Presenter(s)
J. S. Kim1,2, X. Ray1, C. C. Conlin1,3, R. Manger1, I. Dragojevic1, S. Tate1, J. Vuong3, F. Cruz1, T. Calhoun1, T. Pawlicki1, and T. M. Seibert1,4; 1Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, CA, 2Loma Linda University School of Medicine, Loma Linda, CA, 3Department of Radiology, University of California San Diego, La Jolla, CA, 4Department of Bioengineering, University of California San Diego, La Jolla, CA
Purpose/Objective(s): Using a synthetic CT (sCT) generated from MRI streamlines the workflow for MRI-based prostate RT planning. While dosimetric differences between sCT and CT-based plans have been reported to be negligible, logistical challenges remain in implementing sCT-based planning. We describe the implementation of an FDA-cleared, commercially available sCT solution in a setting where the MR scanner is operated by the Department of Radiology, independent of the RT Department.
Materials/Methods: A clinical diagnostic MRI scanner with a flat tabletop was used for image acquisition. sCT generation was achieved using vendor-provided software. Patient selection for MR sim was limited to patients undergoing RT to the prostate (+/- focal boost) or prostate fossa, with no rectal spacer or hip implants present. Because the MR scanner is operated outside the RT department, radiation therapy technologists (RTT) were absent from MR sims, requiring a solution to standardize patient setup and documentation. Diagnostic radiology MR techs were trained on bladder filling and RT patient setup protocols. Patients were not given setup marks during the MR sim; initial setup at treatment was performed using a surface-guided RT system, and RTTs were trained to make fine adjustments by aligning the cone beam CT to the contoured interface of the CTV and rectum. To ensure reliability and consistency of MR-only planning, fusion conventions, contouring approach, and export of image/contours to the treatment planning system were standardized. A standard operating procedure was written to communicate the MR sim workflow, quality assurance expectations, and exceptions and escalation paths. Diagnostic MR QA was performed by Radiology staff in concordance with ACR accreditation. Additionally, monthly image distortion QA was performed by RT physicists using a standard MRI QA phantom and associated software. Once the workflow was in place, we compared key dosimetric parameters between CT- and sCT-based RT plans. Treatment planning was performed in the standard clinical treatment planning system, and treatments were completed on departmental C-arm and O-ring Linacs.
Results: Median difference in Hounsfield Units between CT and sCT plans in the prostate and rectum was 5.4 and 19.0, respectively. Across 6 patients for focal boost, the median dose difference in the GTV (D98%) was 0.3%, while D0.03 cc in the rectum was 5.4%. Distortion QA results over a 6-month period showed median distortion of 1.17 mm within 10–15 cm of the bore center, increasing to 4.61 mm at distances greater than 25 cm from the bore center. For patients treated to date, setup and treatment times are comparable between CT-based and sCT-based cases.
Conclusion: No clinically meaningful differences were found in key dosimetric parameters or monitor units between the plans. The MRI-only workflow was successfully implemented, and patients have begun treatment using MRI-only–based plans.