Initial clinical experience with infrared-reflecting skin markers in the positioning of patients treated by conformal radiotherapy for prostate cancer

Purpose : To evaluate an infrared (IR) marker-based positioning system in patients receiving conformal radiotherapy for prostate cancer. Methods and materials : During 553 treatments, the ability of the IR system to automatically position the isocenter was recorded. Setup errors were measured by means of orthogonal verification films and compared to conventional positioning (using skin drawings and lasers) in 184 treatments. Results : The standard deviation of anteroposterior (AP) and lateral setup errors was significantly reduced with IR marker positioning compared to conventional: 2 vs. 4.8 mm AP ( p < 0.01) and 1.6 vs. 3.5 mm laterally ( p < 0.01). Longitudinally, the difference was not significant (3.5 vs. 3.0 mm). Systematic errors were on the average smaller AP and laterally for the IR method: 4.1 vs. 7.8 mm AP ( p = 0.01) and 3.1 vs. 5.6 mm lateral ( p = 0.07). Longitudinally, the IR system resulted in somewhat larger systematic errors: 5.0 vs. 3.4 mm for conventional positioning ( p = 0.03). The use of an off-line correction protocol, based on the average deviation measured over the first four fractions, allowed virtual elimination of systematic errors. Inability of the IR system to correctly locate the markers, leading to an executional failure, occurred in 21% of 553 fractions. Conclusion : IR marker-assisted patient positioning significantly improves setup accuracy along the AP and lateral axes. Executional failures need to be reduced.