PHYSICS CONTRIBUTION ASSESSMENT OF PLANNING TARGET VOLUME MARGINS FOR INTENSITY-MODULATED RADIOTHERAPY OF THE PROSTATE GLAND: ROLE OF DAILY INTER- AND INTRAFRACTION MOTION

Purpose: To determine planning target volume margins for prostate intensity-modulated radiotherapy based on inter- and intrafractionmotion using four daily localization techniques: three-point skinmark alignment, volumet-ric imaging with bony landmark registration, volumetric imaging with implanted fiducial marker registration, and implanted electromagnetic transponders (beacons) detection. Methods and Materials: Fourteen patients who underwent definitive intensity-modulated radiotherapy for pros-tate cancer formed the basis of this study. Each patient was implanted with three electromagnetic transponders and underwent a course of 39 treatment fractions. Daily localization was based on three-point skinmark alignment followed by transponder detection and patient repositioning. Transponder positioning was verified by volumetric imaging with cone-beam computed tomography of the pelvis. Relative motion between the prostate gland and bony anatomy was quantified by offline analyses of daily cone-beam computed tomography. Intratreatment organ mo-tion was monitored continuously by the Calypso! System for quantification of intrafraction setup error. Results: As expected, setup error (that is, inter- plus intrafractionmotion, unless otherwise stated) was largest with skinmarkalignment, requiringmargins of 7.5mm, 11.4mm, and 16.3mm, in the lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. Margin requirements accounting for intrafraction motion were smallest for transponder detection localization techniques, requiring margins of 1.4 mm (LR), 2.6 mm (SI), and 2.3 mm (AP)