Room: Stars at Night Ballroom 2-3
Purpose: Elekta Unity MR-Linac adapts treatment plan online in each fraction before beam delivery. The online adaptation makes patient-specific QA extremely challenging but imperative for patient safety. We described our clinical experience of patient-specific QA in treating patients using the Unity MR-Linac.
Methods: Two patients have been treated with the Unity MR-Linac in our institution: (1) femur metastasis treated to 8Gy in 1 fraction; and (2) iliac lymph node metastasis treated to 30Gy in 5 fractions. Both patients were treated with step-and-shoot IMRT. Pretreatment QA included an independent secondary MU check at a dose point using RadCalc and an IMRT QA measurement using ArcCheck. Each adaptive plan was first compared with the reference plan to ensure the maintenance of dosimetric criteria. An online secondary MU check was then performed before beam delivery. After each treatment, an IMRT QA measurement was performed using ArcCheck for posttreatment verification.
Results: For the reference plan, the point dose accuracy of RadCalc was within 5% for both patients, though the maximum difference for a single beam was 9.3% and 8.8%, which was likely due to RadCalc not including the effects from magnetic field. IMRT QA had a passing rate of 100% for both patients at 3%/3mm gamma criteria. All adaptive plans maintained initial dosimetric criteria. Online RadCalc calculation took about 5 minutes for each fraction, with maximum difference for a single beam about 18% in both patients. The deviation from TPS calculation had a similar pattern as that for the reference plans though. All posttreatment IMRT QA for the adaptive plans had a passing rate about 100%, except the final fraction of patient 2 (93%).
Conclusion: Our patient-specific QA procedure ensured a safe delivery of online adaptive plan using the Unity MR-Linac. Our future plan is to replace RadCalc with RayStation as secondary dose verification.
Funding Support, Disclosures, and Conflict of Interest: MD Anderson is a founding member of the Elekta Unity MRL consortium.