Purpose: The LIAC intraoperative electron radiotherapy (IOERT) accelerator is commissioned for clinical use mostly based on the manufacturer’s SWL-LIAC simulation software, which substantially reduces the number of difficult-to-set-up measurements needed. Moreover, in clinical IOERT practice, the appropriate beam energy is chosen based on the target tissue thickness using the beam data modeled by this software. We aimed to independently validate the accuracy of SWL-LIAC’s calculated outputs, namely, percentage depth-doses (PDDs), off-axis dose profiles and relative output factors (ROFs), against experimental data.
Methods: For all the available energies (6, 8, 10, 12 MeV), PDD measurements were made for all sizes of the flat-ended applicators (3 to 10 cm) and several beveled applicators. Profiles and ROFs were measured for the more frequently used applicators in clinical practice and the reference 10 cm diameter applicator. Meticulous measurements were made using a p-type electron diode in a water phantom, aided by inhouse custom-made positioning tools and confirmed using radiochromic film dosimetry. The measured and calculated scans were compared using multi-criteria one-dimensional gamma analysis.
Results: Eighty-five percent of the calculated ROFs showed a ≤2% agreement with the measurements. The average gamma passing rate for PDDs was 100% using a 2mm/1% (global dose difference) criterion. Repeated measurements showed that the measured in-plane profiles were somewhat tilted, which was confirmed by the manufacturer. The average gamma passing rate of all profiles was 95.5% (2mm/3%); Without considering the in-plane profiles, this value was 97.6%. Most of the profile points that failed the gamma criteria were in the peripheral region of the inner field and the shoulder regions of profiles.
Conclusion: The accuracy of SWL-LIAC calculations can be deemed acceptable for most clinical applications. However, assuming isotropic off-axis profiles is unwarranted and measuring profiles in both directions for all energies at installation and acceptance testing is indicated.
Funding Support, Disclosures, and Conflict of Interest: This work is part of a postgraduate thesis by R. Rashidfar, financially supported by the Vice-Chancellery of Research, Shiraz University of Medical Sciences (project number 98-01-10-19770). Radio-oncology Department of Shiraz University of Medical Sciences is a Reference Hospital for PTW-Freiburg.