P284] Dosimetry audits in radiotherapy: 15 years of experience and lessons learnt

Abstract

Purpose Dosimetry audits (DA) are useful for the evaluation of the dosimetric quality, planned dose accuracy, treatment complexity and deliverability of radiotherapy (RT) procedures, aiming to consult on detected discrepancies and improve standards and reliability of the RT centers. Since 2002, Greek Atomic Energy Commission (EEAE) provides dosimetry auditing services of level I and II and recently III, IV in MV photon and electron beams in all RT centers of the country. Methods DAs were conducted by means of on-site visits. The audit levels applied in sequence were: Mechanical and functional tests of the RT machine, absolute and relative dosimetry, beam and treatment planning system (TPS) output measurements under reference and non-reference conditions, end-to-end tests for the assessment of the TPS dose calculations, in basic and advanced radiotherapy techniques through 1D, 2D and 3D dosimetry. Measurements were compared to the stated by the RT center values according to acceptance and tolerance criteria. Uncertainties were estimated according to GUM. Results 98% for photons and 95% for electrons beams fulfilled dosimetry accuracy acceptance criteria (±3%) under reference conditions, improving gradually for the latter. The assessment of TPS output for 1134 beams (790 open, 344 wedged) was satisfactory in 95% and 84% of open and wedged beams, respectively. Non-acceptable results decreased progressively through time. End-to-end dosimetry tests, applied in 727 beams, exhibited acceptable, tolerated and non-acceptable results in 94%, 3% and 3% cases, respectively. Results for IMRT, VMAT and SRS were acceptable in all cases, stressing the need for systematic review of procedures for the whole chain of the clinical practice. Conclusions Dosimetry audit was proved to be a valuable tool for the improvement of quality in radiotherapy. Dosimetry accuracy is verified in the gross majority of RT centers. Discrepancies and bad practices were detected, presenting a decreasing incidence over the years. Guidance and recommendations provided, improved dosimetry practices and verified that the quality assurance of procedures in an institution is essential for achieving the required accuracy and safe operation. Results of this study imply challenges in TPS commissioning, especially for small fields, and challenges in multicenter comparison for complex dose distributions.