Assessing a set of optimal user interface parameters for intensity-modulated proton therapy planning

The purpose was to identify an optimal set of treatment planning parameters and a minimal necessary dose matrix resolution for treatment planning with spot-scanned protons. Treatment plans based on different combinations of planning parameters and dose grid resolutions (DG) were calculated in a homogeneous geometric phantom for three cubic targets of different size: 8, 64 and 244 cm(3). The proton dose was delivered by one single beam. Treatment plans were compared in terms of dose profiles parallel to and perpendicular to the central beam axis, as well as by dose homogeneity and conformity measures. Irrespective of target size, the dose homogeneity and conformity were comparable if the distance between spot layers was in the order of the width of a single Bragg peak, and the lateral distance between spots did not exceed two times the spot sigma. If the distance between spot layers was considerably larger than the width of the Bragg peak, the homogeneity index increased. For the small target, this index escalated from values around 5% to 12% in extreme, and to more than 20% for the two larger targets. Furthermore, the width of the 95% isodose increased. Similar results were found for the variation of the parameter determining the lateral spacing between proton dose spots. The average difference of dose profiles with respect to the profile for a DG of 1 mm was below 3% for all considered settings up to a DG of 6 mm. However, a DG of less than 2-3 mm is required to keep the maximum deviation below this limit. The tests performed in this study are necessary to prevent systematic errors from spot-scanning proton therapy planning. A separation of dose spots in the dimensions of the Bragg peak in the longitudinal direction and no more than two times the spot sigma in the lateral direction were found to be adequate for IMPT treatment planning in a homogeneous phantom. A DG of 2-3 mm is necessary to accurately resolve the steep dose gradients of proton beams.