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 height-based patient set-up method in combination with a technologist-driven off-line correction protocol in nonimmobilized radiotherapy of prostate patients. Methods and Materials: A three-dimensional shrinking action level correction protocol is applied in two consecutive patient cohorts with different set-up methods: the traditional 'laser set-up' group (n=43) and the 'couch height set-up' group (n=112). For all directions, left-right, ventro-dorsal, and cranio-caudal, random and systematic set-up deviations were measured. Results: The couch height set-up method improves the patient positioning compared to the laser set-up method. Without application of the correction protocol, 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, systematic 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 significantly during the course of treatment from 1.1 to 0.6 by the couch height set-up method. The treatment time was not prolonged by application of the correction protocol. Conclusions: The couch height set-up method improves the set-up significantly, especially in the ventro-dorsal direction. Combination of this set-up method with an off-line correction strategy, executed by technologists, reduces the number of set-up corrections required