Physics Contribution Day-to-Day Reproducibility of Prostate Intrafraction Motion Assessed by Multiple kV and MV Imaging of Implanted Markers During Treatment Theodore F. Mutanga, M.Sc., Hans C.J. de Boer, Ph.D., Vinayakrishnan Rajan, Ph.D., Maarten L.P. Dirkx, Ph.D., Luca Incrocci, M.D., Ph.D., and Ben J.M. Heijmen, Ph.D. Department of Radiation Oncology, Erasmus MCeDaniel den Hoed Cancer Center, Rotterdam, The Netherlands Received Nov 25, 2010, and in revised form May 4, 2011. Accepted for publication May 22, 2011 Summary This paper presents an anal- ysis of the intrafraction motion of the prostate gland in the sagittal plane using (nearly) lateral kilovoltage (kV) and megavoltage (MV) images of prostate implanted markers obtained at several time points during treatment for 108 prostate patients treated with intensity modulated radio- therapy and on-line pre-frac- tion positioning. The effective intrafraction motion was obtained as the time-weighted mean displacement during a treatment fraction. For a large subgroup of patients the systematic component of prostate infraction motion was significant and could be pre- dicted by measurements from initial fractions. Purpose: When one is performing online setup correction for prostate positioning displacements prior to daily dose delivery, intrafraction motion can become a limiting factor to prostate target- ing accuracy. The aim of this study was to quantify and characterize prostate intrafraction motion assessed by multiple kilovoltage (kV) and megavoltage (MV) imaging of implanted markers during treatment in a large patient group. Methods and Materials: Intrafraction motion in the sagittal plane was studied by retrospective analysis of displacements of implanted gold markers on (nearly) lateral kV and MV images ob- tained at various time points during the treatment fractions (mean, 27 per patient) in 108 consec- utive patients. The effective prostate motion in a fraction was defined as the time-weighted mean displacement. Results: Prostate displacements in the sagittal plane increased during the fraction (mean, 0.2  0.2 mm/min). Forty percent of patients had a systematic (i.e., appearing in all fractions) effective displacement in the sagittal plane greater than 2 mm. Observed effective population mean-of- means (meff) þ/ systematic (Seff) intrafraction motion (m eff  S eff ) was 0.9  1.1 mm and 0.6  1.0 mm for the anterioreposterior and superior inferior directions, respectively. Corre- sponding random motion (s eff ) was 1.2 mm and 1.1 mm. Mean effective prostate motion in the first 5 fractions was predictive for mean effective displacement in the remaining fractions (p < 0.001). Conclusion: For a large subgroup of patients, the systematic component of intrafraction prostate motion was substantial. Intrafraction motion correction prior to each beam delivery or offline corrections could likely be beneficial for the subgroup of patients with significant motion. The systematic component is well predicted by measurements in the initial fractions. Ó 2012 Elsevier Inc. Keywords: Implanted markers, Systematic intrafraction motion, Prostate, IGRT, Geometric uncertainties Reprint requests to: Ben J.M. Heijmen, PhD, Department of Radiation Oncology, Division of Medical Physics, Erasmus MC-Daniel den Hoed Cancer Centre, P.O. Box 5201, 3008 AE Rotterdam, the Netherlands. Tel: þ31 10 7041304; Fax: þ31 10 7041012; E-mail: b.heijmen@erasmusmc.nl Presented in part at the 10th Biennial European Society for Therapeutic Radiology and Oncology Meeting on Physics and Radiation Technology for Clinical Radiotherapy, Maastricht, The Netherlands, August 30eSeptember 3, 2009. Conflict of interest: none. AcknowledgmentdThe authors acknowledge the contributions of Marjo- lein van Os and Mascha van de Laar on manual marker identification and technical support given by Jaco Barnhoorn. Int J Radiation Oncol Biol Phys, Vol. 83, No. 1, pp. 400e407, 2012 0360-3016/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.ijrobp.2011.05.049 Radiation Oncology International Journal of biology physics www.redjournal.org