National pilot-study on verification of calculated absorbed dose distribution to be delivered in advanced radiotherapy treatment

Abstract

Dosimetry audits in radiation oncology are indicated in the Council Directive 2013/59/EURATOM as one of several measures to ensure high quality of absorbed dose delivery in radiation therapy. Therefore, a national pilot-study was prepared and conducted for an on-site dosimetry audit on the 2D pre-treatment verification methodology. The aim was to explore the possibility of introducing such audits at the national level and examine the feasibility of extending such a concept.The main activities performed were developing and verifying methodology to be applied elsewhere, assembling a survey related to the application of advanced radiotherapy techniques, point and 2D absorbed dose measurements, and acquiring data for on-site and off-site data validation. The measurements of 2D absorbed dose distributions of different complexities were performed using local and auditor's (reference) equipment. The beam output, dose linearity, and field output factors were determined using a reference set of equipment only.Eight high-energy photon beams were evaluated. The participating centres fulfilled the recommended criterion of ±2% for beam output. Seven beams fulfilled the dose linearity criterion. Comparison of field output factors exhibited out-of-tolerance discrepancies only for the smallest field size of 2 × 2 cm2.For the evaluation of 2D absorbed dose distributions, the results obtained with local equipment showed that all absorbed dose distributions had gamma (γ) passing rate ≥95% for the 3%/2 mm tolerance criterion. However, the γ passing rates obtained with the reference equipment were lower for most verified beams. Some of the analysed absorbed dose distributions did not fulfil the recommended γ passing rate acceptance criterion. γ passing rates have shown a statistically significant difference for 3%/2 mm and 2%/2 mm criteria with p = 0.0117 and p = 0.0212, respectively, comparing the results obtained by reference and local equipment.External audits related to the 2D dosimetrical pre-treatment verification of calculated dose distributions provide the advantage of being independent of the equipment and methodology used locally. Such audits can be a valuable tool for the evaluation and improvement of local practices in the application of advanced radiotherapy techniques.