Department for Radiotherapy, Division of Medical Radiation Physics, Medical University of Vienna, Vienna, Austria.
J Appl Clin Med Phys. 2010 Aug 20;11(4):3219. doi: 10.1120/jacmp.v11i4.3219.
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 cm3. 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 1mm 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.
目的是为点状扫描质子治疗计划确定最佳的一组治疗计划参数和最小必要剂量矩阵分辨率。在均匀几何体模中，针对三个不同大小的立方靶区（8、64 和 244cm³），基于不同的计划参数和剂量网格分辨率（DG）组合计算了治疗计划。质子剂量由单个射束传输。通过剂量分布平行于和垂直于中央射束轴的方式以及剂量均匀性和适形性度量来比较治疗计划。无论靶区大小如何，如果点层之间的距离在单个布拉格峰宽度的量级，并且点之间的侧向距离不超过点 sigma 的两倍，则剂量均匀性和适形性可以进行比较。如果点层之间的距离明显大于布拉格峰的宽度，则均匀性指数会增加。对于小靶区，该指数从极端情况下的 5%左右上升到 12%，对于两个较大的靶区，则超过 20%。此外，95%等剂量线的宽度也会增加。对于确定质子剂量点之间侧向间距的参数变化，也会出现类似的结果。对于所有考虑的设置，直到 DG 为 6mm 时，相对于 1mmDG 的剂量分布的平均差异均低于 3%。然而，需要 DG 小于 2-3mm 才能将最大偏差保持在该限制以下。本研究中进行的测试对于防止点状扫描质子治疗计划中的系统误差是必要的。在均匀体模中，发现沿布拉格峰的纵向方向将剂量点分开，并且在侧向方向上不超过点 sigma 的两倍，可以为 IMPT 治疗计划提供足够的条件。需要 DG 为 2-3mm 才能准确解析质子束的陡峭剂量梯度。
