Purpose: Quantification of set-up errors is necessary to assess the accuracy of patient positioning and define set-up margins. In this article, we describe the analysis of two different set-up verification and correction procedures in pelvic irradiation for rectal cancer patients treated on a belly-board device.
Methods: First, we conducted a retrospective study in ten patients. Skin marks were used for set-up and the position was verified and corrected at the start of treatment by portal imaging. Second, we analysed the implementation of a more rigorous verification and correction procedure in ten patients. The same set-up procedure was used, but verification was performed during the first three sessions and on a weekly basis thereafter. In both studies, systematic and random errors were linked with possible patient-related, treatment-unit-related and time-related factors.
Results: The pooled data showed a significant reduction in systematic and random error in favour of the second verification procedure (p < 0.05). This resulted in a reduction in the size of the safety margin of more than 3 mm in all directions. Time trends were significant in four patients in the first analysis and in three patients in the second analysis. In six patients in the first and seven patients in the second study, a significant correlation was found between the vertical couch movement and the antero-posterior set-up error. Analysis of patient-related factors demonstrated a relationship between the abdominal contour and rotational errors in both studies.
Conclusion: The results of these set-up analyses show that patient positioning on a belly-board device by laser alignment to skin marks is accurate and reproducible. However, in some patients we recommend the implementation of a fixed vertical couch position. The systematic error should be identified and corrected during the first fractions of treatment. Thereafter, verification should be performed at regular intervals to correct for possible time trends. Positioning of obese patients was found to be more prone to set-up errors and requires online position verification.
