A novel anthropomorphic head phantom for the commissioning of MLC-based stereotactic radiosurgery on a linear accelerator

T. Brown,¹ C. Beck,¹ H. Holloway,¹ J. Kerns,² J. Fagerstrom,¹ D. Kaurin,¹ K. Kielar^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Varian Medical Systems, Palo Alto

ESTRO Annual Meeting, Madrid, Spain (2021).

Purpose/Objective: A new anthropomorphic head phantom has been designed by Varian Medical Systems for the purposes of commissioning MLC-based SRS treatments on TrueBeam and EDGE linear accelerators. The validation of the phantom prototype and development of an end-to-end testing procedure was performed by Northwest Medical Physics Center, a non-profit, clinical physics consulting group, at two independent community cancer centers with active SRS programs.

Material/Methods: The initial phantom prototype was designed to accommodate four interchangeable target cassettes for CT-MRI fusion verification, Winston-Lutz and hidden target tests, an ion chamber, and two perpendicular EBT3 film segments. A 2-cm, contrast-enhanced target located at the center of the ion chamber and film cassettes allowed for single-target SRS verification. Phantom testing and end-to-end procedure development were performed using a Varian TrueBeam and EDGE linear accelerators equipped with Millennium and high-definition MLC, respectively. Treatment plans at 6FFF and 10FFF were designed and tested using a nominal prescription dose of 18 Gy. End-to-end testing comprised of phantom simulation, isocenter size determination, and treatment delivery using a pinpoint ion chamber and EBT3 film.

Results: Initial prototype testing resulted in design changes for a revised, final design, including a change in the Winston-Lutz central marker material, addition of a second (1 cm) target to the film cassette, inclusion of fiducial points to the EBT3 film for registration with DICOM dose planes, and an adjustment to the external dimensions of the film cassette. Pinpoint ion chamber measurements showed agreement with the planned dose to within 3% for all plans tested. DoseLab software was used to perform a relative gamma analysis of the film dose planes compared to extracted data from the treatment planning system. Film irradiations showed gamma-passing results >90% for 3% and 1 mm, using auto-registration shifts ≤1 mm in any direction.

Conclusion: Initial testing of a new anthropomorphic SRS head phantom indicates that the system is robust for verifying end-to-end MLC-based SRS treatments. Ongoing work is now concerned with collecting end-to-end data, using the revised phantom design, from additional clinical sites for the purposes of establishing acceptance criteria for end-to-end measurements.