Off-axis dose equivalent due to secondary neutrons from uniform scanning proton beams during proton radiotherapy

DOSE CALCULATION OF PROTON THERAPY BASED ON MONTE CARLO AND EMPIRICAL FORMULA

In this work, we used the Monte Carlo-based TOPAS simulation software to calculate the ambient dose equivalents and annual effective dose due to the secondary neutron field produced in proton therapy, also we introduced a USTC phantom to access the organ equivalent dose. The ambient dose equivalent and annual effective dose were calculated in several positions of interest inside and outside the facility. The simulation results were compared qualitatively to the results of the Empirical Formula, showing that the Empirical Formula calculations overestimated the dose, 28.95 times higher than the MC simulations, on average, which would lead to over shielding. In addition, the highest equivalent dose rate of a single radiation-sensitive organ simulated by TOPAS was 1.50 × 10-9 mSv/a for the eye lens, 2.36 × 10-3 mSv/a for limbs and 1.01 × 10-3 mSv/a for skin, which also meets the limits. Therefore, MC simulation has great advantages in shielding design and safety evaluation. And this work presents a new method to calculate the dose, introducing a more anthropogenic phantom can get more realistic results.

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.

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.

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.

Conception of a New Recoil Proton Telescope for Real-Time Neutron Spectrometry in Proton-Therapy

Neutrons are the main type of secondary particles emitted in proton-therapy. Because of the risk of secondary cancer and other late occurring effects, the neutron dose should be included in the out-of-field dose calculations. A neutron spectrometer has to be used to take into account the energy dependence of the neutron radiological weighting factor. Due to its high dependence on various parameters of the irradiation (beam, accelerator, patient), the neutron spectrum should be measured independently for each treatment.

The current reference method for the measurement of the neutron energy, the Bonner Sphere System, consists of several homogeneous polyethylene spheres with increasing diameters equipped with a proportional counter. It provides a highresolution reconstruction of the neutron spectrum but requires a time-consuming work of signal deconvolution. New neutron spectrometers are being developed, but the main experimental limitation remains the high neutron flux in proton therapy treatment rooms. A new model of a real-time neutron spectrometer, based on a Recoil Proton Telescope technology, has been developed at the IPHC. It enables a real-time high-rate reconstruction of the neutron spectrum from the measurement of the recoil proton trajectory and energy. A new fast-readout microelectronic integrated sensor, called FastPixN, has been developed for this specific purpose.

A first prototype, able to detect neutrons between 5 and 20 MeV, has already been validated for metrology with the AMANDE facility at Cadarache. The geometry of the new Recoil Proton Telescope has been optimized via extensive Geant4 Monte Carlo simulations. Uncertainty sources have been carefully studied in order to improve simultaneously efficiency and energy resolution, and solutions have been found to suppress the various expected backgrounds. We are currently upgrading the prototype for secondary neutron detection in proton therapy applications.

Spot scanning proton therapy minimizes neutron dose in the setting of radiation therapy administered during pregnancy

This is a real case study to minimize the neutron dose equivalent (H) to a fetus using spot scanning proton beams with favorable beam energies and angles. Minimum neutron dose exposure to the fetus was achieved with iterative planning under the guidance of neutron H measurement. Two highly conformal treatment plans, each with three spot scanning beams, were planned to treat a 25‐year‐old pregnant female with aggressive recurrent chordoma of the base of skull who elected not to proceed with termination. Each plan was scheduled for delivery every other day for robust target coverage. Neutron H to the fetus was measured using a REM500 neutron survey meter placed at the fetus position of a patient simulating phantom. 4.1 and 44.1 44.1 μSv/fraction were measured for the two initial plans. A vertex beam with higher energy and the fetal position closer to its central axis was the cause for the plan that produced an order higher neutron H. Replacing the vertex beam with a lateral beam reduced neutron H to be comparable with the other plan. For a prescription of 70 Gy in 35 fractions, the total neutron H to the fetus was estimated to be 0.35 mSv based on final measurement in single fraction. In comparison, the passive scattering proton plan and photon plan had an estimation of 26 and 70 mSv, respectively, for this case. While radiation therapy in pregnant patients should be avoided if at all possible, our work demonstrated spot scanning beam limited the total neutron H to the fetus an order lower than the suggested 5 mSv regulation threshold. It is far superior than passive scattering beam and careful beam selection with lower energy and keeping fetus further away from beam axis are essential in minimizing the fetus neutron exposure.

PACS number(s): 87.53.Bn, 87.55.D‐, 87.55.N‐