Intrafractional stability of the prostate using a stereotactic radiotherapy technique.

Purpose: To evaluate the stability of the prostate during stereotactic radiation therapy.

Materials And Methods: Forty-seven patients underwent placement of three fiducial markers into the prostate as part of a pilot study of hypofractionated stereotactic radiotherapy. Portal images before and subsequent to 227 radiotherapy fractions were analyzed for prostate movement. Six patients also underwent localizing radiographs at 6-min intervals for 24 min. Relative motion of the bony landmarks and prostate markers was calculated.

Results: Analysis of portal images revealed the undirected average prostate movement of 2.0 mm (superior/inferior), 1.9 mm (anterior/posterior), and 1.4 mm (right/left) with maximum standard deviation (SD) of 2.0. Analysis of radiographs at 6-min intervals showed the greatest undirected average prostate motion between 0-6 min; 1.5 mm (superior/inferior), 1.4 mm (anterior/posterior), and 0.4 mm (right/left). Beyond 6 min, movements decreased to 0.4, 0.9, and 0.8 mm, respectively. Bony landmark motion was 0.9 mm (superior/inferior), 0.9 mm (anterior/posterior), and 0.4 mm (right/left) between 0-6 min. Beyond 6 min, motion decreased to less than 0.5 mm in any direction.

Conclusions: Stereotactic prostate radiotherapy, utilizing fiducial marker localization, resulted in average intrafractional prostate movement of 2.0 mm or less. Most patient and organ movement occurs early and a settling-in period is advisable before treatment.