• Geert M Villeirs, Gert O De Meerleer, Koenraad L Verstraete, and Wilfried J De Neve.
• Department of Radiology, Ghent University Hospital, Ghent, Belgium. Geert.Villeirs@ugent.be
• Int. J. Radiat. Oncol. Biol. Phys. 2004 Dec 1; 60 (5): 1611-21.

PurposeTo measure prostate motion with magnetic resonance imaging (MRI) during a course of intensity-modulated radiotherapy.Methods And MaterialsSeven patients with prostate carcinoma were scanned supine on a 1.5-Tesla MRI system with weekly pretreatment and on-treatment HASTE T2-weighted images in 3 orthogonal planes. The bladder and rectal volumes and position of the prostatic midpoint (PMP) and margins relative to the bony pelvis were measured.ResultsAll pretreatment positions were at the mean position as computed from the on-treatment scans in each patient. The PMP variability (given as 1 SD) in the anterior-posterior (AP), superior-inferior (SI), and right-left (RL) directions was 2.6, 2.4, and 1.0 mm, respectively. The largest variabilities occurred at the posterior (3.2 mm), superior (2.6 mm), and inferior (2.6 mm) margins. A strong correlation was found between large rectal volume (>95th percentile) and anterior PMP displacement. A weak correlation was found between bladder volume and superior PMP displacement.ConclusionsAll pretreatment positions were representative of the subsequent on-treatment positions. A clinical target volume (CTV) expansion of 5.3 mm in any direction was sufficient to ascertain a 95% coverage of the CTV within the planning target volume (PTV), provided that a rectal suppository is administered to avoid rectal overdistension and that the patient has a comfortably filled bladder (<300 mL).

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