PHYSICS CONTRIBUTION OPTIMIZATION OF COLLIMATOR TRAJECTORY IN VOLUMETRIC MODULATED ARC THERAPY: DEVELOPMENTAND EVALUATION FOR PARASPINAL SBRT PENGPENG ZHANG,PH.D.,* LAURA HAPPERSETT, M.S.,* YINGLI YANG,PH.D.,* YOSHIYA YAMADA, M.D., y GIG MAGERAS,PH.D.,* AND MARGIE HUNT, M.S.* Departments of *Medical Physics and y Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY Purpose: To develop a collimator trajectory optimization paradigm for volumetric modulated arc therapy (VMAT) and evaluate this technique in paraspinal stereotactic body radiation therapy (SBRT). Method and Materials: We proposea novel VMAT paradigm, Coll-VMAT, which integrates collimator rotation with synchronized gantry rotation, multileaf collimator (MLC) motion, and dose-rate modulation. At each gantry angle a principal component analysis (PCA) is applied to calculate the primary cord orientation. The collimator angle is then aligned so that MLC travel is parallel to the PCA-derived direction. An in-house VMAToptimization follows the geometry-based collimator trajectory optimization to obtain the optimal MLC position and monitor units (MU) at each gantry angle. A treatment planning study of five paraspinal SBRT patients compared Coll- VMAT to standard VMAT (fixed collimator angle) and static field IMRT plans. Plan evaluation statistics included planning target volume (PTV) V95%, PTV-D95%, cord-D05%, and total beam-on time. Results: Variation of collimator angle in Coll-VMAT plans ranges from 26  to 54  , with a median of 40  . Patient- averaged PTV V95% (94.6% Coll-VMAT vs. 92.1% VMATand 93.3% IMRT) and D95% (22.5 Gy vs. 21.4 Gy and 22.0 Gy, respectively) are highest with Coll-VMAT, and cord D05% (9.8 Gy vs. 10.0 Gy and 11.7 Gy) is lowest. Total beam-on time with Coll-VMAT (5,164 MU) is comparable to standard VMAT (4,868 MU) and substantially lower than IMRT (13,283 MU). Conclusion: Collimator trajectory optimization-based VMAT provides an additional degree of freedom that can improve target coverage and cord sparing of paraspinal SBRT plans compared with standard VMATand IMRT approaches. Ó 2010 Elsevier Inc. Volumetric modulated arc therapy, Treatment planning, Trajectoryoptimization, Paraspinal, SBRT. INTRODUCTION Sweeping window arc therapy (1, 2) or volumetric modu- lated arc therapy (VMAT) (3–6) is receiving broad interest and gaining research and development momentum. Planning studies from various institutions demonstrate that VMAT plans have similar dosimetric quality but much-reduced treatment delivery time compared with the standard inten- sity-modulated radiation therapy (IMRT) approach (7–11). Although shortening daily treatment time is a much desired feature in the context of patient motion management, im- proving the capability of VMAT for dose modulation is crucial for its viability in the clinic. In VMAT delivery, gan- try, dose rate, and multileaf collimator (MLC) are optimized and synchronized to deliver modulated radiation. The linac mechanical axes that are possible but not addressed in the current VMAT optimization approaches are the collimator angle and couch angle. Otto and Clark (12) and Milette and Otto (13) investigated integration of collimator rotation into direct aperture-based IMRT optimization and found that rotating aperture optimization (RAO) can improve plan qual- ity. In RAO approaches, there is either no restriction on collimator angle, or there is an equal angle increment be- tween beams. RAO implicitly includes collimator rotation during optimization rather than explicitly optimizing the collimator in a separate step; therefore, RAO is not easily applicable to VMAT unless an optimal combined collima- tor-gantry trajectory is found. The purpose of this study was to develop a collimator trajectory optimization that can be used with VMAT (Coll-VMAT) to improve the abil- ity of VMAT to modulate dose while maintaining the rapid feature of a single arc therapy. Although most VMAT comparison studies in the litera- ture thus far examined conventional fraction sizes (1.8 or 2 Gy), the shortened treatment time of VMAT could be Reprint requests to: Pengpeng Zhang, Ph.D., Memorial Sloan- Kettering Cancer Center, Medical Physics Department, 1275 York Ave., New York, NY 10065. Tel: (646)888-5616; Fax: (646)888- 5626; E-mail: zhangp@mskcc.org Conflict of interest: Memorial Sloan-Kettering Cancer Center has a research agreement with Varian Medical Systems. Acknowledgment—We thank Yves Archimbault from Varian Med- ical Systems, Palo Alto, CA for his help and discussion regarding Varian’s implementation of RapidArc. Received May 11, 2009, and in revised form Aug 18, 2009. Accepted for publication Aug 20, 2009. 591 Int. J. Radiation Oncology Biol. Phys., Vol. 77, No. 2, pp. 591–599, 2010 Copyright Ó 2010 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/$–see front matter doi:10.1016/j.ijrobp.2009.08.056