Poster - 44: Development and implementation of a comprehensive end-to-end testing methodology for linac-based frameless SRS QA using a modified commercial stereotactic anthropomorphic phantom

Authors


Abstract

Purpose:

An end-to-end testing methodology was designed to evaluate the overall SRS treatment fidelity, incorporating all steps in the linac-based frameless radiosurgery treatment delivery process. The study details our commissioning experience of the Steev (CIRS, Norfolk, VA) stereotactic anthropomorphic head phantom including modification, test design, and baseline measurements.

Methods:

Repeated MR and CT scans were performed with interchanging inserts. MR-CT fusion accuracy was evaluated and the insert spatial coincidence was verified on CT. Five non-coplanar arcs delivered a prescription dose to a 15 mm spherical CTV with 2 mm PTV margin.

Following setup, CBCT-based shifts were applied as per protocol. Sequential measurements were performed by interchanging inserts without disturbing the setup. Spatial and dosimetric accuracy was assessed by a combination of CBCT hidden target, radiochromic film, and ion chamber measurements. To facilitate film registration, the film insert was modified in-house by etching marks.

Results:

MR fusion error and insert spatial coincidences were within 0.3 mm. Both CBCT and film measurements showed spatial displacements of 1.0 mm in similar directions. Both coronal and sagittal films reported 2.3 % higher target dose relative to the treatment plan. The corrected ion chamber measurement was similarly greater by 1.0 %. The 3 %/2 mm gamma pass rate was 99% for both films

Conclusions:

A comprehensive end-to-end testing methodology was implemented for our SRS QA program. The Steev phantom enabled realistic evaluation of the entire treatment process. Overall spatial and dosimetric accuracy of the delivery were 1 mm and 3 % respectively.

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