Clinical Implementation of Proton Therapy Using Pencil-Beam Scanning Delivery Combined With Static Apertures

Abstract

Proton therapy makes use of the favorable depth-dose distribution with its characteristic Bragg peak to spare normal tissue distal of the target volume. A steep dose gradient would be desired in lateral dimensions, too. The widespread spot scanning delivery technique is based, however, on pencil-beams with in-air spot full-widths-at-half-maximum of typically 1 cm or more. This hampers the sparing of organs-at-risk if small-scale structures adjacent to the target volume are concerned. The trimming of spot scanning fields with collimating apertures constitutes a simple measure to increase the transversal dose gradient. The current study describes the clinical implementation of brass apertures in conjunction with the pencil-beam scanning delivery mode at a horizontal, clinical treatment head based on commercial hardware and software components. Furthermore, clinical cases, which comprised craniopharyngiomas, re-irradiations and ocular tumors, were evaluated. The dosimetric benefits of 31 treatment plans using apertures were compared to the corresponding plans without aperture. Furthermore, an overview of the radiation protection aspects is given. Regarding the results, robust optimization considering range and setup uncertainties was combined with apertures. The treatment plan optimizations followed a single-field uniform dose or a restricted multi-field optimization approach. Robustness evaluation was expanded to account for possible deviations of the center of the pencil-beam delivery and the mechanical center of the aperture holder. Supplementary apertures improved the conformity index on average by 15.3%. The volume of the dose gradient surrounding the PTV (evaluated between 80 and 20% dose levels) was decreased on average by 17.6%. The mean dose of the hippocampi could be reduced on average by 2.9 GyRBE. In particular cases the apertures facilitated a sparing of an organ-at-risk, e.g. the eye lens or the brainstem. For six craniopharyngioma cases the inclusion of apertures led to a reduction of the mean dose of 1.5 GyRBE (13%) for the brain and 3.1 GyRBE (16%) for the hippocampi.

Treatment of ocular tumors through a novel applicator on a conventional proton pencil beam scanning beamline

“…Although PBS allows for lateral collimation of the beam through the placement of spots conforming to the target extents, it has been shown that a sharper lateral dose fall-off 15 , 16 could be achieved by a patient specific aperture. The use of aperture for PBS adds another layer of complexity for isocenter verification.…”

Optimization of proton pencil beam positioning in collimated fields

“…The fact that the benefit of LP from overscanning decreases with depth can be explained by the lateral scattering of protons increasing with depth. Likewise, additional lateral scattering of the proton beam in combination of collimated PBS with range shifters for shallow tumors as described in various studies 3–6,9,13,25 diminishes the effect. Future research should investigate the overscanning and spot positioning effect in potential clinical setups such as multiple energies, setups with multiple fields, in different range shifter materials and thicknesses, as well as changing the order of range shifter and aperture 3,25 …”

“…The LP is influenced by the initial spot size, multiple Coulomb scattering in matter (the patient), and optional beam shaping devices in the treatment head (‘nozzle'), such as range shifter or aperture. A sharper LP could be achieved when combining PBS with apertures or multi‐leaf collimators 3–8 . Winterhalter et al 9 .…”

Technical note: Impact of beam properties for uveal melanoma proton therapy—An in silico planning study

“…The pencil-beam scanning (PBS) technique with apertures can in principle be applied as well. [9][10][11] The DFO of a generated SOBP is impacted by the energy spread at the treatment nozzle exit. Here a balance between high dose rate, maximum field size, and energy spread needs to be defined.…”

“…The lateral spreading of the proton beam is typically achieved by a single‐ or double‐scattering systems. The pencil‐beam scanning (PBS) technique with apertures can in principle be applied as well 9–11 …”