Clinical Investigation: Genitourinary Cancer An Endorectal Balloon Reduces Intrafraction Prostate Motion During Radiotherapy Robert Jan Smeenk, M.D.,* Robert J.W. Louwe, Ph.D.,* Katja M. Langen, Ph.D., y Amish P. Shah, Ph.D., y Patrick A. Kupelian, M.D., y Emile N.J.Th. van Lin, M.D., Ph.D.,* and Johannes H.A.M. Kaanders, M.D., Ph.D.* From the *Department of Radiation Oncology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands; and y Department of Radiation Oncology, M.D. Anderson Cancer Center Orlando, FL Received Dec 15, 2010, and in revised form Jun 15, 2011. Accepted for publication Jul 11, 2011 Summary This study investigated the effect of an endorectal balloon (ERB) on prostate motion during radiotherapy. Thirty patients were treated either with an ERB inserted daily or without. Inter- fraction variation was not significantly different between the two groups. Intrafraction motion, however, was significantly reduced by ERB, in partic- ular after 150 seconds. ERBs may therefore be beneficial for long treatment sessions such as hypofractionated radiotherapy. Purpose: To investigate the effect of endorectal balloons (ERBs) on intrafraction and interfrac- tion prostate motion during radiotherapy. Methods and Materials: Thirty patients were treated with intensity-modulated radiotherapy, to a total dose of 80 Gy in 40 fractions. In 15 patients, a daily-inserted air-filled ERB was applied. Prostate motion was tracked, in real-time, using an electromagnetic tracking system. Interfrac- tion displacements, measured before each treatment, were quantified by calculating the system- atic and random deviations of the center of mass of the implanted transponders. Intrafraction motion was analyzed in timeframes of 150 s, and displacements >1 mm, >3 mm, >5 mm, and >7 mm were determined in the anteroposterior, left-right, and superoinferior direction, and for the three-dimensional (3D) vector. Manual table corrections, made during treatment sessions, were retrospectively undone. Results: A total of 576 and 567 tracks have been analyzed in the no-ERB group and ERB group, respectively. Interfraction variation was not significantly different between both groups. After 600 s, 95% and 98% of the treatments were completed in the respective groups. Signif- icantly fewer table corrections were performed during treatment fractions with ERB: 88 vs. 207 (p Z 0.02). Intrafraction motion was significantly reduced with ERB. During the first 150 s, only negligible deviations were observed, but after 150 s, intrafraction deviations increased with time. This resulted in cumulative percentages of 3D-vector deviations >1 mm, >3 mm, >5 mm, and >7 mm that were 57.7%, 7.0%, 0.7%, and 0.3% in the ERB-group vs. 70.2%, 18.1%, 4.6%, and 1.4% in the no-ERB group after 600 s. The largest reductions in the ERB group were observed in the AP direction. These data suggest that a 5 mm CTV-to-PTV margin is sufficient to correct for intrafraction prostate movements when using an ERB. Reprint requests to: Robert Jan Smeenk, M.D., Radboud University Nijmegen Medical Centre, Department of Radiation Oncology, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. Tel: þ31-24-3614515; Fax: þ31-24-3610792; E-mail: r.smeenk@rther.umcn.nl Presented at the 52nd annual ASTRO meeting, October 31-November 4, San Diego, CA, USA. Supported by grant KUN 2008-4239 from the Dutch Cancer Society. Conflict of interest: M.D. Anderson Cancer Center Orlando has received research funding from Calypso Medical Technologies, Inc.. K.M.L. and A.P.S. served as research consultants to Calypso Medical Technologies. All other authors declare no conflicts of interest. Int J Radiation Oncol Biol Phys, Vol. 83, No. 2, pp. 661e669, 2012 0360-3016/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.ijrobp.2011.07.028 Radiation Oncology International Journal of biology physics www.redjournal.org