Search

Intensity modulated proton therapy (IMPT) in the form of pencil beam scanning (PBS) has shown improvement in treatment plan quality as compared to conventional proton and photon-based radiotherapy techniques. However, in IMPT maintaining a sharp lateral dose falloff is crucial for sparing organs at risk (OARs), especially when they are in close proximity to the target volume. The most common approach to improve lateral dose falloff is through the use of physical beam shaping devices, such as brass apertures or collimator-based systems. This work has shown that IMPT can be further improved by implementation of advanced spot placement techniques by moving away from traditional grid-based placements to boundary contoured techniques. We have developed a new optimized spot placement algorithm that provides robust spot distributions inside the target volume by making use of various geometric construction techniques in other fields and developed a unique spot placement technique that provides both high conformality and uniformity in a robust manner for arbitrarily complex target geometries. This approach achieves the boundary conformity of a recently proposed concentric-contours based approach and uses a fast-iterative method to distribute the interior spots in a highly uniform fashion in an attempt to improve both the lateral dose falloff and uniformity. We performed the treatment plan quality comparison for five spot placement techniques using customized homogeneous phantoms. These include two grid-based (rectilinear/hexagonal) and three boundary-contoured (concentric-contours, hybrid and optimized) techniques. Treatment plans were created for two different target volumes, (conical and spherical). An optimal set of planning parameters was defined for all treatment plans and the impact of spot placement techniques on the plan quality was studied in terms of lateral & distal dose falloff, normal tissue sparing, conformity & homogeneity of dose distributions, and total number of spots. For grid-based spot placement techniques, dose conformity is dependent on the target cross sectional shape, which changes for each proton energy. This variable conformity problem is shown to be mitigated by using boundary contoured techniques. However, in the case of concentric contours, the conformity is improved but at the cost of decreased homogeneity. Hybrid and optimized spot placement techniques show more uniform dose distributions while maintaining the improved dose conformity. The optimized spot placement technique is shown to provide robust treatment plans with improved target coverage, homogeneity of dose, and minimal spots count. These results highlight that plan quality in PBS proton therapy may be improved for many patients, without the need for expensive delivery equipment updates, simply by providing additional spot placement techniques in commercial treatment planning software (TPS).