title: Set-up variation of patients treated with radiotherapy to the prostate measured with an electronic portal imaging device.
abstract: The set-up variation of 11 patients treated supine with radical radiotherapy for carcinoma of the prostate was measured with an electronic portal imaging device to determine the adequacy of set-up techniques and current margins, as well as the need for immobilization. During the treatments 172 images of the anterior fields and 159 images of the left-lateral fields were taken and the errors in treatment placement were measured by template matching. The variation in the superior-inferior direction was small, 1.4-1.6 mm (1 SD), while the medio-lateral variation was 2.8 mm (1 SD). The anterior-posterior variation was largest, 4.6 mm (1 SD) with an offset of 3.3 mm anterior. This anterior offset and large anterior-posterior variation suggests that set-up techniques were not optimal for this direction. The 1 cm margin used was adequate for set-up variation except in a small number of cases, which was mainly due to the anterior trend. Random (treatment-to-treatment) variations were small (1.1-2.3 mm; 1 SD), indicating that immobilization would result in only modest improvement in reproducibility for these supine patients.
authors.affiliation: Oncology Department, Auckland Hospital, New Zealand. pgreer@cancer.rah.sa.gov.au
authors.forename: P B
authors.initials: PB
authors.lastname: Greer
authors.forename: C C
authors.initials: CC
authors.lastname: Jose
authors.forename: J H
authors.initials: JH
authors.lastname: Matthews
mesh_terms: D006801:Humans; D007103:Immobilization; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011859:Radiography; D011881:Radiotherapy, Computer-Assisted; D011882:Radiotherapy, High-Energy / Q000379:methods*