Reduction of the secondary neutron dose in passively scattered proton radiotherapy, using an optimized pre-collimator/collimator

Real-time gated proton therapy with a reduced source to imager distance: Commissioning and quality assurance

INTRODUCTION

Real-time gated proton therapy (RGPT) is a motion management technique unique to the Hitachi particle therapy system. It uses pulsed fluoroscopy to track an implanted fiducial marker. There are currently no published guidelines on how to conduct the commissioning and quality assurance. In this work we reported on our centre's commissioning workflow and our daily and monthly QA procedures.

METHODS

Six commissioning measurements were designed for RGPT. The measurements include imaging qualities, fluoroscopic exposures, RGPT marker tracking accuracy, temporal gating latency, fiducial marker tracking fidelity and an end-to-end proton dosimetry measurement. Daily QA consists of one measurement on marker localization accuracy. Four months daily QA trends are presented. Monthly QA consists of three measurementson the gating latency, fluoroscopy imaging quality and dosimetry verification of gating operation with RGPT.

RESULTS

The RGPT was successfully commissioned in our centre. The air kerma rates were within 15 % from specifications and the marker tracking accuracies were within 0.245 mm. The gating latencies for turning the proton beam on and off were 119.5 and 50.0 ms respectively. The 0.4x10.0 mm2 Gold AnchorTM gave the best tracking results with visibility up to 30 g/cm2. Gamma analysis showed that dose distribution of a moving and static detectors had a passing rate of more than 95 % at 3 %/3mm. The daily marker localization QA results were all less than 0.2 mm.

CONCLUSION

This work could serve as a good reference for other upcoming Hitachi particle therapy centres who are interested to use RGPT as their motion management solution.

The Potential Therapeutic Effects of Low-Dose Ionizing Radiation in Alzheimer's Disease

Dementia is an umbrella term used to describe a loss of cognitive function which results in the interference of an individual's daily life and activities. The most common form of dementia is Alzheimer's disease. Alzheimer’s is classified as a progressive, debilitating neurodegenerative disease that results in disturbances to a patient’s higher executive function, memory, language, and visuospatial orientation. Despite extensive research on Alzheimer’s dementia, including both available and potential therapeutic modalities, this neurodegenerative disease is incurable and will continue to pose a major public health concern. Current treatment options for Alzheimer’s focus on symptom management and/or delaying the progression of the disease. Therefore, new treatment strategies must be developed to combat such a deadly disease. One field of medicine that has garnered significant interest from researchers to potentially treat Alzheimer’s is low-dose ionizing radiation. Various reports suggest that the brain’s exposure to low doses of ionizing radiation may serve as a therapeutic modality for combating neurodegenerative diseases, including Alzheimer’s dementia. This article serves as a review of the current available treatments for Alzheimer’s disease and discusses recent studies that provide evidence for the potential use of low-dose ionizing radiation as a therapeutic in the treatment of Alzheimer’s disease.

Investigation of the effects of biomaterials on proton Bragg peak and secondary neutron production by the Monte Carlo method in the slab head phantom

Interest in proton therapy has increased in the last decade, as protons are effective to treat deeply located tumors, cause less damage to healthy tissue and allow controlling the energy to be transferred in a target-oriented manner (or energy transfer within target limits). It is known that secondary particles such as neutrons are produced by a result of nuclear interactions of protons with the target. Secondary neutrons can cause an uncontrolled dose increase in healthy tissue near the target site, and because they have a high radiobiological effectiveness, they raise the risk of secondary cancer. There are not enough studies examining the effect of biomaterials on secondary neutron production (SNP) in proton therapy. This study aims to investigate the effect of biomaterials used as implants instead of cranium in the skull on proton depth dose distribution and SNP with Monte Carlo-based PHITS code. Therefore, Bragg peaks and SNPs for 40-140 MeV energy protons were calculated and compared with the literature in a slab head phantom containing stainless steel, CoCrMo (CCM) alloy, alumina, polytetrafluoroethylene, Ti alloy, and NiTi alloy biomaterials used in cranioplasty. It was observed that the most compatible biomaterial compared to cranium for all energies is polytetrafluoroethylene. When polytetrafluoroethylene biomaterial was placed instead of the cranium in the skull, the Bragg peak position of the 100 MeV protons was decreased by 5.04% compared to that in the cranium. In this case, the energy absorbed in the polytetrafluoroethylene biomaterial increased by approximately 28% compared to the cranium, while it decreased by approx. 4% in the brain tissue. It was also observed that while SNP was 0.0501 in the cranium, it increased by almost 18% in PTFE.

Innovations and the Use of Collimators in the Delivery of Pencil Beam Scanning Proton Therapy

Purpose

The development of collimating technologies has become a recent focus in pencil beam scanning (PBS) proton therapy to improve the target conformity and healthy tissue sparing through field-specific or energy-layer–specific collimation. Given the growing popularity of collimators for low-energy treatments, the purpose of this work was to summarize the recent literature that has focused on the efficacy of collimators for PBS and highlight the development of clinical and preclinical collimators.

Materials and Methods

The collimators presented in this work were organized into 3 categories: per-field apertures, multileaf collimators (MLCs), and sliding-bar collimators. For each case, the system design and planning methodologies are summarized and intercompared from their existing literature. Energy-specific collimation is still a new paradigm in PBS and the 2 specific collimators tailored toward PBS are presented including the dynamic collimation system (DCS) and the Mevion Adaptive Aperture.

Results

Collimation during PBS can improve the target conformity and associated healthy tissue and critical structure avoidance. Between energy-specific collimators and static apertures, static apertures have the poorest dose conformity owing to collimating only the largest projection of a target in the beam's eye view but still provide an improvement over uncollimated treatments. While an external collimator increases secondary neutron production, the benefit of collimating the primary beam appears to outweigh the risk. The greatest benefit has been observed for low- energy treatment sites.

Conclusion

The consensus from current literature supports the use of external collimators in PBS under certain conditions, namely low-energy treatments or where the nominal spot size is large. While many recent studies paint a supportive picture, it is also important to understand the limitations of collimation in PBS that are specific to each collimator type. The emergence and paradigm of energy-specific collimation holds many promises for PBS proton therapy.

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

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.

A comprehensive Monte Carlo study of out-of-field secondary neutron spectra in a scanned-beam proton therapy gantry room

PURPOSE

To simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy.

METHODS

The FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm² fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified.

RESULTS

FLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10^-10 to 10³MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations.

CONCLUSIONS

The results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.

Trimmer sequencing time minimization during dynamically collimated proton therapy using a colony of cooperating agents

The dynamic collimation system (DCS) can be combined with pencil beam scanning proton therapy to deliver highly conformal treatment plans with unique collimation at each energy layer. This energy layer-specific collimation is accomplished through the synchronized motion of four trimmer blades that intercept the proton beam near the target boundary in the beam's eye view. However, the corresponding treatment deliveries come at the cost of additional treatment time since the translational speed of the trimmer is slower than the scanning speed of the proton pencil beam. In an attempt to minimize the additional trimmer sequencing time of each field while still maintaining a high degree of conformity, a novel process utilizing ant colony optimization (ACO) methods was created to determine the most efficient route of trimmer sequencing and beamlet scanning patterns for a collective set of collimated proton beamlets. The ACO process was integrated within an in-house treatment planning system optimizer to determine the beam scanning and DCS trimmer sequencing patterns and compared against an analytical approximation of the trimmer sequencing time should a contour-like scanning approach be assumed instead. Due to the stochastic nature of ACO, parameters where determined so that they could ensure good convergence and an efficient optimization of trimmer sequencing that was faster than an analytical contour-like trimmer sequencing. The optimization process was tested using a set of three intracranial treatment plans which were planned using a custom research treatment planning system and were successfully optimized to reduce the additional trimmer sequencing time to approximately 60 s per treatment field while maintaining a high degree of target conformity. Thus, the novel use of ACO techniques within a treatment planning algorithm has been demonstrated to effectively determine collimation sequencing patterns for a DCS in order to minimize the additional treatment time required for trimmer movement during treatment.

OPTIMIZATION OF AN ADDITIONAL COLLIMATOR IN A BEAM DELIVERY SYSTEM FOR REDUCTION OF THE SECONDARY NEUTRON EXPOSURE IN PASSIVE CARBON-ION THERAPY

The aim of this work is to optimize an additional collimator in a beam delivery system to reduce neutron exposure to patients in passive carbon-ion therapy. All studies were performed by Monte Carlo simulation assuming the beam delivery system at Heavy-Ion Medical Accelerator in Chiba. We calculated the neutron ambient dose equivalent at patient positions with an additional collimator, and optimized the position, aperture size and material of the collimator to reduce the neutron ambient dose equivalent. The collimator located 125 and 470 cm upstream from the isocenter could reduce the dose equivalent near the isocenter by 35%, while the collimator located 813 cm upstream from the isocenter was ineffective. As for the material of the collimator, iron and nickel could conduct reduction slightly better than aluminum and polymethyl methacrylate. The additional collimator is an effective method for the reduction of the neutron ambient dose equivalent near the isocenter.

STUDY FOR A PASSIVE SCATTERING LINE DEDICATED TO RADIOBIOLOGY EXPERIMENTS AT THE TRENTO PROTON THERAPY CENTER

The recent worldwide spread of Proton Therapy centers paves the way to new opportunities for basic and applied research related to the use of accelerated proton beams. Clinical centers make use of proton beam energies up to about 230 MeV. This represents an interesting energy range for a large spectrum of applications, including detector testing, radiation shielding and space research. Additionally, radiobiology research might benefit for a larger availability of proton beams, especially in those centers where a room dedicated to research activities also exists. Here, we describe the initial activities for the setup of a radiobiology irradiation facility at the Trento Proton Therapy Center. Data referring to the characterization of the beam in air are essential to that purpose and will be presented. A basic setup for large field irradiation will be also proposed, which is needed for the majority of in vitro and in vivo radiobiology experiments.

Proton versus photon-based radiation therapy for prostate cancer: emerging evidence and considerations in the era of value-based cancer care

BACKGROUND

Advances in radiation technology have transformed treatment options for patients with localized prostate cancer. The evolution of three-dimensional conformal radiation therapy and intensity-modulated radiation therapy (IMRT) have allowed physicians to spare surrounding normal organs and reduce adverse effects. The introduction of proton beam technology and its physical advantage of depositing its energy in tissue at the end-of-range maximum may potentially spare critical organs such as the bladder and rectum in prostate cancer patients. Data thus far are limited to large, observational studies that have not yet demonstrated a definite benefit of protons over conventional treatment with IMRT. The cost of proton beam treatment adds to the controversy within the field.

METHODS

We performed an extensive literature review for all proton treatment-related prostate cancer studies. We discuss the history of proton beam technology, as well as its role in the treatment of prostate cancer, associated controversies, novel technology trends, a discussion of cost-effectiveness, and an overview of the ongoing modern large prospective studies that aim to resolve the debate between protons and photons for prostate cancer.

RESULTS

Present data have demonstrated that proton beam therapy is safe and effective compared with the standard treatment options for prostate cancer. While dosimetric studies suggest lower whole-body radiation dose and a theoretically higher relative biological effectiveness in prostate cancer compared with photons, no studies have demonstrated a clear benefit with protons.

CONCLUSIONS

Evolving trends in proton treatment delivery and proton center business models are helping to reduce costs. Introduction of existing technology into proton delivery allows further control of organ motion and addressing organs-at-risk. Finally, the much-awaited contemporary studies comparing photon with proton-based treatments, with primary endpoints of patient-reported quality-of-life, will help us understand the differences between proton and photon-based treatments for prostate cancer in the modern era.

Exploring the feasibility of a clinical proton beam with an adaptive aperture for pre-clinical research

OBJECTIVE:

To investigate whether the Mevion S250i with HYPERSCAN clinical proton system could be used for pre-clinical research with millimetric beams.

METHODS:

The nozzle of the proton beam line, consisting of an energy modulation system (EMS) and an adaptive aperture (AA), was modelled with the TOPAS Monte Carlo Simulation Toolkit. With the EMS, the 230 MeV beam nominal range can be decreased in multiples of 2.1 mm. Monte Carlo dose calculations were performed in a mouse lung tumour CT image. The AA allows fields as small as 5 × 1 mm² to be used for irradiation. The best plans to give 2 Gy to the tumour were derived from a set of discrete energies allowed by the EMS, different field sizes and beam directions. The final proton plans were compared to a precision photon irradiation plan. Treatment times were also assessed.

RESULTS:

Seven different proton beam plans were investigated, with a good coverage to the tumour (D95 > 1.95 Gy, D5 < 2.3 Gy) and with potentially less damage to the organs at risk than the photon plan. For very small fields and low energies, the number of protons arriving to the target drops to 1-3%, nevertheless the treatment times would be below 5 s.

CONCLUSION:

The proton plans made in this study, collimated by an AA, could be used for animal irradiation.

ADVANCES IN KNOWLEDGE:

This is one of the first study to demonstrate the feasibility of pre-clinical research with a clinical proton beam with an adaptive aperture used to create small fields.

Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation

PURPOSE

Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments.

METHODS AND MATERIALS

Secondary neutron dose distributions were calculated for both a dynamically collimated and an uncollimated, dual-field chordoma treatment plan and compared with previously published neutron dose rates from other contemporary scanning treatment modalities. Monte Carlo N-Particle transport code was used to track all primary and secondary particles generated from nuclear reactions within the DCS during treatment through a model of the patient geometry acquired from the computed tomography planning data set. Secondary neutron ambient dose equivalent distributions were calculated throughout the patient using a meshgrid with a tally resolution equivalent to that of the treatment planning computed tomography.

RESULTS

The median healthy-brain neutron ambient dose equivalent for a dynamically collimated intracranial chordoma treatment plan using a DCS was found to be 0.97 mSv/Gy for the right lateral SFUD field, 1.37 mSv/Gy for the apex SFUD field, and 1.24 mSv/Gy for the composite intensity modulated proton therapy distribution from 2 fields.

CONCLUSIONS

These results were at least 55% lower than what has been reported for uniform scanning modalities with brass apertures. However, they still reflect an increase in the excess relative risk of secondary cancer incidence compared with an uncollimated PBS treatment using only a graphite range shifter. Regardless, the secondary neutron dose expected from the DCS for these PBS proton therapy treatments appears to be on the order of, or below, what is expected for alternative collimated proton therapy techniques.

Independent application of an analytical model for secondary neutron equivalent dose produced in a passive-scattering proton therapy treatment unit

The purpose of this study was to independently apply an analytical model for equivalent dose from neutrons produced in a passive-scattering proton therapy treatment unit, H. To accomplish this objective, we applied the previously-published model to treatment plans of two pediatric patients. Their model accounted for neutrons generated by mono-energetic proton beams stopping in a closed aperture. To implement their model to a clinical setting, we adjusted it to account for the area of a collimating aperture, energy modulation, air gap between the treatment unit and patient, and radiation weighting factor. We used the adjusted model to estimate H per prescribed proton absorbed dose, D _Rx , for the passive-scattering proton therapy beams of two children, a 9-year-old girl and 10-year-old boy, who each received intracranial boost fields as part of their treatment. In organs and tissues at risk for radiation-induced subsequent malignant neoplasms, T, we calculated the mass-averaged H, H _T , per D _Rx . Finally, we compared H _T /D _Rx values to those of previously-published Monte Carlo (MC) simulations of these patients' fields. H _T /D _Rx values of the adjusted model deviated from the MC result for each organ on average by 20.8  ±  10.0% and 44.2  ±  17.6% for the girl and boy, respectively. The adjusted model underestimated the MC result in all T of each patient, with the exception of the girl's bladder, for which the adjusted model overestimated H _T /D _Rx by 3.1%. The adjusted model provided a better estimate of H _T /D _Rx than the unadjusted model. That is, between the two models, the adjusted model reduced the deviation from the MC result by approximately 37.0% and 46.7% for the girl and boy, respectively. We found that the previously-published analytical model, combined with adjustment factors to enhance its clinical applicability, predicted H _T /D _Rx in out-of-field organs and tissues at risk for subsequent malignant neoplasms with acceptable accuracy. This independent application demonstrated that the analytical model may be useful broadly for clinicians and researchers to calculate equivalent dose from neutrons produced externally to the patient in passive-scattering proton therapy.

Influence of 68Ga-DOTATOC on sparing of normal tissue for radiation therapy of skull base meningioma: differential impact of photon and proton radiotherapy

Background

To evaluate the impact of ⁶⁸Ga-DOTATOC-PET on treatment planning and sparing of normal tissue in the treatment of skull base meningioma with advanced photons and protons.

Methods

From the institutional database consisting of 507 skull base meningiomas 10 patients were chosen randomly for the present analysis. Target volume definition was performed based on CT and MRI only, as well as with additional ⁶⁸Ga-DOTATOC-PET. Treatment plans were performed for Intensity Modulated Radiotherapy (IMRT) and proton therapy using active raster scanning on both target volumes. We calculated doses to relevant organs at risk (OAR), conformity indices as well as differences in normal tissue sparing between both radiation modalities based on CT/MRI planning as well as CT/MRI/PET planning.

Results

For photon treatment plans, PET-based treatment plans showed a reduction of brain stem D_max and D_median for different levels of total dose. At the optic chiasm, use of ⁶⁸Ga-DOTATOC significantly reduces D_max; moreover, the D_median is reduced in most cases, too. For both right and left optic nerve, reduction of dose by addition of ⁶⁸Ga-DOTATOC-PET is minimal and depends on the anatomical location of the meningioma. In protons, the impact of ⁶⁸Ga-DOTATOC-PET is minimal compared to photons.

Conclusion

Addition of ⁶⁸Ga-DOTATOC-PET information into treatment planning for skull base meningiomas has a significant impact on target volumes. In most cases, PET-planning leads to significant reductions of the treatment volumes. Subsequently, reduced doses are applied to OAR. Using protons, the benefit of additional PET is smaller since target coverage is more conformal and dose to OAR is already reduced compared to photons. Therefore, PET-imaging has the greatest margin of benefit in advanced photon techniques, and combination of PET-planning and high-precision treatment leads to comparable treatment plans as with protons.

GEANT4 models for the secondary radiation flux in the collimation system of a 300MeV proton microbeam

In Harbin, we are developing a 300MeV proton microbeam for many applications in space science including upset studies in microelectronic devices, radiation hardness of materials for satellites and radiation effects in human tissues. There are also applications of this facility proposed for proton therapy. The microbeam system will employ a purpose-built proton synchrotron to provide the beam. However there are many challenges to be addressed in the design, construction and operation of this facility. Here we address two important design aspects for which we apply GEANT4 modeling. First, the high energy proton beam interacts strongly with beam line materials, especially the collimation slits, to produce showers of secondary particles which could introduce significant background signals and degrade the resolution of the proton microbeam. Second, the beam transport within the residual vacuum of the beam line may also introduce undesirable background radiation. In both cases mitigation strategies need to be incorporated during the design phase of the new system. We study the use of a dipole magnet following the aperture collimator to reduce the flux of secondary particles incident on the analysis chamber. Monte Carlo simulations are performed using GEANT4 and SRIM. By inserting the dipole magnet, we find as expected a significant reduction in the scattering of protons and other particles, such as neutrons and gamma rays, at the collimation system exit position. Secondary radiation from the residual gas pressure within the beam line vacuum system are also modelled and found to be negligible under the standard operating conditions.

Toward improved target conformity for two spot scanning proton therapy delivery systems using dynamic collimation

PURPOSE

To quantify improvement in target conformity in brain and head and neck tumor treatments resulting from the use of a dynamic collimation system (DCS) with two spot scanning proton therapy delivery systems (universal nozzle, UN, and dedicated nozzle, DN) with median spot sizes of 5.2 and 3.2 mm over a range of energies from 100 to 230 MeV.

METHODS

Uncollimated and collimated plans were calculated with both UN and DN beam models implemented within our in-house treatment planning system for five brain and ten head and neck datasets in patients previously treated with spot scanning proton therapy. The prescription dose and beam angles from the clinical plans were used for both the UN and DN plans. The average reduction of the mean dose to the 10-mm ring surrounding the target between the uncollimated and collimated plans was calculated for the UN and the DN. Target conformity was analyzed using the mean dose to 1-mm thickness rings surrounding the target at increasing distances ranging from 1 to 10 mm.

RESULTS

The average reductions of the 10-mm ring mean dose for the UN and DN plans were 13.7% (95% CI: 11.6%-15.7%; p < 0.0001) and 11.5% (95% CI: 9.5%-13.5%; p < 0.0001) across all brain cases and 7.1% (95% CI: 4.4%-9.8%; p < 0.001) and 6.3% (95% CI: 3.7%-9.0%; p < 0.001), respectively, across all head and neck cases. The collimated UN plans were either more conformal (all brain cases and 60% of the head and neck cases) than or equivalent (40% of the head and neck cases) to the uncollimated DN plans. The collimated DN plans offered the highest conformity.

CONCLUSIONS

The DCS added either to the UN or DN improved the target conformity. The DCS may be of particular interest for sites with UN systems looking for a more economical solution than upgrading the nozzle to improve the target conformity of their spot scanning proton therapy system.

Nuclear physics in particle therapy: a review

Charged particle therapy has been largely driven and influenced by nuclear physics. The increase in energy deposition density along the ion path in the body allows reducing the dose to normal tissues during radiotherapy compared to photons. Clinical results of particle therapy support the physical rationale for this treatment, but the method remains controversial because of the high cost and of the lack of comparative clinical trials proving the benefit compared to x-rays. Research in applied nuclear physics, including nuclear interactions, dosimetry, image guidance, range verification, novel accelerators and beam delivery technologies, can significantly improve the clinical outcome in particle therapy. Measurements of fragmentation cross-sections, including those for the production of positron-emitting fragments, and attenuation curves are needed for tuning Monte Carlo codes, whose use in clinical environments is rapidly increasing thanks to fast calculation methods. Existing cross sections and codes are indeed not very accurate in the energy and target regions of interest for particle therapy. These measurements are especially urgent for new ions to be used in therapy, such as helium. Furthermore, nuclear physics hardware developments are frequently finding applications in ion therapy due to similar requirements concerning sensors and real-time data processing. In this review we will briefly describe the physics bases, and concentrate on the open issues.

A dynamic collimation system for penumbra reduction in spot-scanning proton therapy: proof of concept

PURPOSE

In the absence of a collimation system the lateral penumbra of spot scanning (SS) dose distributions delivered by low energy proton beams is highly dependent on the spot size. For current commercial equipment, spot size increases with decreasing proton energy thereby reducing the benefit of the SS technique. This paper presents a dynamic collimation system (DCS) for sharpening the lateral penumbra of proton therapy dose distributions delivered by SS.

METHODS

The collimation system presented here exploits the property that a proton pencil beam used for SS requires collimation only when it is near the target edge, enabling the use of trimmers that are in motion at times when the pencil beam is away from the target edge. The device consists of two pairs of parallel nickel trimmer blades of 2 cm thickness and dimensions of 2 cm×18 cm in the beam's eye view. The two pairs of trimmer blades are rotated 90° relative to each other to form a rectangular shape. Each trimmer blade is capable of rapid motion in the direction perpendicular to the central beam axis by means of a linear motor, with maximum velocity and acceleration of 2.5 m/s and 19.6 m/s2, respectively. The blades travel on curved tracks to match the divergence of the proton source. An algorithm for selecting blade positions is developed to minimize the dose delivered outside of the target, and treatment plans are created both with and without the DCS.

RESULTS

The snout of the DCS has outer dimensions of 22.6×22.6 cm2 and is capable of delivering a minimum treatment field size of 15×15 cm2. Using currently available components, the constructed system would weigh less than 20 kg. For irregularly shaped fields, the use of the DCS reduces the mean dose outside of a 2D target of 46.6 cm2 by approximately 40% as compared to an identical plan without collimation. The use of the DCS increased treatment time by 1-3 s per energy layer.

CONCLUSIONS

The spread of the lateral penumbra of low-energy SS proton treatments may be greatly reduced with the use of this system at the cost of only a small penalty in delivery time.

Risk-optimized proton therapy to minimize radiogenic second cancers

Proton therapy confers substantially lower predicted risk of second cancer compared with photon therapy. However, no previous studies have used an algorithmic approach to optimize beam angle or fluence-modulation for proton therapy to minimize those risks. The objectives of this study were to demonstrate the feasibility of risk-optimized proton therapy and to determine the combination of beam angles and fluence weights that minimizes the risk of second cancer in the bladder and rectum for a prostate cancer patient. We used 6 risk models to predict excess relative risk of second cancer. Treatment planning utilized a combination of a commercial treatment planning system and an in-house risk-optimization algorithm. When normal-tissue dose constraints were incorporated in treatment planning, the risk model that incorporated the effects of fractionation, initiation, inactivation, repopulation and promotion selected a combination of anterior and lateral beams, which lowered the relative risk by 21% for the bladder and 30% for the rectum compared to the lateral-opposed beam arrangement. Other results were found for other risk models.

The physics of proton therapy

The physics of proton therapy has advanced considerably since it was proposed in 1946. Today analytical equations and numerical simulation methods are available to predict and characterize many aspects of proton therapy. This article reviews the basic aspects of the physics of proton therapy, including proton interaction mechanisms, proton transport calculations, the determination of dose from therapeutic and stray radiations, and shielding design. The article discusses underlying processes as well as selected practical experimental and theoretical methods. We conclude by briefly speculating on possible future areas of research of relevance to the physics of proton therapy.

ICRP Publication 127: Radiological Protection in Ion Beam Radiotherapy

The goal of external-beam radiotherapy is to provide precise dose localisation in the treatment volume of the target with minimal damage to the surrounding normal tissue. Ion beams, such as protons and carbon ions, provide excellent dose distributions due primarily to their finite range, allowing a significant reduction of undesired exposure of normal tissue. Careful treatment planning is required for the given type and localisation of the tumour to be treated in order to maximise treatment efficiency and minimise the dose to normal tissue. Radiation exposure in out-of-field volumes arises from secondary neutrons and photons, particle fragments, and photons from activated materials. These unavoidable doses should be considered from the standpoint of radiological protection of the patient. Radiological protection of medical staff at ion beam radiotherapy facilities requires special attention. Appropriate management and control are required for the therapeutic equipment and the air in the treatment room that can be activated by the particle beam and its secondaries. Radiological protection and safety management should always conform with regulatory requirements. The current regulations for occupational exposures in photon radiotherapy are applicable to ion beam radiotherapy with protons or carbon ions. However, ion beam radiotherapy requires a more complex treatment system than conventional radiotherapy, and appropriate training of staff and suitable quality assurance programmes are recommended to avoid possible accidental exposure of patients, to minimise unnecessary doses to normal tissue, and to minimise radiation exposure of staff.

Proton beam radiation therapy for prostate cancer-is the hype (and the cost) justified?

Although in use for over 40 years, proton beam therapy for prostate cancer has only recently come under public scrutiny, due to its increased cost compared to other forms of treatment. While the last decade has seen a rapid accumulation of evidence to suggest that proton beam therapy is both safe and effective in this disease site, a rigorous comparison to other radiotherapy techniques has not yet been completed. In this review, we provide an in-depth look at the evidence both supporting and questioning proton beam therapy's future role in the treatment of prostate cancer, with emphasis on its history, physical properties, comparative clinical and cost effectiveness, advances in its delivery and future promise.

Secondary neutron dose measurement for proton eye treatment using an eye snout with a borated neutron absorber

Background

We measured and assessed ways to reduce the secondary neutron dose from a system for proton eye treatment.

Methods

Proton beams of 60.30 MeV were delivered through an eye-treatment snout in passive scattering mode. Allyl diglycol carbonate (CR-39) etch detectors were used to measure the neutron dose in the external field at 0.00, 1.64, and 6.00 cm depths in a water phantom. Secondary neutron doses were measured and compared between those with and without a high-hydrogen–boron-containing block. In addition, the neutron energy and vertices distribution were obtained by using a Geant4 Monte Carlo simulation.

Results

The ratio of the maximum neutron dose equivalent to the proton absorbed dose (H(10)/D) at 2.00 cm from the beam field edge was 8.79 ± 1.28 mSv/Gy. The ratio of the neutron dose equivalent to the proton absorbed dose with and without a high hydrogen-boron containing block was 0.63 ± 0.06 to 1.15 ± 0.13 mSv/Gy at 2.00 cm from the edge of the field at depths of 0.00, 1.64, and 6.00 cm.

Conclusions

We found that the out-of-field secondary neutron dose in proton eye treatment with an eye snout is relatively small, and it can be further reduced by installing a borated neutron absorbing material.

Proton beam and prostate cancer: An evolving debate

This paper evaluates the reasons behind the rise in the use of proton beam for prostate cancer, the economics drivers behind it, and the evidence that exists to support it. It concludes that clinical outcome data underlying the notion that this is a superior treatment remains sparse and discusses what is needed to fill in the gaps.

Proton beam therapy and localised prostate cancer: current status and controversies

Proton therapy is a promising, but costly, treatment for prostate cancer. Theoretical physical advantages exist; yet to date, it has been shown only to be comparably safe and effective when compared with the alternatives and not necessarily superior. If clinically meaningful benefits do exist for patients, more rigorous study will be needed to detect them and society will require this to justify the investment of time and money. New technical advances in proton beam delivery coupled with shortened overall treatment times and declining device costs have the potential to make this a more cost-effective therapy in the years ahead.

Results of a multicentric in silico clinical trial (ROCOCO): comparing radiotherapy with photons and protons for non-small cell lung cancer

INTRODUCTION

This multicentric in silico trial compares photon and proton radiotherapy for non-small cell lung cancer patients. The hypothesis is that proton radiotherapy decreases the dose and the volume of irradiated normal tissues even when escalating to the maximum tolerable dose of one or more of the organs at risk (OAR).

METHODS

Twenty-five patients, stage IA-IIIB, were prospectively included. On 4D F18-labeled fluorodeoxyglucose-positron emission tomography-computed tomography scans, the gross tumor, clinical and planning target volumes, and OAR were delineated. Three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT) photon and passive scattered conformal proton therapy (PSPT) plans were created to give 70 Gy to the tumor in 35 fractions. Dose (de-)escalation was performed by rescaling to the maximum tolerable dose.

RESULTS

Protons resulted in the lowest dose to the OAR, while keeping the dose to the target at 70 Gy. The integral dose (ID) was higher for 3DCRT (59%) and IMRT (43%) than for PSPT. The mean lung dose reduced from 18.9 Gy for 3DCRT and 16.4 Gy for IMRT to 13.5 Gy for PSPT. For 10 patients, escalation to 87 Gy was possible for all 3 modalities. The mean lung dose and ID were 40 and 65% higher for photons than for protons, respectively.

CONCLUSIONS

The treatment planning results of the Radiation Oncology Collaborative Comparison trial show a reduction of ID and the dose to the OAR when treating with protons instead of photons, even with dose escalation. This shows that PSPT is able to give a high tumor dose, while keeping the OAR dose lower than with the photon modalities.

Pitfalls of tungsten multileaf collimator in proton beam therapy

PURPOSE

Particle beam therapy is associated with significant startup and operational cost. Multileaf collimator (MLC) provides an attractive option to improve the efficiency and reduce the treatment cost. A direct transfer of the MLC technology from external beam radiation therapy is intuitively straightforward to proton therapy. However, activation, neutron production, and the associated secondary cancer risk in proton beam should be an important consideration which is evaluated.

METHODS

Monte Carlo simulation with FLUKA particle transport code was applied in this study for a number of treatment models. The authors have performed a detailed study of the neutron generation, ambient dose equivalent [H∗(10)], and activation of a typical tungsten MLC and compared with those obtained from a brass aperture used in a typical proton therapy system. Brass aperture and tungsten MLC were modeled by absorber blocks in this study, representing worst-case scenario of a fully closed collimator.

RESULTS

With a tungsten MLC, the secondary neutron dose to the patient is at least 1.5 times higher than that from a brass aperture. The H∗(10) from a tungsten MLC at 10 cm downstream is about 22.3 mSv/Gy delivered to water phantom by noncollimated 200 MeV beam of 20 cm diameter compared to 14 mSv/Gy for the brass aperture. For a 30-fraction treatment course, the activity per unit volume in brass aperture reaches 5.3 × 10⁴ Bq cm(-3) at the end of the last treatment. The activity in brass decreases by a factor of 380 after 24 h, additional 6.2 times after 40 days of cooling, and is reduced to background level after 1 yr. Initial activity in tungsten after 30 days of treating 30 patients per day is about 3.4 times higher than in brass that decreases only by a factor of 2 after 40 days and accumulates to 1.2 × 10⁶ Bq cm(-3) after a full year of operation. The daily utilization of the MLC leads to buildup of activity with time. The overall activity continues to increase due to (179)Ta with a half-life of 1.82 yr and thus require prolonged storage for activity cooling. The H∗(10) near the patient side of the tungsten block is about 100 μSv/h and is 27 times higher at the upstream side of the block. This would lead to an accumulated dose for therapists in a year that may exceed occupational maximum permissible dose (50 mSv/yr). The value of H∗(10) at the upstream surface of the tungsten block is about 220 times higher than that of the brass.

CONCLUSIONS

MLC is an efficient way for beam shaping and overall cost reduction device in proton therapy. However, based on this study, tungsten seems to be not an optimal material for MLC in proton beam therapy. Usage of tungsten MLC in clinic may create unnecessary risks associated with the secondary neutrons and induced radioactivity for patients and staff depending on the patient load. A careful selection of material for manufacturing of an optimal MLC for proton therapy is thus desired.

Risk of second malignant neoplasm following proton versus intensity-modulated photon radiotherapies for hepatocellular carcinoma

Hepatocellular carcinoma (HCC), the sixth most common cancer in the world, is a global health concern. Radiotherapy for HCC is uncommon, largely because of the likelihood of radiation-induced liver disease, an acute side effect that is often fatal. Proton beam therapy (PBT) and intensity-modulated radiation therapy (IMRT) may offer HCC patients a better option for treating the diseased liver tissue while largely sparing the surrounding tissues, especially the non-tumor liver. However, even advanced radiotherapies carry a risk of late effects, including second malignant neoplasms (SMNs). It is unclear whether PBT or IMRT confers less risk of an SMN than the other. The purpose of this study was to compare the predicted risk of developing an SMN for a patient with HCC between PBT and IMRT. For both treatments, radiation doses in organs and tissues from primary radiation were determined using a treatment planning system; doses in organs and tissues from stray radiation from PBT were determined using Monte Carlo simulations and from IMRT using thermo-luminescent dosimeter measurements. Risk models of SMN incidence were taken from the literature. The predicted absolute lifetime attributable risks of SMN incidence were 11.4% after PBT and 19.2% after IMRT. The results of this study suggest that using proton beams instead of photon beams for radiotherapy may reduce the risk of SMN incidence for some HCC patients.

Predicted risks of second malignant neoplasm incidence and mortality due to secondary neutrons in a girl and boy receiving proton craniospinal irradiation

The purpose of this study was to compare the predicted risks of second malignant neoplasm (SMN) incidence and mortality from secondary neutrons for a 9-year-old girl and a 10-year-old boy who received proton craniospinal irradiation (CSI). SMN incidence and mortality from neutrons were predicted from equivalent doses to radiosensitive organs for cranial, spinal and intracranial boost fields. Therapeutic proton absorbed dose and equivalent dose from neutrons were calculated using Monte Carlo simulations. Risks of SMN incidence and mortality in most organs and tissues were predicted by applying risks models from the National Research Council of the National Academies to the equivalent dose from neutrons; for non-melanoma skin cancer, risk models from the International Commission on Radiological Protection were applied. The lifetime absolute risks of SMN incidence due to neutrons were 14.8% and 8.5%, for the girl and boy, respectively. The risks of a fatal SMN were 5.3% and 3.4% for the girl and boy, respectively. The girl had a greater risk for any SMN except colon and liver cancers, indicating that the girl's higher risks were not attributable solely to greater susceptibility to breast cancer. Lung cancer predominated the risk of SMN mortality for both patients. This study suggests that the risks of SMN incidence and mortality from neutrons may be greater for girls than for boys treated with proton CSI.