Prostate motion during standard radiotherapy as assessed by fiducial markers

From November 1993 to August 1994, 55 patients with localized prostate carcinoma had three gold seeds placed in the prostate under transrectal ultrasound guidance prior to the start of radiotherapy in order to track prostate motion. Patients had a planning CT scan before initial simulation and again at about 40 Gy, just prior to simulation of a field reduction. Seed position relative to fixed bony landmarks (pubic symphysis and both ischial tuberosities) was digitized from each pair of orthogonal films from the initial and boost simulation using the Nucletron brachytherapy planning system. Vector analysis was performed to rule out the possibility of independent seed migration within the prostate between the time of initial and boost simulation. Prostate motion was seen in the posterior (mean: 0.56 cm; SD: 0.41 cm) and inferior directions (mean: 0.59 cm; SD: 0.45 cm). The base of the prostate was displaced more than 1 cm posteriorly in 30% of patients and in 11% in the inferior direction. Prostate position is related to rectal and bladder filling. Distension of these organs displaces the prostate in an anterosuperior direction, with lesser degrees of filling allowing the prostate to move posteriorly and inferiorly. Conformal therapy planning must take this motion into consideration. Changes in prostate position of this magnitude preclude the use of standard margins.