Investigations of line scanning proton therapy with dynamic multi-leaf collimator

Highlights

• Reducing penumbra of Scanning proton therapy by using multi-leaf collimator (MLC) is investigated.

• The increasing of neutron dose due to proton interaction with MLC is studied.

• Neutron dose in scanning with MLC is lower than wobbling therapy, but higher than scanning without MLC.

Abstract

Purpose

Scanning proton therapy has dosimetric advantage over passive treatment, but has a large penumbra in low-energy region. This study investigates the penumbra reduction when multi-leaf collimators (MLCs) are used for line scanning proton beams and secondary neutron production from MLCs.

Methods

Scanning beam plans with and without MLC shaping were devised. Line scanning proton plan of 36 energy layers between 71.2 and 155.2 MeV was generated. The MLCs were shaped according to the cross-sectional target shape for each energy layer. The two-dimensional doses were measured through an ion-chamber array, depending on the presence of MLC field, and Monte Carlo (MC) simulations were performed. The plan, measurement, and MC data, with and without MLC, were compared at each depth. The secondary neutron dose was simulated with MC. Ambient neutron dose equivalents were computed for the line scanning with 10 × 10 × 5 cm³ volume and maximum proton energy of 150 MeV, with and without MLCs, at lateral distances of 25–200 cm from the isocenter. The neutron dose for a wobbling plan with 10 × 10 × 5 cm³ volume was also evaluated.

Results

The lateral penumbra width using MLC was reduced by 23.2% on average, up to a maximum of 32.2%, over the four depths evaluated. The ambient neutron dose equivalent was 18.52% of that of the wobbling beam but was 353.1% larger than the scanning open field.

Conclusions

MLC field shaping with line scanning reduced the lateral penumbra and should be effective in sparing normal tissue. However, it is important to investigate the increase in neutron dose.

Introduction

Since the first clinical application of proton therapy to cancer treatment, proton therapy beam delivery methods have evolved, and currently, passive scattering (single and double scattering), passive scanning (uniform scanning and wobbling), and active scanning (spot, raster, and line scanning) are all available for cancer treatment. Spot scanning proton therapy was proposed by Kanai et al. [1] in 1980, and the first patient was treated with 200 MeV protons at the Paul Scherrer institute [2]. Since then, many proton therapy centres have used proton scanning techniques for patient treatment [3], [4], [5], [6], [7], [8], [9]. While conventional scattering proton therapy delivers a large homogenous proton flux, the scanning method adjusts the path of a narrow pencil beam by using steering magnets to position the beamlets. Meanwhile, it modulates the proton energy and intensity to deliver the prescribed dose to the three-dimensional target volume. The cross-sectional shape and spot intensity of a two-dimensional dose can be controlled for each energy layer; this makes intensity modulation proton therapy (IMPT) possible. This technology eliminates the need for the patient-specific aperture and compensator that are necessary in conventional passive scattering and wobbling methods. IMPT is a promising technique for reducing the dose to a closely situated critical organ, while providing better target coverage, or at least similar conformality when compared with scattering proton therapy [10], [11], [12], [13], [14]. In particular, the skin dose is minimised and the neutron dose, produced by protons interacting with high atomic number materials such as the patient aperture, is greatly reduced; this eventually lowers the risk of secondary cancer [15], [16], [17].

However, one drawback of not using an aperture is the larger lateral penumbra of the scanning beams, particularly for low-energy protons. For high-energy protons, the internal scatter contributes significantly to the lateral penumbra rather than the proton spot size. However, for low-energy scanning beams, the initial proton spot size mostly contributes to the lateral penumbra, which is even wider than that for passive scattering [18].

In order to overcome the aforementioned shortcoming, the application of apertures or multi-leaf collimators (MLCs) to scanning proton beams has been studied. The aperture to the scanning proton beam can sharpen the lateral penumbras and reduce the out-of-field dose [19]. Using MLCs instead of an aperture requires an additional hardware design in the nozzle; various studies on this topic have been conducted because it avoids the need for a patient-specific aperture, thus reducing the cost and enhancing the efficiency of patient treatments. Applying MLCs to scanning proton beams has the potential to enhance normal tissue saving adjacent to the field boundary and is comparable to that of a brass aperture [20], [21]. In previous literature, which reports the use of a patient-specific aperture or MLCs, target shaping was determined to cover the largest cross-section in the beam’s eye view (BEV) for one energy layer. Hyer et al. [22] proposed a new concept of a dynamic collimation system (DCS) made of nickel for sharpening the lateral penumbra of spot scanning to reduce the penumbra in three dimensions. It attempts target shaping on all energy layers by using two nickel trimmer blades. The DCS is a lightweight system of 20 kg; however, the maximum treatment field size is limited to 15 × 15 cm², and the motion trigger needs to be initiated for each energy layer to fit the cross-sectional shape; this increases the treatment time by 1–3 s per energy layer.

Rather than the benchtop design, some commercial proton therapy systems now provide MLCs to replace the patient aperture for conventional treatments, such as scattering (IBA system) or wobbling techniques (Sumitomo Heavy Industry). MLC leaf positioning technology has been improved, and thus, the MLCs made for conventional treatment can be dynamically used for scanning beam delivery to reduce the penumbra in three dimensions. Moreover, a commercial product was recently released that supports adaptive aperture with MLCs in spot scanning treatments. However, no studies have yet reported the drawbacks of using MLCs with scanning proton therapy caused by the additional neutron dose generated by proton interaction with the high atomic number material in MLCs; although neutron dose assessments of MLCs for wobbling proton therapy have been reported [23]. Therefore, the aim of this research is to assess the dosimetric advantage of using dynamic MLCs for line scanning proton therapy, particularly in reducing lateral penumbra, and to determine the additional neutron dose generated from the MLCs.