Prostate and patient intrafraction motion: Impact on treatment time-dependent planning margins for patients with endorectal balloon

Purpose: To investigate intrafraction prostate and patient motion during different radiation therapy treatments as a function of treatment time; included were prostate patients with an endorectal balloon (ERB). Margins accounting for setup uncertainties and intrafraction motion were determined. Methods and Materials: The study included 17 patients undergoing prostate cancer radiation therapy. All patients received 3 fiducial gold markers implanted in the prostate and were then immobilized in the supine position with a knee support and treated with an ERB. Twelve patients with intermediate risk for pelvic lymph node metastases received intensity modulated radiation therapy (IMRT), and 5 patients at low risk received a 4-field box treatment. After setup based on skin marks, patients were imaged with a stereoscopic imaging system. If the marker displacement exceeded a 3-mm tolerance relative to planning computed tomography, patients were shifted and verification images were taken. All patients underwent additional imaging after treatment; IMRT patients also received additional imaging at halftime of treatment. Prostate and bone drifts were evaluated as a function of treatment time for more than 600 fractions, and margins were extracted. Results: Patient motion evaluated by bone match was strongly patient dependent but in general was smallest in the superior-inferior (SI) direction. Prostate drifts were less patient dependent, showing an increase with treatment time in the SI and anterior-posterior (AP) directions. In the lateral (LAT) direction, the prostate stayed rather stable. Mean treatment times were 5.5 minutes for 4-field box, 10 minutes for 5-field boost IMRT, and 15 minutes or more for 9-field boost and 9-field pelvic IMRT treatments. Margins resulted in 2.2 mm, 3.9 mm, and 4.3 mm for 4-field box; 3.7 mm, 2.6 mm, and 3.6 mm for 5-field boost IMRT; 2.3 mm, 3.9 mm, and 6.2 mm for 9-field boost IMRT; and 4.2 mm, 5.1 mm, and 6.6 mm for 9-field pelvic IMRT in the LAT, SI, and AP directions, respectively. Conclusion: Intrafraction prostate and patient displacement increased with treatment time, showing different behaviors for the single directions of movement. Repositioning of the patients during long treatments or shorter treatment times will be necessary to further reduce the treatment margin.