Effectiveness of couch height-based patient set-up and an off-line correction protocol in prostate cancer radiotherapy

Purpose: To investigate set-up improvement caused by applying a couch heigh t-based patient set-up method in combination with a technologist-driven off -line correction protocol in nonimmobilized radiotherapy of prostate patien ts. Methods and Materials: A three-dimensional shrinking action level correctio n protocol is applied in two consecutive patient cohorts with different set -up methods: the traditional "laser set-up" group (n = 43) and the "couch h eight set-up" group (n = 112). For all directions, left-right, ventro-dorsa l, and cranio-caudal, random and systematic set-up deviations were measured . Results: The couch height set-up method improves the patient positioning co mpared to the laser set-up method. Without application of the correction pr otocol, both systematic and random errors reduced to 2.2-2.4 mm (1 SD) and 1.7-2.2 mm (1 SD), respectively. By using the correction protocol, systemat ic errors reduced further to 1.3-1.6 mm (1 SD). One-dimensional deviations were within 5 mm for >90% of the measured fractions. The required number of corrections per patient in the off-line correction protocol was reduced si gnificantly during the course of treatment from 1.1 to 0.6 by the couch hei ght set-up method, The treatment time was not prolonged by application of t he correction protocol. Conclusions: The couch height set-up method improves the set-up significant ly, especially in the ventro-dorsal direction. Combination of this set-up m ethod with an off-line correction strategy, executed by technologists, redu ces the number of set-up corrections required. (C) 2001 Elsevier Science In c.