Analysis of Reference Dosimetry Audits for 6 MV FFF Beams: Comparison of C-Arm Vs O-Ring Linac Results.
Abstract
Purpose
To compare dosimetry audit results for 6 MV flattening-filter-free (FFF) photon beams between conventional C-arm linear accelerators (linacs) and a ring-based architecture (O-ring) linacs using IAEA/WHO postal dosimetry audit results.
Methods
A retrospective analysis was conducted of 6 MV FFF beam dosimetry audits (2017–2025). In each audit, radio-photoluminescent dosimeters (RPLDs) were irradiated to a nominal 2 Gy to verify reference beam output. Percent dose deviation was calculated relative to the IAEA-measured dose (Dratio%) and audits with deviations beyond ±5% were followed up. Results were grouped by linac type: C-arm versus O-ring (TomoTherapy® platform, Accuray Inc., Sunnyvale, CA; Halcyon™ and Ethos™ Therapy Systems, Varian Medical Systems, Palo Alto, CA). Distributions were compared using the non-parametric Mann–Whitney U test, variability was assessed using the median-based Levene’s test. Relative numbers of audits outside the ±5% acceptance limit were compared using a two-proportion z-test.
Results
A total of 754 FFF audits (532 C-arm, 222 O-ring) were analyzed. Mean ± SD Dratio% was −0.6 ± 3.0 for C-arm and −0.2 ± 10.5 for O-ring linacs; the difference in means was not statistically significant (p = 0.60). O-ring linacs exhibited significantly greater variability than C-arm linacs (p < 0.001). The proportion of out-of-tolerance audits was 2.4% for C-arm and 8.6% for O-ring linacs (p < 0.001). 79% of those followed up were within tolerance in the second attempt, with no evidence of a difference between groups. Audit volume increased by a factor 10 between 2017 and 2025.
Conclusion
The analysis shows no systematic bias between C-arm and O-ring linacs mean results. However, O-ring linacs exhibit greater variability and a higher proportion of results outside tolerance. These findings reinforce the need for independent reference dosimetry audits, particularly as new technologies are rapidly introduced in the clinic.