• Tara Rosewall, Peter Chung, Andrew Bayley, Gina Lockwood, Hamideh Alasti, Robert Bristow, Vickie Kong, Michael Milosevic, and Charles Catton.
• Radiation Medicine Program, Princess Margaret Hospital, Toronto, Ont., Canada. Tara.Rosewall@rmp.uhn.on.ca
• Radiother Oncol. 2008 Jul 1; 88 (1): 88-94.

Background And PurposeTo quantify inter- and intrafraction prostate motion in a standard VacLok (VL) immobilization device or in the BodyFix (BF) system incorporating a compression element which may reduce abdominal movement.Materials And MethodsThirty-two patients were randomly assigned to VL or BF. Interfraction prostate motion >3 mm was corrected pre-treatment. EPIs were taken daily at the start and end of the first and last treatment beams. Interfraction and intrafraction prostate motion were measured for centre of mass (COM) and individual markers.ResultsThere were no significant differences in interfraction (p0.002) or intrafraction (p0.16) prostate motion with or without abdominal compression. Median intrafraction motion was slightly smaller than interfraction motion in the AP (7.0 mm vs. 7.6 mm) and SI direction (3.2 mm vs. 4.7 mm). The final image captured the maximal intrafraction displacement in only 40% of fractions. Our PTV incorporated >95% of total prostate motion.ConclusionsIntrafraction motion became the major source of error during radiotherapy after online correction of interfraction prostate motion. The addition of 120 mbar abdominal compression to custom pelvic immobilization influenced neither interfraction nor intrafraction prostate motion.

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