Fast Neutron Therapy Facility Research Articles

Commissioning of intensity modulated neutron radiotherapy (IMNRT)

Intensity modulated neutron radiotherapy (IMNRT) has been developed using inhouse treatment planning and delivery systems at the Karmanos Cancer Center∕Wayne State University Fast Neutron Therapy facility. The process of commissioning IMNRT for clinical use is presented here. Results of commissioning tests are provided including validation measurements using representative patient plans as well as those from the TG-119 test suite. IMNRT plans were created using the Varian Eclipse optimization algorithm and an inhouse planning system for calculation of neutron dose distributions. Tissue equivalent ionization chambers and an ionization chamber array were used for point dose and planar dose distribution comparisons with calculated values. Validation plans were delivered to water and virtual water phantoms using TG-119 measurement points and evaluation techniques. Photon and neutron doses were evaluated both inside and outside the target volume for a typical IMNRT plan to determine effects of intensity modulation on the photon dose component. Monitor unit linearity and effects of beam current and gantry angle on output were investigated, and an independent validation of neutron dosimetry was obtained. While IMNRT plan quality is superior to conventional fast neutron therapy plans for clinical sites such as prostate and head and neck, it is inferior to photon IMRT for most TG-119 planning goals, particularly for complex cases. This results significantly from current limitations on the number of segments. Measured and calculated doses for 11 representative plans (six prostate∕five head and neck) agreed to within -0.8 ± 1.4% and 5.0 ± 6.0% within and outside the target, respectively. Nearly all (22∕24) ion chamber point measurements in the two phantom arrangements were within the respective confidence intervals for the quantity [(measured-planned)∕prescription dose] derived in TG-119. Mean differences for all measurements were 0.5% (max = 7.0%) and 1.4% (max = 4.1%) in water and virtual water, respectively. The mean gamma pass rate for all cases was 92.8% (min = 88.6%). These pass rates are lower than typically achieved with photon IMRT, warranting development of a planar dosimetry system designed specifically for IMNRT and∕or the improvement of neutron beam modeling in the penumbral region. The fractional photon dose component did not change significantly in a typical IMNRT plan versus a conventional fast neutron therapy plan, and IMNRT delivery is not expected to significantly alter the RBE. All other commissioning results were considered satisfactory for clinical implementation of IMNRT, including the external neutron dose validation, which agreed with the predicted neutron dose to within 1%. IMNRT has been successfully commissioned for clinical use. While current plan quality is inferior to photon IMRT, it is superior to conventional fast neutron therapy. Ion chamber validation results for IMNRT commissioning are also comparable to those typically achieved with photon IMRT. Gamma pass rates for planar dose distributions are lower than typically observed for photon IMRT but may be improved with improved planar dosimetry equipment and beam modeling techniques. In the meantime, patient-specific quality assurance measurements should rely more heavily on point dose measurements with tissue equivalent ionization chambers. No significant technical impediments are anticipated in the clinical implementation of IMNRT as described here.

Dosimetry measurements at the fast neutron therapy facility in Seattle

Abstract The fast neutron therapy facility at the University of Washington has been in routine clinical use for 25 years. 50.5 MeV protons produce neutrons in a beryllium target mounted on an isocentric gantry. Beam shaping is accomplished with a 40-leaf collimator. Dosimetry measurements for treatment planning and calibration are performed with tissue equivalent ion chambers. A layered phantom of alternating Solid Water ® and Plastic Water ® slabs has been developed for rapid dose verification measurements. The neutron field in the room has been used for radiation testing of electronic components.

Preliminary investigations of Monte Carlo simulations of neutron energy and let spectra for fast neutron therapy facilities

No fast neutron therapy facility has been built with optimized beam quality based on a thorough understanding of the neutron spectrum and its resulting biological effectiveness. A study has been initiated to provide the information necessary for such an optimization. Monte Carlo studies will be used to simulate neutron energy spectra and LET spectra. These studies will be bench-marked with data taken at existing fast neutron therapy facilities. Results will also be compared with radiobiological studies to further support beam quality optimization. These simulations, anchored by this data, will then be used to determine what parameters might be optimized to take full advantage of the unique LET properties of fast neutron beams. This paper will present preliminary work in generating energy and LET spectra for the Fermilab fast neutron therapy facility.

Analytical Description of the d(14)+Be Neutron Beam Source at the Essen Fast Neutron Therapy Facility

Monte Carlo simulation of accelerated ions is a standard method in radiation protection. Such simulations have been used to calculate photon and neutron production in a beryllium target of the Essen d(14)+Be Fast Neutron Therapy Facility. In the deuteron case the predominant part of the neutrons is produced by breakup of the input particle, a decay that is not foreseen in standard versions of Monte Carlo codes. Thus, the calculation yields results that are different from measured ones. For simulations of the neutron beam at such facilities, an input description containing the spectral and geometric properties of the neutron and eventually photon beams produced in the target is needed. For the Essen neutron beam, such a description has been obtained by comparison of MCNPX simulations with published data and measurements at a static beam geometry having no background radiation. The validation of the neutron beam input description was obtained by comparing measured and calculated dose distributions in a water phantom using a standard collimator at the treatment gantry.

Filter, collimator and moderating material to achieve boron neutron capture enhanced fast neutron therapy

The combination of fast neutron therapy and boron neutron capture therapy is currently being studied as a possible treatment for some radio-resistant brain tumours. In an attempt to design a boron-enhanced fast neutron therapy beam for the Fermilab Fast Neutron Therapy Facility, the use of moderating material surrounding the patient's head has been investigated. Graphite, polyethylene, water and heavy water were studied as moderating materials, using MCNP. The use of tungsten, iron, lead and bismuth as materials for a small filter and collimator near the patient's head was investigated. Calculations showed that a filter and collimator made of tungsten with a graphite moderator was capable of producing a dose enhancement of 17.3 +/- 0.6% for a 100 microg g(-1) loading of 10B for a 5.6 cm diameter beam while delivering 1.5 Gy in 7 min.

Design considerations for a computer controlled multileaf collimator for the Harper Hospital fast neutron therapy facility.

The d(48.5) + Be neutron beam from the Harper Hospital superconducting cyclotron is collimated using a unique multirod collimator (MRC). A computer controlled multileaf collimator (MLC) is being designed to improve efficiency and allow for the future development of intensity modulated radiation therapy with neutrons. For the current study the use of focused or unfocused collimator leaves has been studied. Since the engineering effort associated with the leaf design and materials choice impacts significantly on cost, it was desirable to determine the clinical impact of using unfocused leaves in the MLC design. The MRC is a useful tool for studying the effects of using focused versus unfocused beams on beam penumbra. The effects of the penumbra for the different leaf designs on tumor and normal tissue DVHs in two selected sites (prostate and head and neck) was investigated. The increase in the penumbra resulting from using unfocused beams was small (approximately 1.5 mm for a 5 x 5 cm2 field and approximately 7.6 mm for a 25 x 25 cm2 field at 10 cm depth) compared to the contribution of phantom scatter to the penumbra width (5.4 and 20 mm for the small and large fields at 10 cm depth, respectively). Comparison of DVHs for tumor and critical normal tissue in a prostate and head and neck case showed that the dosimetric disadvantages of using an unfocused rather than focused beam were minimal and only significant at shallow depths. For the rare cases, where optimum penumbra conditions are required, a MLC incorporating tapered leaves and, thus, providing focused collimation in one plane is necessary.

Monte Carlo calculations to characterize the source for neutron therapy facilities.

Modern radiation treatment planning for photons includes full 3D modeling of the adsorbed dose distribution, accurate inclusion of the patient anatomy, and consideration of significant changes in material density and composition. Such efforts are founded in an accurate description of the radiation source and the beam delivery system. Modern fast neutron therapy facilities employ highly penetrating beams and isocentric beam delivery. Treatment planning is largely based on analytic models adapted from photon codes and interaction cross sections normalized to macroscopic attenuation. However, the recent PEREGRINE initiative at Lawrence Livermore Laboratory offers the possibility of fully stochastic modeling if the neutron source can be adequately described. In this article we report neutron source modeling of three high energy facilities. Neutron production is based on the intra-nuclear cascade model of the LAHET code while neutron transport through the beam delivery system is managed by MCNP using cross section libraries extended to 100 MeV neutron energy. PEREGRINE is then used to transport the neutron beam through typical phantoms. The resulting neutron sources are in excellent agreement with the limited experimental information and the measured phantom data are well described by the PEREGRINE transport using the LAHET/MCNP determined neutron sources.

The multileaf collimator for fast neutron therapy at Louvain-la-Neuve

The multileaf collimator of the fast neutron therapy facility at Louvain-la-Neuve is described, as well as some of the physics experiments performed in order to evaluate the attenuation of neutron beams in different materials and thus optimize the composition of the collimator leaves. The multileaf collimator consists of two sets of 22 leaves each, which can be moved independently. They are made of iron and their thickness is 95 cm. Seven borated polyethylene disks are located in the distal part of the leaves in order to absorb more efficiently the low-energy component of the neutron spectrum. The width of the leaves is 1 cm at their distal part. The leaves can move 11 cm outwards and 6 cm inwards from their reference position, and field sizes up to 25.7 x 24.8 cm, as well as irregular field shapes, can be obtained. The inner part of the leaves and their two sides are always focused on the target. The complete multileaf collimator can rotate around the beam axis, from -90 degrees to +90 degrees from the reference position. The width of the penumbra (80-20% isodoses) is 0.64 cm and 1.17 cm at the depth of the maximum buildup and at 10 cm in depth respectively, for a 10 x 10 cm field size. The collimator is adequate for the energy of the p(65)+Be neutron beam of Louvain-la-Neuve and has been adapted to the fixed vertical beam. It has been designed following the original plans of Scanditronix, adjusted and fully assembled at the workshop of the Centre de Recherches du Cyclotron (CRC). Systematic measurements were performed in order to optimize the design and the composition of the leaves. In particular the attenuations of the actual beam and of monoenergetic neutron beams were measured in different materials such as iron and polyethylene. Above (upstream) the multileaf collimator, a fixed pre-collimator (iron thickness 50 cm; section 1 x 1 m) defines a conical aperture aligned on the largest opening of the leaves. It contains the two transmission chambers and a 2 cm thick polyethylene layer used for hardening the beam.

Physical characteristics of a clinical d(48.5)+Be neutron therapy beam produced by a superconducting cyclotron

The Harper Hospital and Wayne State University fast neutron therapy facility is the only one in the world to use a compact superconducting cyclotron and multirod collimator. Neutrons are produced by the interaction of the 48.5-MeV deuteron beam with a thick internal beryllium target and the compact accelerator is gantry mounted to allow full 360 degrees rotation of the neutron beam about the therapy couch. The deuteron beam strikes the beryllium target at a glancing angle. A flattening filter is used to flatten the asymmetric neutron beam which results from this geometry. Details of the flattening filter design and construction are discussed. The physical characteristics of the resulting neutron therapy beam were measured. The central axis depth-dose values are approximately equivalent to those of a 4-MV photon beam. The dose buildup curve reaches its maximum value at a depth of 9 mm in a water phantom and the surface dose is approximately 42%. The beam penumbra produced by the multirod collimator has been measured in terms of the distance between the 20% and 80% isodose lines. The penumbra width for a 10 x 10-cm2 field at a depth of 10 cm in a water phantom is 1.65 +/- 0.1 cm, and is comparable to that achieved with other high-energy neutron beams. The long-term stability of the dose monitoring system has been measured and found to be satisfactory. The physical characteristics of the neutron beam are comparable with those of other modern fast neutron therapy facilities.

A d(16) + Be Fast Neutron Beam for Therapy: Radiotherapy and Oncology, Journal of the European society for Therapeutic Radiology and Oncology, Vol. 10, 1987, No. 1, Sept. 1987, pp. 71–75

A fast neutron therapy facility has been established utilizing the classical solid-pole cyclotron of the National Accelerator Centre (CSIR) in Pretoria. The neutron field is generated from the 9Be(d,n)10B reaction using 16 MeV deuterons on a thick target, yielding 0.51 cGy.min-1.microA-1 at an SSD of 135 cm. Essentially the beam characteristics concur with those measured at other centres with comparable energies.

A d(16)+Be fast neutron beam for therapy

A fast neutron therapy facility has been established utilizing the classical solid-pole cyclotron of the National Accelerator Centre (CSIR) in Pretoria. The neutron field is generated from the 9Be(d,n)10B reaction using 16 MeV deuterons on a thick target, yielding 0.51 cGy.min-1.microA-1 at an SSD of 135 cm. Essentially the beam characteristics concur with those measured at other centres with comparable energies.

Spectral energy measurements in a fast neutron therapy field

Various basic physical parameters are needed to convert a specific dosimeter reading in air to tissue kerma or to absorbed dose in a phantom. In a neutron radiation field, spectral information is required for the evaluation of these parameters. Using NE213 liquid scintiillator as a proton recoil spectrometer, neutron spectra were measured in the collimated d(16)+ Be beam of the Pretoria fast neutron therapy facility. The neutron events wer eidentified by means of pulse shape discrimination and a well established computer code employed to unfold the acquired pulse height data. The analysed neutron spectrum was used to compute the neutron kerma ratio ( K t/ K m) N and the ratio of the average energies W N/ W c required to create an ion pair in an A-150/TE gas ionization chamber. The determined values concur with published data from other centres with similar neutron beams. The mean neutron energy E n of the therapy beam was also calculated from the spectral distribution.

Neutron spectra and neutron kerma derived from activation and fission detector measurements in a d+T neutron therapy beam

Neutron spectra at different positions in a phantom have been derived from activation foil and fission counter measurements for the collimated beam of the Amsterdam d+T fast-neutron therapy facility. The spectra show that the fraction of lower-energy neutrons increases with increasing depth in the phantom as well as with increasing distance from the central axis of the beam. Calculation of the ratios of kerma values in tissue and tissue-equivalent (TE) plastic and of WN values for the spectra at five positions in the phantom, showed that the variations in these quantities are less than 0.5%. Kerma values have been derived from the neutron spectra and from the fission counter measurements separately. These neutron kerma values were compared with the neutron absorbed dose values derived from combined TE ionisation chamber and GM counter measurements. The dose values have been obtained applying recommendations given in the European protocol for neutron dosimetry for external beam therapy. At 50 mm and 100 mm depth along the central axis of the beam in the phantom, the agreement between the three methods was within 2%. In the penumbra region a maximum difference of about 7% has been observed between the three methods. The contribution from thermal neutrons to the total kerma is less than 1% in the beam as well as in the penumbra.

The isocentric fast neutron therapy facility at Edinburgh.

A second fast neutron beam for radiotherapy has been brought into operation at the Western General Hospital, Edinburgh. This beam is isocentrically mounted and can be rotated through +/- 120 deg from the vertical. Neutrons are produced by bombarding a thick beryllium target with 15 MeV deuterons. The provision of an isocentric facility enables some of the problems associated with poor beam penetration to be alleviated. The physical features of the isocentric facility are described with particular reference to the shielding and the methods employed to overcome problems of neutron activation. Both the prompt radiation leakage and the induced activity have been measured. It has been found that for the first eight months of operation the whole body dose-equivalent to radiographers was approximately 6 microSv (0.6 mrem) per patient treated. Dosimetry associated with the facility is discussed and an example of a seven field treatment plan for a bladder is presented.

Evolution of dosage schedules at the fast neutron therapy facilities in the united states

This paper is a review of the development of neutron therapy dosage schedules at the University of Washington, MANTA, and TAMVEC. The tumor doses differ at each institution because of differences in dose distribution and RBE. Initially, the dosage schedules were based on pretherapeutic radiobiology studies and biological intercomparisons with existing neutron therapy facilities. However, adjustments in the tumor dose have been made at each institution on the basis of clinical results. The preliminary analysis indicates that the range of acceptable doses with neutrons is relatively narrow. Clinical experience at MANTA and TAMVEC indicates that the late reactions with fast neutrons are more severe than would be expected on the basis of the acute reactions observed.

Physical characteristics of fast-neutron beams from a compact medical cyclotron.

A fast-neutron therapy facility is planned for a compact medical cyclotron. Energy spectra and yields of neutrons produced in a thick beryllium target by deuterons and protons were investigated. Collimator performance, shielding design, and central-axis depth doses were calculated by means of a Monte Carlo computer program. This facility appears adequate for clinical trials on certain tumors of the head and neck.

Use of a Monte Carlo computer code to design a neutron therapy facility

A complete analogue Monte Carlo computer code is being used to optimize the collimator and shielding design for a proposed fast neutron therapy facility using a 30 inch cyclotron. The program uses as input the available neutron and gamma -ray cross-section data and the recently measured neutron energy and angular distributions from a thick beryllium target.

Summary of general discussion on technical aspects of fast neutron therapy facilities

Summary of general discussion on technical aspects of fast neutron therapy facilities