IOP PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 57 (2012) 2997–3011 doi:10.1088/0031-9155/57/10/2997 Computer-aided analysis of star shot films for high-accuracy radiation therapy treatment units Tom Depuydt 1, 7 , Rudi Penne 2 , Dirk Verellen 1 , Jan Hrbacek 3 , Stephanie Lang 3 , Katrien Leysen 1 , Iwein Vandevondel 1 , Kenneth Poels 1 , Truus Reynders 1 , Thierry Gevaert 1 , Michael Duchateau 1 , Koen Tournel 1 , Marlies Boussaer 1 , Dorian Cosentino 4 , Cristina Garibaldi 5 , Timothy Solberg 6 and Mark De Ridder 1 1 Radiotherapy Department, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium 2 Departement Wiskunde, Universiteit Antwerpen en Departement IWT, Karel de Grote-Hogeschool, Antwerpen, Belgium 3 Universit¨ atsSpital Z¨ urich, Z¨ urich, Switzerland 4 Ospedale Sant’Anna, Via Ravona, 22020 San Fermo della Battaglia, Como, Italy 5 Radiation Oncology Department, European Institute of Oncology, Milan, Italy 6 University of Texas Southwestern Medical Center, Dallas, Texas, USA E-mail: tom.depuydt@uzbrussel.be Received 4 November 2011, in final form 4 March 2012 Published 26 April 2012 Online at stacks.iop.org/PMB/57/2997 Abstract As mechanical stability of radiation therapy treatment devices has gone beyond sub-millimeter levels, there is a rising demand for simple yet highly accurate measurement techniques to support the routine quality control of these devices. A combination of using high-resolution radiosensitive film and computer- aided analysis could provide an answer. One generally known technique is the acquisition of star shot films to determine the mechanical stability of rotations of gantries and the therapeutic beam. With computer-aided analysis, mechanical performance can be quantified as a radiation isocenter radius size. In this work, computer-aided analysis of star shot film is further refined by applying an analytical solution for the smallest intersecting circle problem, in contrast to the gradient optimization approaches used until today. An algorithm is presented and subjected to a performance test using two different types of radiosensitive film, the Kodak EDR2 radiographic film and the ISP EBT2 radiochromic film. Artificial star shots with a priori known radiation isocenter size are used to determine the systematic errors introduced by the digitization of the film and the computer analysis. The estimated uncertainty on the isocenter size measurement with the presented technique was 0.04 mm (2σ ) and 0.06 mm (2σ ) for radiographic and radiochromic films, respectively. As an application of the technique, a study was conducted to compare the mechanical stability of O-ring gantry systems with C-arm-based gantries. In 7 Author to whom any correspondence should be addressed. 0031-9155/12/102997+15$33.00 © 2012 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 2997