Results of IAEA supported TPS audit in Europe Eduard Gershkevitsh North Estonia Medical Center Tallinn, Estonia

Principles of operation TPS audit uses IAEA TECDOC 1583 methodology IAEA provides CIRS Thorax phantom on loan for 6 month to the Member State IAEA together with national audit coordinator (Medical Physics group, RT department nominated by the national authorities) organise one day workshop in the country National audit coordinator’s centre is audited by independent auditor National audit coordinator is performing audits through on site visits

IAEA TECDOC 1583 methodology Based on anthropomorphic phantom To verify that logistic chain: CT scanning Treatment planning Data transfer Dose delivery is operational and leads to desired results with sufficient accuracy Employs ionisation chamber measurements

IAEA TECDOC 1583 methodology Eight test cases with 15 measurement points Single 10x10cm2 field at nominal SSD Tangential field with wedge Corner blocks 4-field “box” Customised blocking Oblique incidence with L-shape block Half fields with wedges Non-coplanar field arrangement Agreement criteria 2-5% depending on complexity

National TPS audit coordinators B. Petrovic – Institute of Oncology Vojvodina, Sremska Kamenica, Serbia C. Pesznyak – National Institute of Oncology, Budapest, Hungary K. Chelminski – M. Skłodowska-Curie Memorial Cancer Center and Institute of Oncology, Warsaw, Poland J. Grezdo – St. Elizabeth Institute of Oncology, Bratislava, Slovakia M. do Carmo Lopes – Portuguese Institute of Oncology, Coimbra, Portugal E. Gershkevitsh – North Estonia Medical Centre, Tallinn, Estonia

Participants of the TPS Audit 8 countries 61 centres 195 datasets (combination of algorithms and beam quality)

CT to RED conversion CT to RED conversion curve required adjustment in 2/3 of centres (criteria used adopted from IAEA TRS 430 – for the same electron density the variation should not exceed ± 20HU for all materials except water (± 5HU)

Reasons for deviations Dosimetry problems Discrepancies requiring intervention and not related to algorithm limitations were found in approximately 9% of datasets Reasons for deviations

Calibration Use of chamber with outdated calibration factor Use of plastic phantom instead of water for calibration Incorrect value in TPS

Input beam data & model fitting Typographic errors Use of standard data Quality of measurement data Sub-optimal beam fitting

Problems with model beam fitting

Problems with treatment unit New couch top was installed and unaccounted. This lead to 8% underdose from posterior fields Loose mechanical wedge

Accuracy achievable With corrected data and advanced algorithms the majority of the measurements are within agreement criteria

Conclusion Input beam data and suboptimal beam modelling were the largest contributors to observed deviations CT to RED conversion is customised in minority of centres The majority of observed deviations have been corrected Contribution to better understanding of TPS performance and its limitations

Acknowledgement Stanislav Vatnitsky for drafting a proposal for TPS audit Joanna Izewska for support and implementation of the TPS audit at IAEA To national audit coordinators To medical physicist at audited RT departments

Thank you for attention! More details: Gershkevitsh E, Pesznyak C, Petrovic B, Grezdo J, Chelminski K, do Carmo Lopes M, Izewska J, Van Dyk J. Dosimetric inter-institutional comparison in European radiotherapy centres: Results of IAEA supported treatment planning system audit. Acta Oncol. 2014 May;53(5):628-36.