Physics Contribution Dosimetric Impact of Intrafraction Motion During RapidArc Stereotactic Vertebral Radiation Therapy Using Flattened and Flattening Filter-Free Beams Chin Loon Ong, PhD, Max Dahele, MBChB, PhD, FRCR, FRCP, Johan P. Cuijpers, PhD, Suresh Senan, PhD, MRCP, FRCR, Ben J. Slotman, MD, PhD, and Wilko F.A.R. Verbakel, PhD Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands Received May 22, 2012, and in revised form Nov 22, 2012. Accepted for publication Dec 26, 2012 Summary RapidArc can reduce the delivery time of vertebral stereotactic body radiation therapy (SBRT). The dosi- metric consequences of intrafraction shifts of varying magnitude and duration, all less than the delivery time of a full arc, were modeled for 9 patients and compared for flattening filter-free (FFF) and flattened beams. Such shifts can result in consider- ably higher spinal cord dose, especially with FFF beams. Accurate patient positioning, monitoring, and correction of unexpected motions throughout vertebral SBRT are important. Purpose: To study the dosimetric impact of relatively short-duration intrafraction shifts during a single fraction of RapidArc delivery for vertebral stereotactic body radiation therapy (SBRT) using flattened (FF) and flattening filter-free (FFF) beams. Methods and Materials: The RapidArc plans, each with 2 to 3 arcs, were generated for 9 patients using 6-MV FF and 10-MV FFF beams with maximum dose rates of 1000 and 2400 MU/min, respectively. A total of 1272 plans were created to estimate the dosimetric consequences in target and spinal cord volumes caused by intrafraction shifts during one of the arcs. Shifts of 1, 2, and 3 mm for periods of 5, 10, and 30 seconds, and 5 mm for 5 and 10 seconds, were modelled during a part of the arc associated with high doses and steep dose gradients. Results: For FFF plans, shifts of 2 mm over 10 seconds and 30 seconds could increase spinal cord D max by up to 6.5% and 13%, respectively. Dosimetric deviations in FFF plans were approximately 2-fold greater than in FF plans. Reduction in target coverage was <1% for 83% and 96% of the FFF and FF plans, respectively. Conclusion: Even short-duration intrafraction shifts can cause significant dosimetric deviations during vertebral SBRT delivery, especially when using very high dose rate FFF beams and when the shift occurs in that part of the arc delivering high doses and steep gradients. The impact is greatest on the spinal cord and its planning-at-risk volume. Accurate and stable patient positioning is therefore required for vertebral SBRT. Ó 2013 Elsevier Inc. Reprint requests to: Chin Loon Ong, PhD, Department of Radiation Oncology, VU University Medical Center, De Boelelaan 1117, 1081HV Amsterdam, The Netherlands. Tel: (þ31) 20-4440841; E-mail: c.ong@ vumc.nl This research is partly funded by a grant from Varian Medical Systems. Conflict of interest: The VU University Medical Center has research collaborations with Varian Medical Systems and BrainLAB AG. Int J Radiation Oncol Biol Phys, Vol. 86, No. 3, pp. 420e425, 2013 0360-3016/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ijrobp.2012.12.028 Radiation Oncology International Journal of biology physics www.redjournal.org