ESTRO 2020 Abstract Book

S211 ESTRO 2020

visit takes place. Measurements in SHANE are performed with an ion chamber in four positions and a film. Currently, the national audit run has taken place in 7 countries, with another 12 countries planned for 2020 and 2021. The first results showed high pass rates with only two out of 51 linacs checked having results out of tolerance. The most challenging part of remote IMRT audits is finding the root cause for an identified discrepancy. For both the Imaging and Oncology Core (IROC) Houston QA Centre and IAEA IMRT audit methodologies, positioning accuracy plays a major role, and it can remain unidentified until the dosimeter readout is performed. On-site audits, on the other hand, allow for immediate analysis of the results and have an additional educational value for the staff at the audited institution. An experienced auditor may give valuable advice and point towards potentially problematic areas, which coincides with the main goal of any radiotherapy audit being an assistance in improving quality of radiation treatments. Additionally, end-to-end on-site IMRT audit methodologies include checking of parameters usually not available for remote audits, such as the Hounsfield units to relative electron density (HU-to-RED) conversion curve. For the IAEA on-site IMRT audits, the HU- to-RED curve measured was off by more than 100 HU for at least one point (usually, in the higher density area) in 16/38 audits, and in one case, the HU-to-RED curve did not include densities higher than water. Clearly, this discrepancy had to be rectified before proceeding with the measurements in the phantom. With the further advancement of imaging technologies and the introduction of artificial intelligence (AI) tools into the radiotherapy routine, new quality assurance and external audit procedures need to be developed. Kilovoltage image guidance became a reality long ago but it still is not a part of most auditing procedures and is normally used without checking its own geometrical accuracy. On-board magnetic resonance imaging (MRI) capabilities affect both dose calculation and delivery, and with the first clinical trials, new audit methodologies will appear. MRI compatibility of dosimeters used by DANs needs to be verified and MRI visualizable phantoms need to be developed. Implementation of AI-supported adaptive radiotherapy drives the need for audit phantoms with variable and reproducible internal geometry. The same applies to motion tracking which is in clinical use but is not widely supported by audits. Apparently, keeping abreast with the new technologies will make DANs busy for the years ahead. Abstract text Dose calculations in lungs have always been difficult for calculation algorithms in treatment planning systems (TPS). It has been shown that simple algorithms, even if they perform quite well in reasonably homogeneous parts of the patients, can be weak for calculations involving interphases between very different tissues [1], [2], [3]. This issue becomes crucial for stereotactic treatments where it is necessary to deliver high doses on small volumes delivered in a few fractions. A few audits have been developed to control the delivered dose in the case of lung stereotactic body radiotherapy (SBRT) [4], [5], [6], [7]. They will be reviewed. The Centre for Nuclear Technology, NuTeC, from Hasselt University is presently developing a mailed audit program for lung SBRT using a SP-0394 Audits for SBRT B. Reniers 1 , B. Yalvac 2 , N. Reulens 2 1 Universiteit Hasselt, Nutec, Diepenbeek, Belgium; 2 Hasselt University, Nutec, Diepenbeek, Belgium

combination of radiochromic film and alanine/EPR dosimetry. The project is supported by the Belgian College of Radiation Oncology and FOD healthcare. The audit program from UHasselt uses the IMRT Virtual Water Dose Verification Phantom (Standard Imaging Inc., Middleton, Wi, USA) containing a set of lung inserts and a bone equivalent plug for the spine. Films can be placed in the coronal orientation. The lung inserts are modified with a 3D printed PLA box with 20% infill containing an imprint derived from the real contour of a lung tumour and filled with silicone to simulate tumour tissue. The silicone contains a hole for an alanine pellet. Films can also be placed through the tumour box and on top of one lung at the interface between lung substitute and solid water. The audit procedure was evaluated by performing End-To-End (E2E) tests with different TPS. The films were compared with the TPS using gamma analysis [8] with 5%/1mm criteria, global normalisation and 10 % threshold. The audit is now ready to be proposed to the Belgian centres.

We would like to thank the Cancer Plan, FOD Healthcare and the College of Medicine for the financial and scientific support. Besides, we would like to thank the members from the steering committee and the physicists from LOC and Cliniques Universitaires Saint-Luc for their time and support. [1] . J. J. Ojala et al. , "Performance of dose calculation algorithms from three generations in lung SBRT: comparison with full Monte Carlo-based dose distributions," (in en), J Appl Clin Med Phys, vol. 15, no. 2, p. 4662, Mar 6 2014 [2]. A. Fogliata et al. , "Dose calculation algorithm accuracy for small fields in non-homogeneous media: The lung SBRT case," Phys Med, vol. 44, pp. 157-162, Dec 2017 [3]. M. Josipovic et al. , "Advanced dose calculation algorithms in lung cancer radiotherapy: Implications for SBRT and locally advanced disease in deep inspiration breath hold," Phys Med, vol. 56, pp. 50-57, Dec 2018 [4] . G. Distefano et al. , "A national dosimetry audit for stereotactic ablative radiotherapy in lung," Radiother Oncol, vol. 122, no. 3, pp. 406-410, Mar 2017 [5]. C. H. Clark et al. , "The role of dosimetry audit in lung SBRT multi-centre clinical trials," (in en), Phys Med, vol. 44, pp. 171-176, Dec 2017 [6]. M. L. Lambrecht et al. , "Results of a multicentre dosimetry audit using a respiratory phantom within the EORTC LungTech trial," Radiother Oncol, vol. 138, pp. 106- 113, Jun 25 2019 [7] . J. Lye, J. Kenny, J. Lehmann, L. Dunn, T. Kron, A. Alves, et al. A 2D ion chamber array audit of wedged and asymmetric fields in an inhomogeneous lung phantom. Med Phys vol. 41 pp. 101712. 2014 [8]. D. A. Low, W.B. Harms, S Mutic and J.A.Purdy, A technique for the quantitative evaluation of dose distributions, Med. Phys. Vol 25(5) pp. 656-661, March 1998.