ESTRO 2024 - Abstract Book

S4846

Physics - Quality assurance and auditing

ESTRO 2024

Purpose/Objective:

An audit was performed for the first 150 patients treated with the new SunCHECK "PerFRACTION" transit dosimetry system at Coventry. Transit dosimetry calculates dose using transmitted radiation through patients and detects issues that could impact treatment safety. These include changes that affect patient dose such as weight loss, tissue swelling, tumour shrinkage, density changes, position errors, machine faults, or errors in the planning process [1]. The system was audited to ensure that all clinically relevant issues were detected, to evaluate the suitability of tolerances, and to identify potential improvements to the workflow.

Material/Methods:

SunCHECK recalculates planned doses from the Treatment Planning System (TPS) using an independent dose algorithm and beam model, validated against linac and patient test data. Transit dose is then calculated using EPID images measured in-situ during radiotherapy treatment. Clinical 6FFF and 6MV volumetric modulated arc therapy (VMAT) patients were evaluated, including head and neck, lung, and pelvis treatments. Baseline tolerances were derived during commissioning using data for a range of treatment scenarios and artificially induced errors, such as machine faults, anatomical changes, and set-up errors. The sensitivity of the system was tuned in order to trigger further review in cases where measured dose deviations were significant (3%/3mm for brain or head and neck plans, 5%/3mm otherwise).

Results:

94.0% of patients had in-tolerance “Level 1” results by the end of treatment. 2.0% of plans had no measured transit result owing to technical or procedural error and were instead investigated by alternative methods and categorised as “Level 1” after review. 3.3% had out-of-tolerance “Level 2” results, and for 0.7% the measurement was not done. Underlying causes for all “Level 2” results were identified by evaluating differences between the fused planning CT and Cone beam CT (CBCT) images. For two patients there were pockets of air in the bowel that had not been present in the CT scan. Two patients had fluid in their lungs (one of these patients also had additional weight-loss). In the last patient, a large piece of wax bolus was placed on the skin to increase the surface dose; the CBCT image indicated the bolus position was incorrect by 1-2cm. No out-of-tolerance results due to machine error were identified. Doses were recalculated in the TPS using the CBCT and where dose difference exceeded the minimum threshold, they were referred for clinical review. In these cases, treatment was continued without adjustment.