Electronic portal image assisted reduction of systematic set-up errors in head and neck irradiation

Purpose: To quantify systematic and random patient set-up errors in head and neck irradiation and to investigate the impact of an off-line correction protocol on the systematic errors. Material and methods: Electronic portal images were obtained for 31 patients treated for primary supra-glottic larynx carcinoma who were immobilised using a polyvinyl chloride cast. The observed patient set-up errors were input to the shrinking action level (SAL) off-line decision protocol and appropriate set-up corrections were applied. To assess the impact of the protocol, the positioning accuracy without application of set-up corrections was reconstructed. Results: The set-up errors obtained without set-up corrections (1 standard deviation (SD) = 1.5-2 mm for random and systematic errors) were comparable to those reported in other studies on similar fixation devices. On an average, six fractions per patient were imaged and the set-up of half the patients was changed due to the decision protocol. Most changes were detected during weekly check measurements, not during the first days of treatment. The application of the SAL protocol reduced the width of the distribution of systematic errors to 1 mm (1 SD), as expected from simulations. A retrospective analysis showed that this accuracy should be attainable with only two measurements per patient using a different off-line correction protocol, which does not apply action levels. Conclusions: Off-line verification protocols can be particularly effective in head and neck patients due to the smallness of the random set-up errors. The excellent set-up reproducibility that can be achieved with such protocols enables accurate dose delivery in conformal treatments.

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