ESTRO 2020 Abstract book

S228 ESTRO 2020

quality assurance (QA) procedures and supported by external audits. Broad introduction of intensity-modulated radiation therapy (IMRT) into clinical practice in the early 2000s led to the development of several remote and on-site audit methodologies of these techniques. Almost simultaneously, several dosimetry audit networks (DANs) in Europe and USA developed audit methodologies for IMRT audits. They mostly utilized custom made phantoms of different geometries mimicking specific anatomical sites, with the dose delivered measured with films and thermoluminescent (TLDs) or other solid-state dosimeters placed in critical locations. The IAEA initiated the development of IMRT audits in 2012, under the coordinated research project (CRP) “Development of Quality Audits for Advanced Technology in Radiotherapy Dose Delivery”. Since the aim was to build expertise in the national DANs participating in the CRP, it gradually covered topics of auditing MLC performance, small field dosimetry, single-field IMRT checking, and finally tested an audit methodology following an end-to- end approach. The latter audit methodology utilized a custom-made solid water phantom containing the planned target volume (PTV) and the organ at risk (OAR) located close to each other and accommodating TLDs within the structures and a film in the cross-section. The audit was implemented in 6 countries with an average pass rate of 66%. An on-site audit methodology was then developed by the IAEA, which also employed the end-to-end approach. The phantom for the audit was developed and is now commercially available as the “Shoulders, Head and Neck, End-to-end” phantom (SHANE, CIRS Inc.). The audit includes a pre-visit phase which allows to the collection and analyses of relevant linac commissioning, treatment planning and MLC performance data before the on-site 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 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. 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

We would like to thank the Cancer Plan, FOD Healthcare and the College of Medicine for the financial and scientific