This study aimed to investigate the influence of calibration field size on the gamma passing rate (GPR) in patient-specific quality assurance (PSQA).

Two independent detectors, PTW OCTAVIUS 4D (4DOCT) and Arc Check, were utilised in volumetric modulated arc therapy plans for 26 patients (14 with Arc Check and 12 with 4DOCT). Plans were administered using Varian Unique machine (with 4DOCT) and Varian TrueBeam (with Arc Check), each employing different calibration factors (CFs): 4 × 4, 6 × 6, 8 × 8, 10 × 10, 12 × 12 and 15 × 15 cm2 field sizes. Gamma analysis was conducted with 2%2mm, 2%3mm and 3%3mm gamma criteria.

GPR exhibited variations across different CFs. GPR demonstrated an increasing trend below 10 × 10 cm² CFs, while it displayed a decreasing trend above 10 × 10 cm². Both detectors exhibited similar GPR patterns. The correlation between 4DOCT and Arc Check was strong in tighter criteria (2%2mm) with an R² value of 0·9957, moderate criteria (2%3mm) with an R² value of 0·9868, but reduced in liberal criteria (3%3mm) with an R² value of 0·4226.

This study demonstrates that calibration field sizes significantly influence GPR in PSQA. This study recommends the plan specific calibration field must obtain to calibrate the QA devices for modulated plans.

The complexity associated with the treatment planning and delivery of stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) volumetric modulated arc therapy (VMAT) plans which employs continuous dynamic modulation of dose rate, field aperture and gantry speed necessitates diligent pre-treatment patient-specific quality assurance (QA). Numerous techniques for pre-treatment VMAT treatment plans QA are currently available with the aid of several different devices including the electronic portal imager (EPID). Although several studies have provided recommendations for gamma criteria for VMAT pre-treatment QA, there are no specifics for SRS/SRT VMAT QA. Thus, we conducted a study to evaluate intracranial SRS/SRT VMAT QA to determine clinical action levels for gamma criteria based on the institutional estimated means and standard deviations.

We conducted a retrospective analysis of 118 EPID patient-specific pre-treatment QA dosimetric measurements of 47 brain SRS/SRT VMAT treatment plans using the integrated Varian solution (RapidArcTM planning, EPID and Portal dosimetry system) for planning, delivery and EPID QA analysis. We evaluated the maximum gamma (γmax), average gamma (γave) and percentage gamma passing rate (%GP) for different distance-to-agreement/dose difference (DTA/DD) criteria and low-dose thresholds.

The gamma index analysis shows that for patient-specific SRS/SRT VMAT QA with the portal dosimetry, the mean %GP is ≥98% for 2–3 mm/1–3% and Field+0%, +5% and +10% low-dose thresholds. When applying stricter spatial criteria of 1 mm, the mean %GP is >90% for DD of 2–3% and ≥88% for DD of 1%. The mean γmax ranges: 1·32 ± 1·33–2·63 ± 2·35 for 3 mm/1–3%, 1·57 ± 1·36–2·87 ± 2·29 for 2 mm/1–3% and 2·36 ± 1·83–3·58 ± 2·23 for 1 mm/1–3%. Similarly the mean γave ranges: 0·16 ± 0·06–0·19 ± 0·07 for 3 mm/1–3%, 0·21 ± 0·08–0·27 ± 0·10 for 2 mm/1–3% and 0·34 ± 0·14–0·49 ± 0·17 for 1 mm/1–3%. The mean γmax and mean γave increase with increased DTA and increased DD for all low-dose thresholds.

The establishment of gamma criteria local action levels for SRS/SRT VMAT pre-treatment QA based on institutional resources is imperative as a useful tool for standardising the evaluation of EPID-based patient-specific SRS/SRT VMAT QA. Our data suggest that for intracranial SRS/SRT VMAT QA measured with the EPID, a stricter gamma criterion of 1 mm/2% or 1 mm/3% with ≥90% %GP could be used while still maintaining an in-control QA process with no extra burden on resources and time constraints.