0.80Â±0.13 for abdominal sites (liver, pancreas, gastric) and 1.02Â±0.07 for pelvis sites (rectum). Breast had the greatest MI difference 0.67Â±0.20, potentially due to interfractional changes with time between CTsim and MRdaily, (average time 33 days). The MAE (N=604 ROIs) was 38.8Â±40.9HU for thoracic ROIs (lung, esophagus;N=116), 30.2Â±38.6HU for thoracic/abdominal soft tissue ROIs (heart, pancreas, kidney, liver, breast;N=274), 36.1Â±29.7HU for bone ROIs (chestwall, spinal column, femur, pelvis;N=57), 16.3Â±19.9 HU for gastrointestinal ROIs (bowel, duodenum, stomach, sigmoid, rectum;N=143), and 9.3Â±15.6 HU for genitourinary ROIs (bladder;N=14). A correlation was observed between MI and the requirement for clinical on-table MRgoART.
Conclusion: CTsim-MRsim-MRdaily deformation was less accurate for abdominal sites (MI=0.80, p=0.0013) compared to thoracic and pelvic sites for MRgoART, potentially due to large interfractional abdominal changes observed from CTsim to MRdaily; nevertheless resulting electron density differences (MAE=16.3 HU) were negligible for gastrointestinal tissues. MI may be a valuable clinical tool for indication of MRgoART, due to sensitivity of metric to interfractionial changes.
Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees and consulting fees from ViewRay Inc. during conduct of this study. Dr. Bayouth reports membership on scientific advisory board of ViewRay Inc. during conduct of this study.