INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 49 (2004) N139–N146 PII: S0031-9155(04)76363-0 NOTE Dose resolution in gel dosimetry: effect of uncertainty in the calibration function J V Trapp 1,2 , G Michael 2 , P M Evans 1 , C Baldock 2,3 , M O Leach 4 and S Webb 1 1 Joint Department of Physics, The Institute of Cancer Research and the Royal Marsden Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT, UK 2 School of Physical and Chemical Sciences, Queensland University of Technology, Gardens Point, Brisbane QLD 4000, Australia 3 Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia 4 Cancer Research UK Clinical MR Research Group, The Institute of Cancer Research and the Royal Marsden Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT, UK E-mail: jtrapp@icr.ac.uk Received 18 February 2004 Published 4 May 2004 Online at stacks.iop.org/PMB/49/N139 DOI: 10.1088/0031-9155/49/10/N02 Abstract Dose resolution, D p  , is becoming a common method for characterizing the performance of a gel dosimeter. In this note we examine how the goodness of fit of the calibration function affects D p  and show that its inclusion in the calculation of D p  is essential to avoid overestimating the performance of the gel. (Some figures in this article are in colour only in the electronic version) 1. Introduction Gel dosimetry is the only truly three-dimensional radiotherapy dosimeter (DOSGEL’99 1999, McJury et al 2000, DOSGEL 2001) and has recently been used in clinical practice (Trapp et al 2004). To characterize the performance of a gel system the concept of dose resolution, D p  , in gel dosimetry was introduced by Baldock et al (2001) and is defined as ‘the minimal difference between two absorbed doses that allows them to be distinguished with a specified level of confidence, p’. Baldock et al stated that a first-order Taylor approximation should be used when calculating the contribution of calibration uncertainty to D p  . Since then several papers have been published where D p  has been used to characterize the performance of a gel dosimeter. In this technical note we examine D p  taking into account goodness of fit of the calibration curve. 0031-9155/04/100139+08$30.00 © 2004 IOP Publishing Ltd Printed in the UK N139