IOP PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 54 (2009) 187–206 doi:10.1088/0031-9155/54/2/002 Predicting the clonogenic survival of A549 cells after modulated x-ray irradiation using the linear quadratic model Regina Bromley 1,3 , Lyn Oliver 1,3 , Ross Davey 2 , Rozelle Harvie 2 and Clive Baldock 3 1 Northern Sydney Cancer Centre, Radiation Oncology, Royal North Shore Hospital, Sydney, NSW 2065, Australia 2 Department of Medical Oncology, Bill Walsh Cancer Research Laboratories, Royal North Shore Hospital, Sydney, NSW 2065, Australia 3 Institute of Medical Physics, School of Physics, Sydney University, NSW 2006, Australia Received 30 April 2008, in final form 6 November 2008 Published 16 December 2008 Online at stacks.iop.org/PMB/54/187 Abstract In this study we present two prediction methods, mean dose and summed dose, for predicting the number of A549 cells that will survive after modulated x- ray irradiation. The prediction methods incorporate the dose profile from the modulated x-ray fluence map applied across the cell sample and the linear quadratic (LQ) model. We investigated the clonogenic survival of A549 cells when irradiated using two different modulated x-ray fluence maps. Differences between the measured and predicted surviving fraction were observed for modulated x-ray irradiation. When the x-ray fluence map produced a steep dose gradient across the sample, fewer cells survived in the unirradiated region than expected. When the x-ray fluence map produced a less steep dose gradient across the sample, more cells survived in the unirradiated region than expected. Regardless of the steepness of the dose gradient, more cells survived in the irradiated region than expected for the reference dose range of 1–10 Gy. The change in the cell survival for the unirradiated regions of the two different dose gradients may be an important factor to consider when predicting the number of cells that will survive at the edge of modulated x-ray fields. This investigation provides an improved method of predicting cell survival for modulated x-ray radiation treatment. It highlights the limitations of the LQ model, particularly in its ability to describe the biological response of cells irradiated under these conditions. Introduction The aim of radiotherapy is to eliminate malignant cells while preserving normal tissues. The ability to deliver a high enough dose to eliminate all malignant cells, or to achieve maximum tumour control, is limited by the radiation dose the normal surrounding tissues and organs 0031-9155/09/020187+20$30.00 © 2009 Institute of Physics and Engineering in Medicine Printed in the UK 187