Intrafraction Motion of the Prostate During External-Beam Radiation Therapy: Analysis of 427 Patients with Implanted Fiducial Markers

Conclusions: Intrafraction motion of the prostate occurs frequently during external-beam irradiation on a time scale of 5-7 min. Margins of 2 mm account for these intrafraction motions. However, larger margins are required in practice to accommodate other uncertainties in the treatment.