Purpose: To develop a patient-specific QA protocol for robustly optimized mixed electron-photon beam radiotherapy (MBRT) utilizing trajectory log files and MapCHECK.
Methods: An MBRT plan for a soft-tissue sarcoma patient was optimized. The plan consisted of an arc delivery of a 6MV photon beam combined with step-and-shoot deliveries of 5 electron beam energies. The SAD of the photon delivery was 100cm, and that of the electron delivery was 80cm to minimize the effect of scattering. The plan was delivered on a Varian TrueBeam linac and trajectory log files were collected. The axis positions and monitor unit (MU) at each control point were retrieved from the log files to recalculate the 3D dose distribution in the patient anatomy by EGSnrc. The planned and the calculated mean dose to the CTV and 3 organs at risk were compared. The plan was also delivered to the MapCHECK with collapsed gantry angles. A MapCHECK Monte Carlo phantom was modelled on EGSnrc using vendor-supplied technical drawings. The dose to the diode detectors were scored in a DOSXYZnrc simulation. The agreement between measured and calculated dose distribution was evaluated using gamma analysis.
Results: The log file-recalculated and the planned mean dose to the CTV differed by less than 0.1%. The deviation between the recalculated and the planned mean dose to the OARs varied between -0.01% to +1.87%. The comparison between the measured and the calculated overall dose distribution to the MapCHECK demonstrated a high gamma passing rate of 96.4% with the gamma criteria of 2%/2mm.
Conclusion: Both the log file-recalculation and MapCHECK measurement were in excellent agreement with the planned dose distribution. The results demonstrated the feasibility to conduct MBRT plan verification with log file analysis and MapCHECK. Ideally, both procedures are performed in a single delivery to provide comprehensive QA for this complex plan.
Funding Support, Disclosures, and Conflict of Interest: Funding support: Canadian Institutes of Health Research, Foundation Grant FDN-143257
Quality Assurance, Monte Carlo, Treatment Verification
TH- External Beam- Electrons: Development (new technology and techniques)