T 978, British Journal of Radiology,
51,122-126
Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy By B. J. Mijnheer, Ph.D. Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands J. Zoetelief, M.Sc, and J. J. Broerse, Ph.D. Radiobiological Institute TNO, Rijswijk, The Netherlands {Received August, 1977 and in revisedform October, 1977) ABSTRACT
Build-up and central axis depth-dose curves have been, obtained for d(50) + Be, p(42) + Be and d + T neutron beams. Measurements carried out with the collimator opening covered with a layer of lead showed that for all three neutron beams the entrance dose is approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams will be approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves have been established for d(50) + Be neutrons at 129 cm SSD, for 60 p(42) + Be neutrons at 125 cm SSD and d + T neutrons and Co y rays at 80 cm SSD. The 50% dose values in a water phantom are at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm respectively, for field sizes of approximately 15 cm X 20 cm. Insertion of a 6 cm thick nylon filter in the p(42) + Be beam increases this value from 12.0 cm to 13.5 cm. The gamma component for the d + T neutron beam is higher than for the cyclotron beams.
on beryllium (p-f-Be). This last reaction has been proposed as an alternative to the d+Be reaction, since the same particle energy can be obtained with a cyclotron of smaller size (Johnsen et al., 1976). A linear accelerator employing the p(66)-j-Be reaction is already in preliminary use for treatment of patients (Cohen and Awschalom, 1976). Extensive information on physical characteristics of p-f-Be neutron beams, including microdosimetry and determinations of energy spectra, has recently become available (Amols et al., 1978). However, these studies did not yield information concerning the y-ray contribution and the effect of filtering the neutron beam.
For radiotherapy purposes, the depth-dose distribution in tissue and the build-up of absorbed dose in the skin and subcutaneous tissue are two important physical characteristics. Central axis depth-dose curves are available for a number of different neutron beams (ICRU, 1977). However, comparison of measurements of different authors is complex, due to differences in field size, source-surface distance (SSD), phantom material and the quantity measured (total absorbed dose or neutron absorbed dose). The build-up of secondary charged-particle equilibrium is also dependent on the experimental conditions. For any one specific neutron beam, the curve representing the variation in ionization with increasing thickness of build-up material is dependent on field size, the presence of phantom material behind the ionization chamber, the use of air or tissue-equivalent (TE) gas flushing the chamber and the application of an absorber to remove charged particles from the beam (Zoetelief et ah, 1978). It was decided therefore to determine central axis depth-dose curves and charged particle build-up for different neutron beams with one set of detectors under similar experimental conditions. The neutron sources studied were the reactions of deuterons on tritium ( d + T ) and 50 MeV deuterons on beryllium (d(50)+Be), which have both been in clinical use for some years, and the reaction of protons
The d(50)+Be neutrons have been obtained from the isochronous cyclotron at Louvain-la-Neuve, Belgium, made by CSF. A multilayer collimator 105 cm thick, of steel, acrylic plastic (Perspex) and lead provided a 20 cm X 20 cm field at 129 cm distance from the target (Octave-Prignot et al., 1976). The p(42)-|-Be neutrons were produced by the synchrocyclotron at the Institute for Nuclear Physics Research, IKO, Amsterdam. Protons accelerated to 52 MeV were degraded in energy to 42 MeV by means of a 3.7 mm thick aluminium foil and fully stopped in a Be layer 11 mm thick. A proton energy of 42 MeV was chosen as this energy seems to be well suited for a cyclotron therapy system. According to Jungerman et al. (1971) the neutron production cross sections for protons on a thin aluminium target are much lower than for protons of equal incident energy on a thin beryllium target, and in addition the resulting average neutron energy is lower for aluminium than for beryllium. Consequently the contribution of neutrons resulting from the aluminium foil will not have much influence on the build-up and depth-dose distributions. The beam was collimated by means of a tapered multilayer collimator consisting of 25 cm steel and 25 cm borated paraffin with a circular steel
EXPERIMENTAL TECHNIQUES
122
FEBRUARY 1978
Build-up and depth-dose characteristics of different fast neutron beams
insert providing a field with a diameter of about 17 cm at 94 cm distance from the target; this is approximately equivalent to a square field of 15 cm X 15 cm. During some of the experiments, a 6 cm thick nylon filter (density, 1.15 g cm"3) was inserted in the collimator opening near the target. The d-f-T neutrons of the Amsterdam fast neutron therapy facility were produced with a sealed tube shielded by approximately 30 cm of steel and 19 cm of polythene followed by 3 cm of lead (Mijnheer
51,122-126
Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy By B. J. Mijnheer, Ph.D. Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands J. Zoetelief, M.Sc, and J. J. Broerse, Ph.D. Radiobiological Institute TNO, Rijswijk, The Netherlands {Received August, 1977 and in revisedform October, 1977) ABSTRACT
Build-up and central axis depth-dose curves have been, obtained for d(50) + Be, p(42) + Be and d + T neutron beams. Measurements carried out with the collimator opening covered with a layer of lead showed that for all three neutron beams the entrance dose is approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams will be approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves have been established for d(50) + Be neutrons at 129 cm SSD, for 60 p(42) + Be neutrons at 125 cm SSD and d + T neutrons and Co y rays at 80 cm SSD. The 50% dose values in a water phantom are at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm respectively, for field sizes of approximately 15 cm X 20 cm. Insertion of a 6 cm thick nylon filter in the p(42) + Be beam increases this value from 12.0 cm to 13.5 cm. The gamma component for the d + T neutron beam is higher than for the cyclotron beams.
on beryllium (p-f-Be). This last reaction has been proposed as an alternative to the d+Be reaction, since the same particle energy can be obtained with a cyclotron of smaller size (Johnsen et al., 1976). A linear accelerator employing the p(66)-j-Be reaction is already in preliminary use for treatment of patients (Cohen and Awschalom, 1976). Extensive information on physical characteristics of p-f-Be neutron beams, including microdosimetry and determinations of energy spectra, has recently become available (Amols et al., 1978). However, these studies did not yield information concerning the y-ray contribution and the effect of filtering the neutron beam.
For radiotherapy purposes, the depth-dose distribution in tissue and the build-up of absorbed dose in the skin and subcutaneous tissue are two important physical characteristics. Central axis depth-dose curves are available for a number of different neutron beams (ICRU, 1977). However, comparison of measurements of different authors is complex, due to differences in field size, source-surface distance (SSD), phantom material and the quantity measured (total absorbed dose or neutron absorbed dose). The build-up of secondary charged-particle equilibrium is also dependent on the experimental conditions. For any one specific neutron beam, the curve representing the variation in ionization with increasing thickness of build-up material is dependent on field size, the presence of phantom material behind the ionization chamber, the use of air or tissue-equivalent (TE) gas flushing the chamber and the application of an absorber to remove charged particles from the beam (Zoetelief et ah, 1978). It was decided therefore to determine central axis depth-dose curves and charged particle build-up for different neutron beams with one set of detectors under similar experimental conditions. The neutron sources studied were the reactions of deuterons on tritium ( d + T ) and 50 MeV deuterons on beryllium (d(50)+Be), which have both been in clinical use for some years, and the reaction of protons
The d(50)+Be neutrons have been obtained from the isochronous cyclotron at Louvain-la-Neuve, Belgium, made by CSF. A multilayer collimator 105 cm thick, of steel, acrylic plastic (Perspex) and lead provided a 20 cm X 20 cm field at 129 cm distance from the target (Octave-Prignot et al., 1976). The p(42)-|-Be neutrons were produced by the synchrocyclotron at the Institute for Nuclear Physics Research, IKO, Amsterdam. Protons accelerated to 52 MeV were degraded in energy to 42 MeV by means of a 3.7 mm thick aluminium foil and fully stopped in a Be layer 11 mm thick. A proton energy of 42 MeV was chosen as this energy seems to be well suited for a cyclotron therapy system. According to Jungerman et al. (1971) the neutron production cross sections for protons on a thin aluminium target are much lower than for protons of equal incident energy on a thin beryllium target, and in addition the resulting average neutron energy is lower for aluminium than for beryllium. Consequently the contribution of neutrons resulting from the aluminium foil will not have much influence on the build-up and depth-dose distributions. The beam was collimated by means of a tapered multilayer collimator consisting of 25 cm steel and 25 cm borated paraffin with a circular steel
EXPERIMENTAL TECHNIQUES
122
FEBRUARY 1978
Build-up and depth-dose characteristics of different fast neutron beams
insert providing a field with a diameter of about 17 cm at 94 cm distance from the target; this is approximately equivalent to a square field of 15 cm X 15 cm. During some of the experiments, a 6 cm thick nylon filter (density, 1.15 g cm"3) was inserted in the collimator opening near the target. The d-f-T neutrons of the Amsterdam fast neutron therapy facility were produced with a sealed tube shielded by approximately 30 cm of steel and 19 cm of polythene followed by 3 cm of lead (Mijnheer
