Physics Contribution The Use of Photon Beams of a Flattening Filter-free Linear Accelerator for Hypofractionated Volumetric Modulated Arc Therapy in Localized Prostate Cancer Daniel R. Zwahlen, M.D., Stephanie Lang, M.Sc., Jan Hrbacek, M.Sc., Christoph Glanzmann, M.D., Stephan Kloeck, Ph.D., Yousef Najafi, M.D., Tino Streller, Ph.D., Gabriela Studer, M.D., Kathrin Zaugg, M.D., Ph.D., and Urs M. Luetolf, M.D. Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland Received May 2, 2011, and in revised form Sep 23, 2011. Accepted for publication Oct 4, 2011 Summary Hypofractionated volumetric modulated arc therapy combined with unflattened photon beams for prostate cancer was tested in this planning study. Unflattened and flattened beams resulted in similar dose distributions to the planning target volumes. Unflattened 10 MV beams showed advantages in sparing rectum and bladder as well as keeping whole- body dose low. The use of unflattened beams reduced treatment time for single arc treatment or when gantry speed was a limiting factor for faster treatment delivery. Purpose: To evaluate the potential usage of flattening filter-free (FFF) photon beams in the treat- ment of prostate cancer. Methods and Materials: Volumetric-modulated arc therapy (VMAT) treatment planning was performed for 7 patients using TrueBeam Ò linear accelerator and photon beams with (X6, X10) and without (X6FFF, X10FFF) flattening filter. Prescribed dose was 19  3 Gy Z 57 Gy. One or two 360  arcs with dose rate of 600 MU/min for flattened beams, and 1,200 MU/min for FFF beams were used. Results: No difference was detected between the four beams in PTV coverage, conformity, and homogeneity. Mean body dose and body volume receiving 50% of the prescribed dose decreased with increasing mean energy (r 2 Z 0.8275, p < 0.01). X6FFF delivered 3.6% more dose compared with the X6 (p < 0.01). X10FFF delivered 3.0% (p < 0.01), and the X10 5.8% (p < 0.01) less mean body dose compared with X6. There was a significant increase in the mean dose to the rectum for the X10 compared with X6 (2.6%, p < 0.01). Mean dose to the bladder increased by 1.3% for X6FFF and decreased by 2.3% for X10FFF. Using a single arc and FFF, treatment time was reduced by 35% (2 SD Z 10%). Conclusion: FFF beams resulted in dose distributions similar to flattened beams. X10FFF beam provided the best solution, sparing rectum and bladder and minimizing whole-body dose. FFF beams lead to a time efficient treatment delivery, particularly when combined with hypofractio- nated VMAT. Ó 2012 Elsevier Inc. Keywords: Prostate cancer, Flattening filter-free, Hypofractionation, VMAT, TrueBeam Ò , Treat- ment time Reprint requests to: Daniel R. Zwahlen, M.D., Department of Radi- ation Oncology, University Hospital Zurich, CH-8091, Zurich, Switzerland. Tel: þ41 (0) 44 255 2930; Fax: þ41 (0) 44 255 4435; E- mail: daniel.zwahlen@gmx.ch D.R.Z. and S.L. contributed equally to this publication. Conflict of interest: none. Int J Radiation Oncol Biol Phys, Vol. 83, No. 5, pp. 1655e1660, 2012 0360-3016/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.ijrobp.2011.10.019 Radiation Oncology International Journal of biology physics www.redjournal.org