P158] Analysis of different overlap of patched spot scanned proton fields

Abstract

Purpose Proton treatment fields can be larger than the effective field size of the delivery machine. In this case, delivery is composed of multiple patched sub-fields, where the couch is moving between the patches. As our delivery machine uses parallel scanning, patched fields are currently delivered without overlap, but there is a possibility to change the overlap between patches to a defined distance to improve robustness. In this work we have evaluated the influence of different overlaps. Methods Measurements of patched fields with different amount of overlap have been performed with a CCD camera. 13 homogenous box fields of different sizes with and without preabsorber, have been measured with the CCD camera, each with 0 cm, 1 cm, 2 cm and 3 cm overlap. Offsets of ± 1 mm between the patches were introduced to evaluate the possible error of an unexpected movement between patches. The images have been analyzed using gamma analysis (1% local dose difference and 1 mm distance to agreement) as well as comparing profiles over the patched area. By comparing the different overlaps, each of them was ranked (from 1 to 4) for every set of samples. A rank of 1 was given to the overlap which had the best fit of measured data to the calculated data and a rank of 4 given to the sample with the worst fit. Results Smaller dose differences are seen with a pre-absorber in the field than without, and all results are consistent for different measurement depths. With offsets, the dose difference is smaller for greater overlaps. The ranking over all overlaps shows that the optimal overlap would be a 2 cm overlap (ranking score of 44), followed by a 3 cm overlap (45), 1 cm (59) and 0 cm (75). If measurements with offsets are excluded, the 2 cm overlap is still the best (22), whereas 1 cm (31) and 3 cm (31) overlaps are equally effective. Again the worst results were obtained with 0 cm overlap (36). Conclusions Based on the measurements taken for this project, the optimal overlap setting for patched fields is 2 cm.