ESTRO 2020 Abstract Book

S212 ESTRO 2020

SP-0395 Audits in Light-Ion Beam Therapy A. Carlino 1 , H. Palmans 1,2 , S. Vatnitsky 1 , M. Stock 1 1 EBG Medaustron Gmbh, Medical Physics, Wiener Neustadt, Austria ; 2 national Physical Laboratory, Dosimetry, London, United Kingdom Abstract text Light-ion beam therapy (LIBT) becomes more popular with an increase access to this type of radiotherapy and with about 90 LIBT centers currently in operation and 45 under construction worldwide. The majority of recently established LIBT facilities, mostly proton therapy facilities, is implementing pencil beam scanning (PBS) technology that places particular demands on comprehensive quality assurance programmes. It is therefore desirable to ensure that each center delivers clinically comparable treatments with the required accuracy of dose delivery. Dosimetry audits for LIBT are more demanding compared to the audits in photon and electron beam therapy. In particular, almost all commercial anthropomorphic phantoms are made of materials which are designed for photon dosimetry and are not compatible for protons and carbon ions. Moreover, the solid state detectors (e.g. TLDs, OSLDs, films, alanine) need specific modeling and corrections based on the knowledge of the particle energy spectra at the detector position (so-called “quenching” effect) when applied to LIBT dosimetry. The models to predict the “quenching” are often not available in literature or are very complex to apply in clinical cases. Few institutions provide dosimetry audit for LIBT worldwide. In United States, the Imaging and Radiation Oncology Core Houston Quality Assurance center (IROC-H, Huston, Texas) is auditing the proton therapy facilities which applied to participate in the National Cancer Institute-sponsored clinical trials utilizing proton radiation therapy. Point doses and planar doses are measured using thermoluminescent dosimeters (TLD) and Gafchromic films placed in different anthropomorphic phantoms. The result of the study for 17 proton therapy centers in US showed that the phantom dosimetry results pass rate (overall, 79%) was high for simple phantoms and lower for phantoms that introduced higher levels of complexity, such as motion, multiple targets, or increased heterogeneity [1]. In 2014, the Japan Carbon-ion Radiation Oncology Study group (J-CROS) was established to obtain clinical evidence through a multicenter carbon ions clinical trial. The QA team within the J-CROS group organizes external reference dosimetry audit based on a single point-dose measured with an ionization chamber [2]. The audits were performed for delivery of two treatment plans at each institution and the results were within ± 3 % for all the six audited facilities in Japan [2]. Since 2016, the MedAustron Ion Beam Therapy Center (MA, Austria) in collaboration with the National Physical Laboratory (NPL, UK) developed a new dosimetry audit methodology based on end-to-end (E2E) testing with alanine dosimeters, ionization chamber and Gafchromic films [3]. The purpose of the E2E testing is to confirm that the entire logistic chain of a radiation treatment starting from CT imaging, treatment planning, patient positioning and verification and beam delivery is adequately implemented resulting in sufficient accuracy of planned dose delivery. Five proton therapy facilities were audited so far in Europe and several audits are planned in the near future. For all the tests the mean of the local differences of the absolute dose to water determined with the alanine pellets compared to the predicted dose from the treatment planning system installed at the audited institution was -0.1 ± 1.0 % and maximum deviations were

within ± 3.0 % [4]. The measurements carried out with the ionization chambers were consistent with the dose determined by the alanine pellets with a mean deviation of -0.5 ± 0.6 %. The same methodology is under development at MA for carbon ion beams and a dosimetry audit service based on E2E testing for carbon ion beams will be available in the near future. The MA-NPL experience shows that alanine pellets are also suitable detectors for dosimetric audit based on E2E testing in LIBT and the developed procedures can be used to support the implementation of upcoming new LIBT facilities in Europe and also may serve as dosimetric credentialing for clinical trials in the future. [1] Paige Taylor et al. “Results From the Imaging and Radiation Oncology Core Houston's Anthropomorphic Phantoms Used for Proton Therapy Clinical Trial Credentialing” International Journal of Radiation Oncology, Biology, Physics, 2016, Volume 95, Issue 1, 242 - 248 [2] Hideyuki Mizuno et al., “External dosimetry audit for quality assurance of carbon-ion radiation therapy clinical trials”. J Appl Clin Med Phys 2019; 20:1:31–36 [3] Antonio Carlino et al. “End-to-end tests using alanine dosimetry in scanned proton beams.” Physics in medicine and biology. 2018; 63:055001. [4] Antonio Carlino et al. “Novel independent dosimetry audit based on end-to-end testing for proton beam therapy”, Radiotherapy and Oncology, Submitted SP-0396 Novel approaches to auditing J. Lehmann 1 1 Calvary Mater Newcastle, Department Of Radiation Oncology, Newcastle- Nsw, Australia Abstract text Modern radiation therapy requires novel approaches to auditing. As new treatment technology and techniques are being introduced constantly, new audits are needed. The fast pace of change calls for an equally increased pace of audit development or adoption as well as for a fast audit turnaround for a potentially large number of facilities. Ever shrinking budgets necessitate lower audit costs. Additionally, large distances between audited facilities in some jurisdictions as well as the global nature of clinical trials compel new thinking towards dosimetry audits. Phantom based audits have a proven track record and will remain essential for end to end tests. Likewise, onsite audits with expert personnel present generally enable the most accurate measurements. They also allow for immediate discussion of any found problems and thereby a fast return to optimal patient care. Significant experience exists with postal audits, where phantoms with detectors are shipped to the audited facility, irradiated and shipped back for analysis. Advancing remote auditing, electronic portal imaging device based audits that do not use a physical phantom have been introduced recently. The audits are accurate and comprehensive in assessing intensity modulated treatments. Not requiring the shipment of any equipment these audits are less expensive and faster accessible as well as more environmentally friendly. Not depending on the availability of phantoms many facilities can perform the audit measurements in parallel and upload their calculations and measurements (images) for central processing and analysis. Similarly, the use of log file analysis in remote audits has been investigated and implemented. A new audit concept is the assessment of the sensitivity of patient specific quality assurance (PSQA) tools. Such audit