Range optimization for mono- and bi-energetic proton modulated arc therapy with pencil beam scanning

Abstract

The development of rotational proton therapy plans based on a pencil-beam-scanning (PBS) system has been limited, among several other factors, by the energy-switching time between layers, a system-dependent parameter that ranges between a fraction of a second and several seconds. We are investigating mono- and bi-energetic rotational proton modulated arc therapy (PMAT) solutions that would not be affected by long energy switching times. In this context, a systematic selection of the optimal proton energy for each arc is vital. We present a treatment planning comparison of four different range selection methods, analyzing the dosimetric outcomes of the resulting treatment plans created with the ranges obtained. Given the patient geometry and arc definition (gantry and couch trajectories, snout elevation) our in-house treatment planning system (TPS) FoCa was used to find the maximum, medial and minimum water-equivalent thicknesses (WETs) of the target viewed from all possible field orientations. Optimal ranges were subsequently determined using four methods: (1) by dividing the max/min WET interval into equal steps, (2) by taking the average target midpoints from each field, (3) by taking the average WET of all voxels from all field orientations, and (4) by minimizing the fraction of the target which cannot be reached from any of the available angles. After the range (for mono-energetic plans) or ranges (for bi-energetic plans) were selected, the commercial clinical TPS in use in our institution (Varian Eclipse) was used to produce the PMAT plans using multifield optimization. Linear energy transfer (LET) distributions of all plans were also calculated using FoCa and compared among the different methods. Mono- and bi-energetic PMAT plans, composed of a single 180° arc, were created for two patient geometries: a C-shaped target located in the mediastinal area of a thoracic tissue-equivalent phantom and a small brain tumor located directly above the brainstem. All plans were optimized using the same procedure to (1) achieve target coverage, (2) reduce dose to OAR and (3) limit dose hot spots in the target. Final outcomes were compared in terms of the resulting dose and LET distributions. Data shows little significant differences among the four studied methods, with superior results obtained with mono-energetic plans. A streamlined systematic method has been implemented in an in-house TPS to find the optimal range to maximize target coverage with rotational mono- or bi-energetic PBS rotational plans by minimizing the fraction of the target that cannot be reached by any direction.

MultiRBE: Treatment planning for protons with selective radiobiological effectiveness

“…Besides, a recent study in prostate cancer has found that it is not possible to simultaneously meet the tumor and rectal constrains for both physical and variable RBE‐weighted dose, the latter being estimated based on in vitro cell survival data. In fact, other groups have decided to avoid the sometimes controversial concept of proton RBE completely and have tackled this problem by performing the optimization directly on LET distributions or proposing new beam orientations or treatment modalities …”

“…In fact, other groups have decided to avoid the sometimes controversial concept of proton RBE completely and have tackled this problem by performing the optimization directly on LET distributions [35][36][37][38][39] or proposing new beam orientations or treatment modalities. [40][41][42] Our proposed solution is the use of a mixed RBE model (MultiRBE) for plan optimization, where a uniform RBE is used in the target contours to ensure an adequate tumor coverage in terms of physical dose, but a variable RBE is used elsewhere. This solution incorporates the benefits of both approaches: it produces a quantifiable, numeric RBE quantity that can be used for weighting of physical dose, but it also ensures appropriate coverage of the target with a flat physical dose distribution.…”

Teaching treatment planning for protons with educational open‐source software: experience with FoCa and matRad

“…After being used successfully in several research projects, FoCa and matRad were utilized for teaching purposes in two courses during academic year 2016/2017 at the our University: Nuclear Physics Applied to Medicine (from the MSc in Nuclear Physics) and Hadron therapy (from the Summer School on Advanced Topics in Medical Physics), with a total of over 60 students. The objective of the learning experiences was the familiarization of the students with treatment planning techniques, pencil beam modeling, and radiobiology applied to treatment planning.…”

Proton relative biological effectiveness (RBE): a multiscale problem

“…Beam number and beam angle selection will also be important considerations: wide "hinge-angles" between beams offer greater scope for LET optimisation and even proton arc-based solutions have been proposed. 4 But ultimately, RBE is more than an end-of-range problem that can be easily mitigated by incorporating physics-based surrogates into treatment plan optimisation. Even after physics-based mitigation, RBE introduces considerable biological uncertainty into predictions of treatment response.…”