What is the prostate movement during fractionated irradiation?

Purpose: To find the largest displacement of prostate position during radiotherapy. Methods: We analysed 20 patients treated with radical radiotherapy for localized prostate cancers between October 2004 and July 2005. Patients were immobilized using thermoplastic Orfit masks and Leg Support and Foot Support Orfit System. All patients were treated conformally, using high energy photons. The total dose was 74 Gy, delivered in 37 fractions during seven and a half weeks. We evaluated the positions of prostate once a week using CT scans. CTs, all planning procedures and irradiation were performed using the same positioning system. Prostates were delineated and the changes in their positions were measured using appropriate device the 'Eclipse' program. All procedures were performed by the same group of persons. Results: We did not observe any major prostate displacement in cranio-caudal and lateral axes. Some movements of prostate were observed in anterior-posterior (AP) axis. The mean of prostate movement in AP axis was 0.57 cm and ranged between 0.0 cm and 2.21 cm. In 89% of measurements the prostate displacement was smaller than 14 mm. In 90% of patients, all measurements of prostate displacement were smaller than 20 mm. Conclusions: During prostate cancer patients radiotherapy, anterior-posterior displacements should be checked because of significant prostate motions. Prostate AP margins (PTV) should be increased to about 1,5-2,0 cm to avoid geographical misses