Epithermal Neutron Beam Collimator Design


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Epithermal Neutron Beam Collimator Design

  1. 1. INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 49 (2004) 2015–2028 PII: S0031-9155(04)75549-9 The design, construction and performance of a variable collimator for epithermal neutron capture therapy beams K J Riley1, P J Binns1, S J Ali2 and O K Harling2 1 Nuclear Reactor Laboratory, Massachusetts Institute of Technology, 138 Albany St. Cambridge, MA 02139, USA 2 Department of Nuclear Engineering, Massachusetts Institute of Technology, 138 Albany St. Cambridge, MA 02139, USA Received 30 January 2004 Published 4 May 2004 Online at stacks.iop.org/PMB/49/2015 DOI: 10.1088/0031-9155/49/10/012 Abstract A patient collimator for the fission converter based epithermal neutron beam (FCB) at the Massachusetts Institute of Technology Research Reactor (MITR- II) was built for clinical trials of boron neutron capture therapy (BNCT). A design was optimized by Monte Carlo simulations of the entire beam line and incorporates a modular construction for easy modifications in the future. The device was formed in-house by casting a mixture of lead spheres (7.6 mm diameter) in epoxy resin loaded with either 140 mg cm−3 of boron carbide or 210 mg cm−3 of lithium fluoride (95% enriched in 6Li). The cone shaped collimator allows easy field placement anywhere on the patient and is equipped with a laser indicator of central axis, beam’s eye view optics and circular apertures of 80, 100, 120 and 160 mm diameter. Beam profiles and the collateral dose in a half-body phantom were measured for the 160 mm field using fission counters, activation foils as well as tissue equivalent (A-150) and graphite walled ionization chambers. Leakage radiation through the collimator contributes less than 10% to the total collateral dose up to 0.15 m beyond the edge of the aperture and becomes relatively more prominent with lateral displacement. The measured whole body dose equivalent of 24 ± 2 mSv per Gy of therapeutic dose is comparable to doses received during conventional therapy and is due principally (60–80%) to thermal neutron capture reactions with boron. These findings, together with the dose distributions for the primary beam, demonstrate the suitability of this patient collimator for BNCT. 1. Introduction Modern radiotherapy uses high-energy photons having exponential depth dose distributions and well-collimated beams with small penumbrae that are not significantly broadened in the 0031-9155/04/102015+14$30.00 © 2004 IOP Publishing Ltd Printed in the UK 2015
  2. 2. 2016 K J Riley et al patient. Applied advances in computing capability have improved the modality by exploiting these characteristics through the placement of multiple, uniquely shaped fields that match the tumour contours to ensure better dose conformity with the planning target volume (PTV) as defined by the ICRU (1999). In boron neutron capture therapy (BNCT) dose conformity is more complex since it is limited more by the preferential accumulation of boron in tumour than the physical properties of the incident beam. The presence of boron modifies the depth dose curve of the incident radiation with a superimposed peak dose at the location of relatively high boron concentrations. Ideally, NCT requires a homogeneous distribution of thermal neutrons in the PTV, which makes the diffusive nature of low-energy neutron interactions in the patient advantageous. Thermal neutrons deposit little energy in tissue allowing the PTV to be safely enlarged to encompass, in addition to the main tumour mass, microscopically infiltrating tumour cells that have also absorbed boron. Precisely defining the size and shape of the radiation flux incident upon the patient is less important for BNCT in limiting the dose to critical normal structures near the target than the steep dose gradients that arise from the selective uptake of boron. Clinical studies are proceeding in the USA to establish the maximum tolerable dose that normal brain can withstand in undergoing BNCT (Chanana et al 1999, Busse et al 2003) while at the same time attempting to mitigate effects of the virtually untreatable brain tumour glioblastoma multiforme. These trials are designed to maximize tumour dose while gradually escalating the absorbed dose to normal brain, which is prescribed as a volume average over both hemispheres and is administered in two fractions of up to three fields. The simple field arrangements applied in these trials are sufficient to study normal brain tolerance while exercising the conceptual advantage of BNCT to selectively deliver dose within a single field. Reactor-based facilities for BNCT use fixed (usually horizontal) beam lines with a very limited number of field sizes. Generally, a beam of epithermal neutrons is obtained from the fission process through a series of trade-offs between intensity, purity and collimation. Though there are advantages to a well-collimated beam, it is often impractical to attain a beam that is nearly unidirectional as in photon therapy. Therefore, the air gap used between the collimator and patient is typically much smaller in BNCT than in conventional radiotherapy to help preserve beam intensity and limit exposure to other parts of the body. The positioning restrictions inherently imposed by a fixed beam line and a small air gap influence the physical form of the collimator which must shield the patient from undesired photons and fast neutrons that inevitably accompany the epithermal neutrons, even in the purest of beams. This must be achieved while ensuring that the primary beam provides a well-defined field with an acceptably uniform intensity profile for radiotherapy. The fission converter based epithermal neutron facility (FCB) at the MIT Research Reactor (MITR-II) was designed to meet the present and future needs of the Harvard–MIT research program for clinical studies in BNCT (Harling et al 2002). The portion of the horizontal beam line nearest the patient confines the radiation field to the PTV and shields tissues that lie outside the primary field. Practical considerations in the design of the collimator, such as the ability to change the size of the field and to conveniently position the patient for cranial irradiations, are essential for the present needs. The physical design of the collimator should also allow for easy patient positioning for other disease sites that are under investigation (Coderre et al 1997, Pignol et al 1998) and incorporate features to assist with patient alignment such as a beam’s eye view and a backpointing laser along the central axis of the beam. Lastly, it was considered desirable to design a beam port that can be readily modified as needed for clinical research. A flexible collimator design permits epithermal flux to be traded for other desirable beam parameters (such as improved collimation or filtration) that may eventually prove useful. To accommodate the high patient throughput that will be needed for these and other studies, the
  3. 3. The design, construction and performance of a variable collimator 2017 Figure 1. Plan view of the epithermal neutron beam line at the MIT fission converter facility. approach at MIT was to adopt a modular design in which the patient collimator is assembled from several removable components that are easily interchangeable from inside the medical room. A patient collimator has been designed and constructed for use in the beam line of the FCB (Ali 2001). The neutronic performance of the field-defining device was optimized using an MCNP model of the entire fission converter beam line. Design studies identified lead as a good material for the collimator walls, because it scatters many epithermal neutrons towards the patient position that might otherwise be lost from the beam (Kiger et al 1999). A lead collimator also provides good shielding to areas outside the PTV from photons that are produced in the beam line. Lead, however, is an inefficient shield for epithermal neutrons and must be used with lighter nuclei to reduce the kinetic energy of neutrons through elastic scattering in combination with elements such as boron or lithium that absorb thermal neutrons. After identifying the best composition for the wall materials, the geometry was optimized to offer the greatest convenience in terms of patient positioning. Attention was given to ease of manufacture throughout the design as the device was to be constructed in-house from readily available materials without the need for special tools or machining. After construction, the collimator was evaluated in situ through a series of measurements performed under clinically relevant irradiation conditions to ensure that the collimator walls provide sufficient shielding and that the resulting epithermal neutron beam is uniform and well defined. 2. Methods and materials 2.1. Epithermal neutron beam The FCB delivers a high-intensity beam of epithermal neutrons to the patient from a source of fission neutrons generated in the uranium fuel of the converter. The converter is driven by thermal neutrons from the reflector region surrounding the core of the MITR-II, which currently operates at a maximum of 5 MW. The present configuration of the FCB is shown in figure 1 and yields an epithermal flux at the patient position of approximately
  4. 4. 2018 K J Riley et al 5 × 109 n cm−2 s−1 (Riley et al 2003). The fission source in the converter is moderated and filtered with aluminium, fluorine, cadmium and lead, resulting in a beam of epithermal neutrons that is virtually free of unwanted contamination from photons as well as fast and thermal neutrons. The filtered beam then enters a tapered portion of the horizontal beam line extending 1.1 m with lead walls that reflect some of the source neutrons (that might otherwise leave the beam) towards the patient and forms a beam collimator. The 40 cm long patient collimator extends this circular cone beyond the shielded beam line into the medical room to provide a beam that is spatially and directionally well defined. 2.2. Computational methods The beam line of the MIT FCB was modelled in detail using the general purpose Monte Carlo transport code MCNP (Kiger et al 1999, Riley 2001, Riley et al 2003). Fast neutron and photon absorbed dose rates (Dfn and Dγ , respectively), epithermal (1 eV–10 keV) neutron flux (φ epi) and the epithermal neutron current to flux ratio (J/φ epi) were calculated in-air at the patient position for collimators of different compositions. A homogeneous mixture of lead and epoxy was assumed for the collimator wall. Flux tallies were convolved with photon and neutron kerma coefficients (ICRU 2000) to determine the absorbed dose to soft muscle tissue. Absorbed dose rates at the end of the patient collimator with circular apertures ranging in diameter from 80 to 160 mm were calculated as a function of radial displacement from the central axis of the beam. The full width at half maximum (FWHM) of this cross plane profile was defined as a figure of merit to compare the effectiveness of the different collimator designs. Circular symmetry for the beam profile was assumed and dose rates were tallied in concentric rings with increasing diameters of 20 mm, which were chosen to match the length of the active volume of the detector used for measurements. To simulate the dose to the patient in areas outside the target volume, a neutron and photon source was written to a 1.8 × 1.4 m2 plane coincident with the end of the patient collimator and transported into a half-body phantom. Calculations used the phantom shown in figure 2 suspended free in-air that comprises of a water filled ellipsoid with major axes of 160, 170 and 200 mm that simulated the head while water filled cylinders of diameter 120 and 360 mm simulated the neck and torso, respectively. The 160 mm axis of the ellipsoid was coincident with the central axis of the beam and no air gap was left between the apex of the ellipsoid and the end plane of the collimator. The neutron and photon absorbed dose rates were averaged over the entire volume of the head and each of the 11 body segments shown in figure 2 to obtain sufficient statistical precision in the dose tallies. The physical absorbed dose rates tallied in the half-body phantom included the effects of leakage radiation from both the walls of the collimator and the beam line shielding in addition to the radiation scattered from the primary beam. 2.3. Construction method and materials From the Monte Carlo calculations a conical collimator extending 0.40 m into the medical room was designed that can readily provide the required circular apertures by attaching truncated conical pieces to its base. A modular design was chosen so that the field-defining aperture could be changed as needed to provide differently sized or shaped apertures by only needing to manufacture an appropriately dimensioned addition to mate with the collimator base. More importantly, this collimator is conveniently accessible from inside the medical room and can be easily changed to give different fields for a particular treatment plan. The collimator can also be more extensively modified if clinical experience dictates improvements in beam delivery.
  5. 5. The design, construction and performance of a variable collimator 2019 Figure 2. Schematic plan view of the conical patient collimator with the 160 mm diameter aperture and the water filled half-body phantom used in MCNP calculations of the collateral dose to the patient. The head is an ellipsoid with major axes of 160, 170 and 200 mm, while the neck and torso comprises cylinders with diameters of 120 and 360 mm, respectively. The patient collimator mounts on a reinforced steel structure within the medical room wall containing an integrated annulus of lead with four fission counters that monitor the neutron fluence delivered to the patient. The collimator itself is fabricated on a steel plate with a concentric circular cut-out and fixtures to hoist and mount the whole assembly (Ali 2001). The steel plate was made to accept a wooden male mould and a rolled sheet of aluminium as a female mould to form the conical inner and outer surfaces of the collimator. The mould was filled with a mixture containing the desired proportions of neutron and photon shielding material. The patient collimator comprises two main segments, which form the 160 mm diameter aperture that is shown in figure 2. Smaller apertures are formed by either extending the length of the collimator with additional conical pieces or by replacing the end cone with one containing the desired aperture. The various pieces mount to each other by bolting into threaded anchors cast into the collimator. The collimator walls are made of lead spheres cast in an epoxy resin loaded with either boron carbide or lithium fluoride (95% enriched in 6Li) to obtain the desired shielding properties. A mixture of 60% lead and 40% epoxy (which yielded the smallest combined penumbra for photons and neutrons) is easily obtained as higher volume fractions of lead can be realized by adding different sized spheres. Lead spheres (99.99% pure) of 7.6 mm in diameter were arranged in the mould before adding epoxy uniformly loaded with boron carbide (10B loading of 22 mg cm−3). The mould was filled in successive layers of approximately 50 mm thickness that were allowed to cure before filling the next to ensure that the epoxy adequately filled the interstitial spaces.
  6. 6. 2020 K J Riley et al Several test pieces with various volume fractions of lead and different amounts of boron carbide or lithium fluoride were poured to ensure an absence of voids and that sufficient structural integrity had been attained. Volume fractions of lead ranging from 50 to 70% had enough strength to support the weight of the collimator. Sufficient neutron attenuation was afforded by loadings of either 140 mg cm−3 boron carbide (19.9% 10B isotopic abundance, grain size 20–120 µm) or 210 mg cm−3 lithium fluoride, which was enriched with 95% 6Li. 2.4. Measurement methods Horizontal and vertical cross plane profiles of the neutron flux were measured across the constructed collimator using a cylindrical fission counter with an active volume of length 20 mm and 5 mm in diameter (LND model 30571). The pulses from fission events were of sufficient magnitude to allow discrimination against the photon background in the beam. Though fission counters are most sensitive to thermal neutrons, the cadmium filter in the beam line reduces the thermal flux to such comparatively low levels that 85% of the registered events in the fission chamber are due to epithermal neutrons (Wilson 2001). A graphite walled ionization chamber with an active volume of 0.1 ml flushed with 99.995% pure CO2 at 20 ml min−1 was used to obtain profiles of the photon absorbed dose rate. This chamber is operated at a bias of +250 V and has virtually no response to neutrons (Riley et al 2003). The fission counter and ionization chamber were separately scanned across the aperture and through the beam using a computer controlled stepper motor remotely operated from outside the medical room. The output of each measurement is normalized to a common beam intensity using one of the four beam monitoring detectors located at the base of the collimator. Measurements of the total absorbed dose rate were performed in-air and in a half-body phantom (ART Alderson/Rando Phantom—Radiation Support Devices, Long Beach, CA) using methods described previously (Rogus et al 1994, Riley et al 2003). The phantom is similar in size to the water phantom used for calculations and comprised 23 layers, each 25.4 mm thick that are constructed of plastic specially formulated to simulate radiation transport through the human body. The phantom was positioned for a lateral irradiation with the brain centred in the 160 mm field, without an air gap. Gold foils (25 mm2 and 0.05 mm thick) with and without cadmium covers (0.5 mm thick) were placed on the midline of several slices to determine the thermal neutron flux that was used to calculate the absorbed dose induced by thermal neutrons captured in 14N and 10B for different displacements from the central axis of the beam. The graphite walled ionization chamber described earlier was used together with a tissue equivalent (A-150) plastic walled chamber flushed with methane-based tissue equivalent gas (64.4% CH4, 32.4% CO2 and 3.2% N2). Measurements with these two chambers are used to separate the photon and neutron absorbed dose components in the mixed field. The portion of the neutron absorbed dose due to 14N(n,p)14C events can be subtracted using the activation foil results, to determine the fast neutron absorbed dose that arises from scattering with hydrogen nuclei. During these measurements one slice of the phantom was replaced with a dummy piece machined of nylon (type 6) to the same size and containing a 7.5 mm diameter hole into which the ionization chambers were inserted. The dose equivalent at each measurement location was determined by applying radiation weighting factors of 10 and 1 for neutrons and photons respectively (ICRP 1990). The nitrogen content of brain is taken as 2.2% by weight, while 3.5% is assumed for all other tissues (ICRU 1989). The dose equivalent from neutron capture in boron is estimated using a boron concentration of 15 µg g−1 for all tissues in the absence of detailed information regarding the uptake of boron in organs other than the brain. A radiation weighting factor of 20 is applied for the densely ionizing products of the boron reaction (ICRP 1990). The dose
  7. 7. The design, construction and performance of a variable collimator 2021 Figure 3. FWHM of the spatial profile for the neutron and photon absorbed dose rates as a function of the fractional percentage volume of epoxy in the collimator wall. The field aperture used in this design study has a nominal diameter of 150 mm. equivalent is reported per unit of weighted tumour dose, which uses weighting factors of 1.0 for photons and 3.2 for neutrons. Parameters representative of the capture compound currently in use, boronated phenylalanine (BPA), were chosen with an assumed boron concentration in tumour of 52.5 µg g−1, and an associated RBE of 3.8 (Coderre et al 1993). 3. Results 3.1. Design studies To determine the optimum volume fraction of epoxy and lead, the spatial profiles of the photon and neutron absorbed dose rates were tallied for various mixtures using a collimator with a nominal diameter of 150 mm. The FWHM for each profile was determined from an interpolation of a least-squares fit to the tallied profile. Figure 3 plots the FWHM versus volume fraction of epoxy mixed with lead in the collimator. The uncertainties shown in figure 3 are derived from the error associated with a linear fit to the data at the edge of the beam profile between 20% and 80% of full beam intensity. Each collimator also included 22 mg cm−3 of 10B. The FWHMs for both the neutron and photon profiles appear to reach a minimum for an epoxy volume fraction of 30–45% and are unaffected by variations in the relative compositions in this range. Spatial profiles and beam characteristics were calculated for fields with diameters of 80 and 100 mm that were formed by extending the length of the collimator without changing the taper angle. These profiles are compared to those obtained by keeping the collimator length constant and changing the taper angle on the inner surface of the piece nearest the patient to obtain the desired diameter. The results from these calculations with their associated statistical uncertainties are summarized in table 1. The term ‘extended’ is used to describe that the taper angle is kept constant and the length of the collimator is altered to produce the desired field size, while ‘fixed’ is used when the collimator length is kept constant and the taper angle is altered to achieve the same final aperture size as illustrated in figure 4. The two collimators of fixed length provide the same nominal intensity within the predicted (1σ ) uncertainties, while the epithermal flux at the patient position is reduced 30–40% for the longer collimators due
  8. 8. 2022 K J Riley et al Figure 4. Schematic illustrating the ‘extended’ and ‘fixed’ approaches to forming apertures smaller than 160 mm diameter. Table 1. Calculated beam characteristics for 80 and 100 mm diameter apertures formed using the two different approaches illustrated in figure 4. Extended collimators are made by lengthening the cone to achieve the desired aperture diameter. Fixed collimators use a different taper for the inner cone near the patient to achieve the desired aperture diameter without changing the length of the beam line. Diameter (mm) 80 (fixed) 80 (extended) 100 (fixed) 100 (extended) (109 φ epi cm−2 s−1) n 5.4 ± 0.2 3.2 ± 0.1 5.1 ± 0.2 3.6 ± 0.1 J/φ 0.80 ± 0.02 0.85 ± 0.02 0.81 ± 0.02 0.84 ± 0.02 Dfn/φ epi (10−13 Gy cm2) 0.93 ± 0.05 1.03 ± 0.05 0.95 ± 0.05 1.02 ± 0.05 Dγ /φ epi (10−13 Gy cm2) 2.6 ± 0.2 2.9 ± 0.2 2.9 ± 0.2 3.0 ± 0.2 Neutron FWHM (mm) 121 ± 3 91 ± 2 132 ± 3 120 ± 3 Photon FWHM (mm) 125 ± 3 94 ± 2 131 ± 3 122 ± 3 to greater geometric attenuation of the beam. The taper angle used for the fixed collimators, however, scatters more of the beam resulting in slightly poorer directionality (J/φ decreases between 3.5% and 6%) and profiles that have penumbrae 10–32% wider than the extended version. Beam purity measured by the specific fast neutron and photon dose rates (Dfn/φ and Dγ /φ) is largely unaffected by the different designs. The effect of the photons produced in the 10B(n,α)7Li capture reaction was studied by replacing the boron in the section of the collimator nearest the patient with a similar quantity of lithium to provide equivalent neutron absorption. A reduction in the specific photon dose rate from 2.8 ± 0.2 to 2.2 ± 0.2 × 10−13 Gy cm2 was calculated if 50 mg cm−3 of 6Li is used instead of 22 mg cm−3 10B for a nominal field size of 150 mm. Further simulations indicated that the same reduction could be achieved by using lithium in only a 38 mm thick layer along the inner surface of the collimator, as shown in figure 2.
  9. 9. The design, construction and performance of a variable collimator 2023 Figure 5. Total (neutron and photon) physical absorbed dose rate measured and calculated under simulated therapy conditions for a lateral irradiation in the 160 mm field. The leakage radiation represented by in-air absorbed dose rates (measured and calculated) is shown for comparison. The data do not include the dose deposited from capture events in boron and no dose weighting factors are applied. 3.2. Completed collimator The overall physical form of the collimator was chosen based on the desire to minimize leakage radiation to portions of the body outside the target volume but with exterior dimensions that allow the desired fields to be conveniently arranged with the fixed beam geometry. Shielding considerations necessitated an outer diameter at the base of the collimator that is large enough to shadow the outer diameter of the lead collimator further upstream in the beam line. The taper angle of the outer cone was then chosen to provide ample clearance around the collimator for positioning the patient while retaining sufficiently thick shielding in the walls of the collimator, for the smallest field sizes. The collimator was constructed using 7.6 mm diameter lead spheres that yielded a 40% volume fraction of epoxy, based on density measurements from the test pieces. A stepped cylindrical access port that penetrates the wall of the cone near its base was formed by casting an acrylic tube inside the mould. 3.3. Calculated and measured performance The total physical absorbed dose rates from neutrons and photons measured in the ART phantom and calculated in water for a lateral irradiation are shown in figure 5 as a function of displacement from the centre of the 160 mm field. Calculations and measurements of the in-air absorbed dose rate obtained with no phantom present are representative of leakage radiation from the beam line and are included in figure 5 for comparison. At any point outside the main beam, neutrons account for no more than 10% of the total absorbed dose from either leakage or scattered radiation. To compare the relative magnitude of radiation leaking from the beam line with that from the primary field, which is scattered within the patient, biological weighting factors were not applied nor was the dose from capture events in boron included. The overall estimated uncertainty for the measurements was 7% and the statistical uncertainties for the calculations range from 2% in the field to a maximum of 10% for the points furthest from the beam centreline. These are omitted from figure 5 for clarity.
  10. 10. 2024 K J Riley et al Figure 6. In-air measurements of photon and epithermal neutron intensities along the central axis of the beam normalized to unity at the patient position for the 160 mm diameter aperture. Good agreement between the measured and calculated quantities is observed in-phantom even though the size of the detector is small (0.1 ml) compared to the large tally volumes (0.5–5 l) that were required. Relative to values measured in-air on the central axis of the beam, dose rates outside the collimator opening are reduced to approximately one-third and 10% at displacements of 3 and 15 cm from the edge of the aperture, respectively. The calculated in-air dose profile shows a much steeper gradient, falling to 10% at 35 mm outside the main beam which is significantly lower than the corresponding measurements. This discrepancy is believed to be due in part to the presence of albedo neutrons from the walls, ceiling and floor of the medical room that are missing from the computational model. The relative intensities of the epithermal neutrons and photons along the central axis of the beam from the base of the collimator to 0.2 m beyond the exit plane of the 160 mm diameter aperture are shown in figure 6. Each profile is normalized to unity at the end of the collimator at the patient position. The intensities of both the photons and epithermal neutrons decrease rapidly along the collimator axis at a rate of approximately 0.7% mm−1 near the patient position. Statistical uncertainties of 1 and 2% associated with the measurements and calculations are smaller than the symbols in the figure. The measurements represent an average over the 20 mm active length of the detector that was oriented along the beam axis and
  11. 11. The design, construction and performance of a variable collimator 2025 Table 2. The dose equivalent per RBE weighted tumour Gy measured along the midline of the half- body phantom at different displacements from the central axis for the 160 mm diameter aperture. Dose equivalent was determined with weighting factors of 1, 10 and 20 for photons, neutrons and the high LET boron reaction products, respectively. A boron concentration of 15 µg g−1 was assumed for normal tissue. Therapeutic dose is determined using RBEs of 1.0 for photons and 3.2 for neutrons. A boron concentration of 52.5 µg g−1 is assumed for tumour with an associated RBE of 3.8. Displacement Dose equivalent per from central Anatomical RBE weighted tumour axis (m) region dose (mSv Gy−1) 0.21 Upper lungs 66 ± 5 0.32 Centre of lungs 19 ± 1.3 0.45 Upper intestines 6.4 ± 0.4 1.15 Feet 2.4 ± 0.2 positioned with an absolute uncertainty of 5 mm. The relative location of each measurement is, however, accurate to within 1 mm. Reasonable agreement is observed between the predictions and the measured profiles with the exception of the photon dose rates outside the field aperture where the measurements show a steeper attenuation. The measured dose equivalent at four different locations along the midline of the half- body phantom is summarized in table 2 and is expressed per RBE weighted Gy of tumour dose delivered at the midline of the phantom during a simulated patient irradiation. The tabulated results for the 160 mm field correlate the measurement positions with the anatomical features in these locations. The estimated uncertainty of 7% for these measurements is due principally to counting statistics of the gold foil activities used to determine the absorbed dose from neutron capture in nitrogen and boron (Riley et al 2003). A whole body dose equivalent of 24 ± 2 mSv per Gy of RBE weighted tumour dose was determined from a volume-weighted sum of the dose equivalents calculated in the neck and torso segments of the phantom. 3.4. Patient positioning The required beam entry point for any treatment field can be conveniently aligned with the central axis of the beam using a vertically mounted laser that is directed along the central axis of the collimator. The mirrors for reflecting the laser are contained in a tube that fits into the cylindrical access port in the collimator wall and includes optics that provide a beam’s eye view for aligning the patient in the fixed beam which would otherwise be difficult with little or no air gap. The laser and viewing system assembly are removed for an irradiation and replaced with a plug that was constructed separately in a manner similar to the collimator. When the plug is removed (and the laser installed), an interlock in the beam control system prevents an irradiation from commencing. Fiducial marks on the back wall of the medical room and on an insert for each aperture verify proper alignment of the laser with the geometric centre of the beam line when it is replaced. A photograph of the completed collimator and the laser/viewing optics are shown in figure 7. The length of the collimator for the 160 mm field extends 0.32 m from the wall of the medical room and is 0.31 m wide at its end, gradually increasing to 0.66 m at the wall. This geometry is well suited to brain irradiations since most patients are positioned supine on the treatment couch perpendicular to the beam, which does not interfere with the collimator. Smaller field sizes provide an even more favourable geometry as the collimator is lengthened with a thinner wall at the field aperture. The centreline of the beam in the medical room is 0.42 m above the floor and is at a comfortable height for medical staff to conduct patient
  12. 12. 2026 K J Riley et al Figure 7. Left: photograph of the collimator mounted inside the FCB medical room with the 120 mm diameter aperture. Right: the laser and optical system are removed from the collimator following patient positioning prior to commencing irradiation. A cross laser is projected along the central axis of the beam when the laser and optics are installed for patient set-up that also allow a beam’s eye view along the central axis. set-up. The height of the beam also allows the possibility of seating the patient in a low chair, with the head reclined into the beam. Positioning for other potential tumour sites such as the torso, limbs and pelvis is also straightforward with this patient collimator. 4. Discussion and summary The design of the patient collimator for the FCB achieves a practical minimum in the increase of the FWHM for both the photon and neutron absorbed dose profiles relative to the geometric aperture size. Replacing a portion of the boron in the collimator with lithium proved effective in reducing the photon contamination produced by neutron capture in the collimator. Smaller apertures formed by adding sections to the end of the collimator have superior spatial profiles but reduced beam intensity by up to 40% compared with those provided by leaving the length of the collimator fixed and changing the taper of the inner surface. Even using the 80 mm diameter aperture with the lowest available intensity, irradiation times comparable to conventional radiotherapy can be achieved with the FCB (<20 min) for a peak dose in brain tissue of 12.5 (RBE) Gy, using BPA infused at 350 mg kg−1. An advantage of the additional space afforded by extending the collimator to form smaller apertures is to provide greater flexibility for patient positioning. The patient collimator was constructed from lead and epoxy with either boron carbide or lithium fluoride (95% enriched in 6Li), which (with the exception of enriched lithium) are inexpensive and readily available materials. The present configuration has a removable end segment that can be easily redesigned and constructed to provide an aperture of any size or shape. In addition, the present collimator can be moved forward several centimetres to allow, for example, the inclusion of a lithium (enriched in 6Li) filter that is now planned. The detectors that serve as beam monitors are positioned upstream of these variable components to maintain continuity in the measured output for all therapy field configurations. The shape of the final collimator and the axial laser that indicates the central axis of the beam and which can be viewed even when the beam aperture is covered makes patient
  13. 13. The design, construction and performance of a variable collimator 2027 positioning easier for any irradiation site. Beam intensity at the patient position steadily reduces with increasing distance from the end of the collimator. The distance from the end of the collimator to the beam entry point on the surface of the patient is reproducible to within 1 mm and contributes an uncertainty of less than 1% in the fluence delivered to the patient. The MCNP model of the fission converter beam line proved a valuable tool for performing the parametric studies necessary to design the patient collimator. Satisfactory agreement is obtained between the absorbed dose rates measured outside the beam aperture and those calculated using MCNP. Similar agreement is observed for the relative epithermal neutron and photon intensities along the central axis of the beam. Leakage radiation through the walls of the collimator or from the beam line itself is only a small fraction of the dose received in regions outside the target volume. Within 0.15 m beyond the geometric edge of the aperture, the absorbed dose from leakage radiation contributes less than 10% of the total collateral dose to the patient. At displacements greater than 0.3 m from the central axis of the beam where the total dose equivalent is relatively low, leakage radiation becomes comparable to that from scatter within the patient. Further reduction of the beam leakage component will not significantly decrease the collateral dose received during therapy that mostly comes from scattering of the beam within the primary target volume. The highest dose equivalent measured was 66 mSv per therapy (RBE weighted) Gy at a position of 0.21 m from the central axis which is considerably greater than the 10 mSv Gy−1 reported near the edge of a 10 × 10 cm2 field for photon therapy (ICRP 1985). However, the average measured whole body dose of 24 mSv per (RBE) Gy to tumour is similar to the 20 mSv Gy−1 for the largest fields in photon therapy (Nath et al 1984) and less than the 37–70 mSv Gy−1 (Followill et al 1997) for advanced tomotherapy. In BPA-mediated BNCT, boron interactions produce between 60% and 80% of the total dose equivalent at all locations observed outside the field and occur principally with neutrons that are scattered from the primary beam. The relatively high dose equivalent attributed to the capture products released from boron makes these results very sensitive to the dose weighting factors and uptake parameters assumed. The simple construction technique and inexpensive materials employed have proved successful in building collimators that provide both desirable beam characteristics and effective patient shielding for epithermal neutron beams. The patient collimator as described in its initial configuration has already been used successfully for clinical brain irradiations at the MIT fission converter beam. Versatility in the design and ease of construction readily facilitate adaptation of this collimator to satisfy the needs of further clinical studies that are envisioned for BNCT. Acknowledgments The authors acknowledge the input of Dr Paul Busse, Jody Kaplan RN BSN, Dr Stead Kiger and Dr Robert Zamenhof in the design of the collimator. This work was supported by the US Department of Energy under contract number DEFG02-96ER62193. References Ali J 2001 The design optimization and construction of a patient collimator for the fission converter beam SM Thesis (Cambridge, MA: Massachusetts Institute of Technology) Busse P M et al 2003 A critical examination of the results from the Harvard–MIT NCT program phase I clinical trial of neutron capture therapy for intracranial disease J. Neurooncol. 62 111–21
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