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In vivo dosimetry (IVD) in radiotherapy is a strong safety recommendation within the UK and a legal requirement in many European countries. Despite this, large-scale deployment of a service-wide IVD programme is challenging in terms of required physics resource associated with calibration and maintenance of dosimetry equipment and troubleshooting out-of-tolerance results. Moreover, traditional point-based measurement devices are becoming obsolete as VMAT and other rotational techniques are increasingly utilised. We describe our experience implementing and sustaining a semi-automated EPID in vivo dosimetry service as a gross safety check within a large radiotherapy centre using the Sun Nuclear SunCHECK system.

Following initial configuration in SunCHECK, 10 Varian linacs (7 x TrueBeam and 3 x 2100 series with aSi1000 or aSi1200 MV panels) were calibrated (July-September 2019) for acquisition of 2D integrated dosimetry images, supporting a patient load of approximately 300 new starts per month across a range of tumour sites and techniques. Subsequently as older linacs were replaced, new machines were calibrated as part of standard pre-clinical commissioning.

Since implementation, patient measurements were acquired as part of routine treatment delivery, against standard exclusion criteria such as field limit (determined with an automated script within the TPS) or collision risk. Patients were typically measured at first fraction or near beginning of treatment, and each measured image (1-6 treatment fields) analysed using 2D global gamma analysis at 20% threshold. Analysis was carried out simultaneously at 10% 5 mm 'Good', 5% 5 mm 'Better', and 3% 3 mm 'Best', but deemed acceptable at >90% passing rate at 'Good' reflecting local policy and regulatory requirement in respect of significant treatment errors. (All patients also received daily IGRT). DICOM query-retrieve and image processing was entirely automated within SunCHECK. Human review and approval of results was carried out daily by the local dosimetry team, with escalation of anomalous results to senior physicists following first-line troubleshooting.

Over three years, 9440 patients were measured comprising 22,532 individual beam measurements. Mean beam gamma passing rate at 10% 5 mm was 96.0 ± 14.6%, with 20,961 fields passing local criteria (93.0%). first line dosimetry review comprised approximately 0.5 – 1 person-hours per day. Common failure modes such as calibration drift, off-axis EPID placement or beam interruptions were easy to identify and correct with follow up measurement. Second-line physics activity, including Medical Physics Expert review, typically comprised 1 person-hour per month. Four patients (0.04%) required further investigation beyond routine troubleshooting protocols. No significant treatment errors were detected.

The SunCHECK system offers an efficient and practical solution for large-scale deployment of IVD as a safety check within a VMAT-first treatment paradigm.