Purpose: The maximum field size on a commercial MR linac is 27x24cm. In the event that a treatment volume exceeds the field dimensions, a multi-isocenter technique can be employed to elongate the total treatment coverage area. We commissioned and clinically implemented a Pseudo 4Pi workflow on an MRgRT linac.
Methods: An eight-coplanar beam, dual isocenter step-and-shoot IMRT plan with 6cm displacement of isocenters across the IEC-Y axis was created. The plan was delivered to a 3D IMRT diode array phantom. In order to treat each isocenter, the treatment couch moves between the isocenters during the treatment workflow. A second plan was developed to perform an end-to-end validation in treatment workflow on a cylindrical water phantom with an A1SL ion chamber in the region of the field junction. The chamber dose was compared between the calculated TPS (i.e., mean dose of A1SL active volume ROI) and the measured dose. Additionally, treatment interrupts with IGRT shifts were performed to validate the couch localization accuracy in the Pseudo 4Pi workflow.
Results: The gamma analysis using 3%/2mm criteria resulted in a passing result of 98%. There was no observed overlap or separation at the dosimetric junction on the diode array measured dose profiles. For the ionization chamber absolute dose measurement, the difference between the measured (10.21±0.06Gy) and calculated (10.10±0.20Gy) dose was 1.04%. The treatment time was twice as long due to the doubling of treatment beams for the two isocenter approach. The couch localized within ±0.1mm during interrupts irrespective of positioned isocenter at respective delivery.
Conclusion: There was no overlap visible in the measured profiles and the difference between the expected and measured dose to the ion chamber was less than 5%. The accuracy of Pseudo 4Pi planning and execution make it a clinically acceptable technique when target volumes exceed the field size dimensions.
Funding Support, Disclosures, and Conflict of Interest: Dr. Mittauer reports personal fees from ViewRay Inc, other from MR Guidance LLC, grants from ViewRay Inc.