A multi-institutional EPID-based 3D dose reconstruction model in a virtual phantom for standardized patient QA, benchmarking and auditing for stereotactic radiosurgery

For single-isocentre multi-target (SIMT) stereotactic radiosurgery (SRS), benchmarking, auditing and inter-institutional standardisation of dose verification remain challenging as they require specialized equipment and expertise. This work aims to develop and validate an electronic portal imaging device (EPID)-based technique to determine 3D dose in a virtual spherical phantom which is applicable across institutions for SIMT SRS dose verification. Small-field output factors from 11 international centres were measured in-water for the TrueBeam linear accelerator, including jaw-defined and high-definition multi-leaf collimator (MLC) fields from 0.5 × 0.5 to 20 × 20 cm and depths from 1.5-20 cm. Matching EPID images were collected at each centre. Utilising the average-centre data, jaw-derived and MLC-derived models were created to convert EPID images to three-dimensional dose in a virtual spherical phantom. Models were validated by comparing EPID-derived dose with film in a three-dimensional-printed spherical phantom for 11 non-coplanar SIMT SRS plans. Output factors estimated from EPID-to-dose models were within 1.8% (jaw-derived) and 1.0% (MLC-derived) of measured factors at 10 cm depth. For SIMT SRS plans, target dose differences were -0.3 ± 2.4% (mean ± standard deviation) and gamma pass rates were 99.6 ± 1.7% (5%/1.5 mm) for the jaw-derived model compared to film. For the MLC-derived model, -0.2 ± 2.1% dose differences and 99.7 ± 1.2% pass rates (5%/1.5 mm) were achieved. The EPID-based three-dimensional dose reconstruction method was modelled using multi-institutional data and validated for static and SIMT SRS deliveries. This method requires no specialised equipment and can be applied across institutions for benchmarking, auditing and standardised patient-specific quality assurance for SRS.