CLINICAL INVESTIGATION Prostate SPOT SCANNING PROTON BEAM THERAPY FOR PROSTATE CANCER: TREATMENT PLANNING TECHNIQUE AND ANALYSIS OF CONSEQUENCES OF ROTATIONAL AND TRANSLATIONAL ALIGNMENT ERRORS J EFF MEYER, M.D., * J AQUES BLUETT, M.S., * RICHARD AMOS , M.S., * LARRY L EVY , M.S., * S EUNGTAEK CHOI , M.D., * Q UYNH-N HU NGUYEN, M.D., * X. R ON Z HU, P H.D., * MICHAEL GILLIN , P H.D., * AND ANDREW LEE, M.D., M.P.H. * From the *University of Texas-M.D. Anderson Cancer Center, Houston, TX Purpose: Conventional proton therapy with passively scattered beams is used to treat a number of tumor sites, including prostate cancer. Spot scanning proton therapy is a treatment delivery means that improves conforma coverage of the clinical target volume (CTV). Placement of individual spots within a target is dependent on traversed tissue density. Errors in patient alignment perturb dose distributions. Moreover,there is a need for a rational planning approach that can mitigate the dosimetric effect of random alignment errors. We propose a treatment planning approach and then analyze the consequences of various simulated alignment errors on p tate treatments. Methods and Materials: Ten control patients with localized prostate cancer underwent treatment planning for sp scanning proton therapy. After delineation of the clinical target volume, a scanning target volume (STV) was created to guide dose coverage. Errors in patient alignment in two axes (rotational and yaw) as well as translat errors in the anteroposterior direction were then simulated, and dose to the CTV and normal tissues were reanalyzed. Results: Coverage of the CTV remained high even in the setting of extreme rotational and yaw misalignments. Changes in the rectum and bladder V45 and V70 were similarly minimal, except in the case of translational erro where, as a result of opposed lateral beam arrangements, much larger dosimetric perturbations were observed Conclusions: The concept of the STV as applied to spot scanning radiation therapy and as presented in this repo leads to robust coverage of the CTV even in the setting of extreme patient misalignments. Ó 2010 Elsevier Inc. Protons, Spot scanning, Prostate cancer. INTRODUCTION The primary advantage of proton beam therapy lies in its reduced integral dose to normal tissues relative to what is achievedwith X-ray therapy.Many groups have implemented the use of proton treatments for patients with localized prostate cancer, in an effort to limit genitourinary, rectal,and soft-tissue toxicity (1,2). Recentmodeling comparisons have shown that, compared with intensity- modulated X-ray therapy, proton treatment plansdeliver less radiation in the low- and mid-dose range to the bladder and rectum, for equal or slightly higher dose in the high- dose range (3). Studies looking at comparisons of acute toxicity have not revealed significant differences between patients treated with X-rays and with protons (2). Whether or notlate toxicity differences will be seen remains to be determined. In conventional passivelyscatteredprotontherapy, conformation of the beam to the distal edge of the clinical target volume (CTV) is made possible by the use of rang compensators. However,with passively scattered proton therapy, tight conformation of dose to the proximal edg the target cannotbe physically achieved. In addition,to account for potential perturbations in dose caused by o motion in directions perpendicular to the beam path or in patient setup, the compensator is manipulated by a ‘ ing’’ algorithm. For setup error or organ motion within the assigned smearing radius, distalcoverage of the target is assured. Finally,uncertainties in location ofthe position where proton beams complete their range in tissue (ran uncertainties) must be incorporated in the treatment pl process (4). Spot scanningprotontherapyuses magneticbeam scanning technology to individually place (scan) proton Reprint requests to: Jeff Meyer, M.D., Department of Radiation Oncology,UniversityofTexas-Southwestern MedicalCenter, 5801 Forest Park,Dallas,TX 75390-9183. Phone:(214)645- 8525; E-mail: jmeye3@utsouthwestern.edu Conflict of interest: none. Received June 5, 2009,and in revised form July 24, 2009. Accepted for publication July 27, 2009. 428 Int. J. Radiation Oncology Biol. Phys., Vol. 78, No. 2, pp. 428–434, 2010 Copyright Ó 2010 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/$–see front matter doi:10.1016/j.ijrobp.2009.07.1696