A-150 Tissue-equivalent Plastic Research Articles

Double differential cross sections of neutron production by 135 and 180 MeV protons on A-150 tissue-equivalent plastic

The double differential cross sections (DDXs) of neutron production by 135 and 180 MeV protons were measured to obtain benchmark data for calculating neutron doses in proton therapy. The incident proton energies correspond to those of protons decelerating in a patient. An A-150 tissue-equivalent plastic was used as the target. The measured DDXs were compared with those calculated using PHITS, FLUKA, and Geant4. The calculated DDXs were found to be underestimated compared with the measured ones, especially at forward angles. In proton therapy, exposure evaluations using one of these simulation codes would underestimate the neutron dose.

A detailed investigation of gamma-ray energy absorption and dose buildup factor for soft tissue and tissue equivalents using Monte Carlo simulation

In the present work, the gamma-ray energy absorption buildup factor for water and dose buildup factor for soft tissue (ICRU 4-component), A-150 tissue-equivalent plastic (A150TEP), and tissue equivalent-gas, methane-based (TEGMB) were studied for 356, 662, 1173, and 1332 keV gamma-ray energy up to depths of 10 mean free path (mfp) extensively using Monte Carlo simulation. The aim in this paper is to investigate the gamma buildup factor for soft-tissue equivalent materials and more specifically to study the relative difference between them at practical gamma energies through the Monte Carlo simulation method. The simulations have been performed by MCNPX code, which could be very useful for calculation of shielding estimates. The results revealed that by using the MCNPX code one can easily calculate the energy absorption and dose buildup factors for different materials and also different energies. The simulation results for water were verified by comparing data with those of experimental works wherever possible. In addition, the simulation results for soft tissue were verified by the G-P fitting method, as well. Moreover, the simulation results of the total mass attenuation coefficients of water, A150TEP, soft tissue, and TEGMB were compared with standard database of NIST-XCOM and a good agreement has been obtained. The materials selected have been studied for the first time in this work with respect to the gamma buildup factors at specific energies mentioned above by the Monte Carlo simulation method.

Development of a new code for stopping power and CSDA range calculation of incident charged particles, part A: Electron and positron

The stopping power of targets against incident charged particles and their range through matter are commonly considered as two important parameters in radiation and health physics studies. The exact calculation of these parameters is very crucial in many applications of radiation. The main objective of this study is the development of a new code for stopping power and range calculation of incident electron and positron projectiles. In this case, the interactions with matter are mainly due to the collision process and the Bremsstrahlung radiation emission. For collision stopping power, the Moller and Bhabha relations were considered for electron and positron, respectively. Some sophisticated correlations based on the Fano model have been selected for density correction term. For the radiative stopping power, a multifunction analytical model was developed based on an extensive literature review. All the equations were consolidated and programmed via the development of a new computational code using the C# programming language. Some simple and compound generic materials were considered for the verification process. Graphite, Silicon and Copper were selected as simple materials; whilst compound materials include air, water and the A-150- Tissue-Equivalent Plastic (A150TEP). The results of the stopping power and the range of incident particles were compared with well-known ICRU reports as well as Monte Carlo simulation. The results are consistent and in agreement with reference data. The maximum relative error of the collision stopping power was about 5% in comparison with ICRU reports for both electron and positron. However, for the positron projectile at low energies, some minor discrepancies were observed between the calculated collision stopping power and the MCNPX data. For the radiative stopping power, some deviations were observed in the low energy region and the model borders. The maximum relative error in the total stopping power was about 5–6% in comparison with ICRU reports and 15–20% in comparison with the MCNPX.

Tissue composition effect on dose distribution in radiotherapy with a 6 MV photon beam of a medical linac.

The aim of this study is to evaluate soft-tissue composition effect on dose distribution for various soft tissues in radiotherapy with a 6 MV photon beam of a medical linac. The compositions of various soft tissues are different which could affect dose calculations. A phantom and Siemens Primus linear accelerator were simulated using MCNPX Monte Carlo code. In a homogeneous cubic phantom, six types of soft tissue and three types of tissue-equivalent materials were defined separately. The soft tissues were muscle (skeletal), adipose tissue, blood (whole), breast tissue, soft tissue (9-component), and soft tissue (4-component). The tissue-equivalent materials included water, A-150 tissue equivalent plastic and perspex. Photon dose relative to dose in 9-component soft tissue at various depths on the beam's central axis was determined for the 6 MV photon beam. The relative dose was also calculated and compared for various MCNPX tallies including *F8, F6, and *F4. The results of the relative photon dose in various materials relative to dose in 9-component soft tissue using different tallies are reported in the form of tabulated data. Minor differences between dose distributions in various soft tissues and tissue-equivalent materials were observed. The results from F6 and F4 were practically the same but differ with the *F8 tally. Based on the calculations performed, the differences in dose distributions in various soft tissues and tissue-equivalent materials are minor but they could be corrected in radiotherapy calculations to upgrade the accuracy of the dosimetric calculations.

Evaluation of the effect of soft tissue composition on the characteristics of spread-out Bragg peak in proton therapy.

The aim of this study is to evaluate the effect of soft tissue composition on dose distribution and spread-out Bragg peak (SOBP) characteristics in proton therapy. Proton beams with nominal energies of 70, 120 and 210 MeV were considered. The soft tissues and tissue equivalent materials implemented in this study are: 9-component soft tissue, 4-component soft tissue, adipose tissue, muscle (skeletal), lung tissue, breast tissue, A-150 tissue equivalent plastic, perspex and water. Each material was separately defined inside a 20 cm × 20 cm × 40 cm phantom. A multilayer phantom was evaluated as well. The effect of tissue composition on the relative dose in SOBP region (relative to the dose in SOBP region in water), range of SOBP, length of SOBP, and uniformity index of SOBP was evaluated. Various soft tissues and tissue equivalent materials have shown different dose level in SOBPs, ranges of SOBPs, lengths of SOBPs and uniformity indices. Based on the obtained results, various soft tissues and tissue equivalent materials have quite different SOBP characteristics. Since in clinical practice with proton therapy, only the range of SOBP is corrected for various tissues, omission of the above effects may result in major discrepancies in proton beam radiotherapy. To improve treatment accuracy, it is necessary to introduce such effects in treatment planning in proton therapy.

Physical design and Monte Carlo simulations of a space radiation detector onboard the SJ-10 satellite

A radiation gene box (RGB) onboard the SJ-10 satellite is a device carrying mice and drosophila cells to determine the biological effects of space radiation environment. The shielded fluxes of different radioactive sources were calculated and the linear energy transfers of γ-rays, electrons, protons and α-particles in the tissue were acquired using A-150 tissue-equivalent plastic. Then, a conceptual model of a space radiation instrument employing three semiconductor sub-detectors for deriving the charged and uncharged radiation environment of the RGB was designed. The energy depositions in the three sub-detectors were classified into 15 channels (bins) in an algorithm derived from the Monte Carlo method. The physical feasibility of the conceptual instrument was also verified by Monte Carlo simulations.

Comparison of plastics used in proportional counters for proton and heavy ion measurements

This study investigates alternative plastics for use in the construction of tissue equivalent gas-filled detectors. Four different alternative plastics (acrylic, nylon, polyethylene, and polystyrene) were tested in comparison with A-150 Five proportional counters were constructed possessing spherical ionization cavities each made of one of the five plastics with wall thickness of 3mm and sensitive diameters of 4.48cm. The detectors operated at 1400V. They were exposed to three different energies of proton beams (87, 162, and 222MeV) and five different heavy ion beams (143MeV/amu He, 265MeV/amu C, 440MeV/amu Si, 430MeV/amu Ar, and 421MeV/amu Fe). The detectors were exposed to bare beams in air and no phantom was used. Lineal energy spectra derived from these measurements of energetic protons and heavy ion beams were used for comparison. Frequency averaged lineal energies and dose averaged lineal energies were calculated from each spectrum. Comparison of the experimental data indicates that the responses of the four alternative plastics tested are very similar to the response of A-150 tissue equivalent plastic.

Tissue equivalency of phantom materials for neutron dosimetry in proton therapy

Previous Monte Carlo and experimental studies involving secondary neutrons in proton therapy have employed a number of phantom materials that are designed to represent human tissue. In this study, the authors determined the suitability of common phantom materials for dosimetry of secondary neutrons, specifically for pediatric and intracranial proton therapy treatments. This was achieved through comparison of the absorbed dose and dose equivalent from neutrons generated within the phantom materials and various ICRP tissues. The phantom materials chosen for comparison were Lucite, liquid water, solid water, and A150 tissue equivalent plastic, These phantom materials were compared to brain, muscle, and adipose tissues. The magnitude of the doses observed were smaller than those reported in previous experimental and Monte Carlo studies, which incorporated neutrons generated in the treatment head. The results show that for both neutron absorbed dose and dose equivalent, no single phantom material gives agreement with tissue within 5% at all the points considered. Solid water gave the smallest mean variation with the tissues out of field where neutrons are the primary contributor to the total dose. Of the phantom materials considered, solid water shows best agreement with tissues out of field.

Design, Construction, and Implementation of Spherical Tissue-Equivalent Proportional Counter

In this work a new single anode spherical tissue-equivalent proportional counter (TEPC) was developed for mixed field radiation dosimetry. The approach used was to divide the cathode into several rings with different thicknesses and adjust the potential difference between each ring and the anode to produce an electric field that is nearly constant along the length of the anode. A-150 tissue-equivalent plastic is used for the detector walls, the insulator material between the cathode rings is low-density polyethylene, and the gas inside the detector is propane. The detector, along with the charge sensitive preamplifier, is encased in a stainless steel vacuum chamber. This 1.8-cm-diam TEPC provides low electronic noise, a gas gain of 550 at 782 V, good spectral resolution (26 keV/μm), and a full-width at half-maximum value of 17%. This spherical TEPC detector system will improve the accuracy of dosimetry in space and as a result improve radiation safety for astronauts.

TU-D-BRB-07: Determination of Neutron Dose Due to a Therapeutic Proton Beam Incident On a Closed Tungsten MLC Using the Dual Hydrogenous/non-Hydrogenous Ionization Chamber Method

Purpose: To measure neutron dose around a tungsten MLC used for proton beam therapy using the dual ionization chamber (IC) method. Method and Materials: ICs were constructed with large 10cc volumes to enable low dose measurements in the mGy range. The chambers are identical aside from wall, electrode, and guard-ring material with one constructed of A150 tissue-equivalent plastic (T) and filled with methane based tissue-equivalent gas and the other magnesium (U) and filled with argon gas. IC(T) has approximately the same sensitivity to neutrons and photons while the non-hydrogenous IC(U) exhibits a lower sensitivity to neutrons. The dose from neutrons and photons can be expressed as DN=(RT-RU)/(1-kU) and DG=(RU- kURT)/(1-kU) where RU,RT are IC readings and kU is the relative sensitivity of IC(U) to neutrons which varies with energy. Measurements were made with 230MeV proton beams incident on a closed MLC with the ICs on the beam axis 21cm from the MLC. Open field measurements were made with attenuation by 0, 0.5, 2, 5, 9, and 10cm of solid water. The ICs were calibrated against a Farmer chamber in air with a 6MV beam and suitable buildup caps. Results: Initial analysis of the measurements suggests an average neutron energy of 2.6MeV corresponding to kU=0.033. Measured neutron dose with the MLC closed and detector geometry stated above is 1.67mGy/Gy where IC(T) was used to determine dose in the open field. Conclusion: Initial measurements of neutron dose from a proton therapy beam incident on a MLC were performed which indicate the validity of the dual chamber method for determining neutron dose around a proton beam. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-07-2-0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.

A comparison of the measured responses of a tissue-equivalent proportional counter to high energy heavy (HZE) particles and those simulated using the Geant4 Monte Carlo code

Monte Carlo simulations of heavy ion interactions using the Geant4 toolkit were compared with measurements of energy deposition in a spherical tissue-equivalent proportional counter (TEPC). A spherical cavity with a physical diameter of 12.7 mm was filled with propane-based tissue-equivalent gas surrounded by a wall of A-150 tissue-equivalent plastic that was 2.54 mm to thick. Measurements and Monte Carlo simulations were used to record the energy deposition and the trajectory of the incident particle on an event-by-event basis for ions ranging in atomic number from 2 ( 4He) to 26 ( 56Fe) and in energy from 200 MeV/nucleon to 1000 MeV/nucleon. In the simulations, tracking of secondary electrons was terminated when the range of an electron was below a specified threshold. The effects of range cuts for electrons at 0.5 μm, 1 μm, 10 μm, and 100 μm were evaluated. To simulate an energy deposition influenced by large numbers of low energy electrons with large transverse momentum, it was necessary to track electrons down to range cuts of 10 μm or less. The Geant4 simulated data closely matched the measured data acquired using a TEPC for incident particles traversing the center of the detector as well as near the gas-wall interface. Values of frequency mean lineal energy and dose mean lineal energy were within 8% of the measured data. The production of secondary particles in the aluminum vacuum chamber had no effect on the response of the TEPC for 56Fe at 1000 MeV/nucleon. The results of this study confirm that Geant4 can simulate patterns of energy deposition for existing microdosimeters and is valuable for improving the design of a new generation of detectors used for space dosimetry and for characterizing particle beams used in hadron radiotherapy.

The role of nonelastic reactions in absorbed dose distributions from therapeutic proton beams in different medium

Many new techniques for delivering radiation therapy are being developed for the treatment of cancer. One of these, proton therapy, is becoming increasingly popular because of the precise way in which protons deliver dose to the tumor volume. In order to achieve this level of precision, extensive treatment planning needs to be carried out to determine the optimum beam energies, energy spread (which determines the width of the spread-out Bragg peak), and angles for each patient's treatment. Due to the level of precision required and advancements in computer technology, there is increasing interest in the use of Monte Carlo calculations for treatment planning in proton therapy. However, in order to achieve optimum simulation times, nonelastic nuclear interactions between protons and the target nucleus within the patient's internal structure are often not accounted for or are simulated using less accurate models such as analytical or ray tracing. These interactions produce high LET particles such as neutrons, alpha particles, and recoil protons, which affect the dose distribution and biological effectiveness of the beam. This situation has prompted an investigation of the importance of nonelastic products on depth dose distributions within various materials including water, A-150 tissue equivalent plastic, ICRP (International Commission on Radiological Protection) muscle, ICRP bone, and ICRP adipose. This investigation was conducted utilizing the GEANT4.5.2 Monte Carlo hadron transport toolkit.

Measurement of the tissue to A-150 tissue equivalent plastic kerma ratio at two p(66)Be neutron therapy facilities

The ICRU tissue to A-150 tissue equivalent plastic kerma ratio is needed for neutron therapy dosimetry. The current ICRU protocol for neutron dosimetry recommends using a common conversion factor of 0.95 at all high-energy neutron therapy facilities. In an effort to determine facility specific ICRU tissue to A-150 plastic kerma ratios, an experimental approach was pursued. Four low pressure proportional counters that differed in wall materials (i.e. A-150, carbon, zirconium and zirconium-oxide) were used as dosimeters and integral kerma ratios were determined directly in the clinical beam. Measurements were performed at two p(66)Be facilities: iThemba LABS near Cape Town and Fermilab near Chicago. At the iThemba facility the clinical neutron beam is routinely filtered by a flattening and hardening filter combination. The influence of beam filtration on the kerma ratio was evaluated. Using two recent gas-to-wall dose conversion factor (rm,g value) evaluations a mean ICRU tissue to A-150 plastic kerma ratio of 0.93 ± 0.05 was determined for the clinical beam at iThemba LABS. The respective value for the Fermilab beam is 0.95 ± 0.05. The experimentally determined ICRU tissue to A-150 plastic kerma ratios for the two clinical beams are in agreement with theoretical evaluations. Beam filtration reduces the kerma ratio by 3 ± 2%.

Effective atomic numbers for photon energy absorption of some low- Z substances of dosimetric interest

Effective atomic numbers for photon energy absorption (Z pea eff) and effective atomic numbers for photon interaction (Z pi eff) of some low- Z substances of dosimetric interest such as A-150 tissue-equivalent plastic (A150TEP), alanine, bakelite, Gafchromic sensor (GS), plastic scintillator (PS), polyethylene, mylar, polystyrene, perspex, radiochromic dye film nylon base (RDF : NB), tissue-equivalent gas-methane based (TEG : MB) and tissue-equivalent gas-propane based (TEG : PB) have been calculated by a direct method in the energy region of 1 keV–20 MeV. Experimental mass attenuation coefficients and Z pi eff of some of these substances at selected photon energies of 26.34, 33.2, and 59.54 keV have been obtained and compared with theoretical values. The Z pea eff and Z pi eff values steadily increases up to 6–15 keV, and then they steadily decrease up to 600–1500 keV for all the substances studied. From 1.5 MeV, the values increases with increase in energy up to 20 MeV. Significant differences up to 33.68% exist between Z pi eff and the Z pea eff in the energy region of 10–150 keV. The single effective atomic numbers obtained using the program XMuDat (Z xmudat eff ) are found to be significantly higher compared to those of Z pea eff and Z pi eff values in the entire energy of interest for all the substances studied. The directly calculated Z pea eff and Z pi eff values vary with energy compared to the energy-independent effective atomic numbers predicted by various theoretical expressions. The effects of absorption edges on effective atomic numbers and their variation with photon energy and the possibility of defining two set values of effective atomic numbers below the absorption edges of elements present in the composite substances are discussed.

Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: applications for neutron capture therapy.

252Cf is a neutron emitting radioisotope which has promise for both standard brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the thermal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of 252Cf applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed manner on multiple workstations using MCNP. Three studies were executed: (1) relative phi(th) as a function of distance from a 252Cf AT source in an A-150 tissue equivalent plastic phantom using thermoluminescent dosimeters (TLDs) of varying 6Li/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two 252Cf AT sources, and (3) calculations of the impact of phantom material composition (e.g., A-150, water, brain, muscle) on phi(th) from moderated 252Cf fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed within 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measurements followed the ASTM E 262-86e protocol, and the ratio of activated plain and Cd encased gold foils (7.31) agreed well with the calculated ratio (7.26). Measured phi(th) at 5 cm (1.70+/-0.10 x 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 x 10(7) n cm(-2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 24% less, respectively, at a distance of 5 cm from the source in a 15 cm diameter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 x 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm.

The potential use of polymer gel dosimetry in boron neutron capture therapy

Polymer gels with and without 60 ppm of 10B were exposed to an epithermal neutron beam produced by the Dynamitron at the University of Birmingham on two separate occasions. Eight vials containing the gel, four with and four without boron, were irradiated in pairs in a water phantom for 5 h. The maximum dose was calculated to be 9 Gy in A-150 tissue equivalent plastic, 4 cm deep in the phantom. Measurements were made of the variation of relaxation rates of the gels with depth in a phantom. These were compared with calculations using the MCNP Monte Carlo program and the gel response followed the general trend of the results of the calculations. The calculations showed that the absence of boron gave 66.1% and 44.3% of the absorbed dose with boron and the measurements showed the response of the gel without boron to give 65±2% and 41±6% of the response with boron for the two halves of the first vial. All the gel measurements showed an enhancement in absorbed dose when boron was added. These results indicate that polymer gels may have a role in measuring the enhancement of absorbed dose due to boron in an epithermal or thermal neutron.

Use of low-pressure tissue equivalent proportional counters for the dosimetry of neutron beams used in BNCT and BNCEFNT.

The absorbed dose in a phantom or patient in boron neutron capture therapy (BNCT) and boron neutron capture enhanced fast neutron therapy (BNCEFNT) is deposited by gamma rays, neutrons of a range of energies and the 10B reaction products. These dose components are commonly measured with paired (TE/Mg) ion chambers and foil activation technique. In the present work, we have investigated the use of paired tissue equivalent (TE) and TE+ l0B proportional counters as an alternate and complementary dosimetry technique for use in these neutron beams. We first describe various aspects of counter operation, uncertainties in dose measurement, and interpretation of the data. We then present measurements made in the following radiation fields: An epithermal beam at the University of Birmingham in the United Kingdom, a d(48.5) + Be fast neutron therapy beam at Harper Hospital in Detroit, and a 252Cf radiation field. In the epithermal beam, our measured gamma and neutron dose rates compare very well with the values calculated using Monte Carlo methods. The measured 10B dose rates show a systematic difference of approximately 35% when compared to the calculations. The measured neutron+gamma dose rates in the fast neutron beam are in good agreement with those measured using a calibrated A-150 TEP (tissue equivalent plastic) ion chamber. The measured 10B dose rates compare very well with those measured using other methods. In the 252Cf radiation field, the measured dose rates for all three components agree well with other Monte Carlo calculations and measurements. Based on these results, we conclude that the paired low-pressure proportional counters can be used to establish an independent technique of dose measurement in these radiation fields.

Dosimetry of the boron neutron capture reaction for BNCT and BNCEFNT.

The use of paired proportional counters, constructed from A-150 tissue equivalent plastic (TEP) and A-150 TEP loaded with an appropriate amount of 10B (50 to 200 ppm), for the dosimetry of the boron neutron capture reaction has been investigated for several years at the Gershenson Radiation Oncology Center. This method has been used for determining the dose components (fast neutron, gamma ray and boron capture product dose) in both Boron Neutron Capture Therapy (BNCT) beams and in beams proposed for boron neutron capture enhancement of fast neutron therapy (BNCEFNT). A disadvantage of this method, when standard 1/2" diameter Rossi type proportional counters are used, is that the beam intensity must be relatively low in order to avoid saturation effects (pulse pile-up) in the counter. This is a major problem if measurements are to be made in a reactor beam, since reducing the beam intensity generally results in a change in the neutron spectrum. In order to overcome this problem, miniature cylindrical proportional counters have been developed which may be used in high intensity beams. The operational characteristics of these counters are compared with the standard 1/2" spherical counters. A further disadvantage of proportional counters is the relatively long time it takes to collect data, particularly if detailed information (depth-dose curves and beam profiles) is required. This problem could be overcome by using a set of ionization chambers (an A-150 TEP chamber, a Mg chamber and a Mg chamber with a 25 microns boron loaded inner wall) which can be scanned in a water phantom. After calibration against the paired proportional counters it should be possible to extract the fast neutron, gamma ray and boron neutron capture product doses from measurements made with these three ionization chambers. A set of such chambers has been used to make preliminary measurements in a fast neutron beam and the results of these measurements are presented.

A consistent set of neutron kerma coefficients from thermal to 150 MeV for biologically important materials.

Neutron cross sections for nonelastic and elastic reactions on a range of elements have been evaluated for incident energies up to 150 MeV. These cross sections agree well with experimental cross section data for charged-particle production as well as neutron and photon production. Therefore they can be used to determine kerma coefficients for calculations of energy deposition by neutrons in matter. Methods used to evaluate the neutron cross sections above 20 MeV, using nuclear model calculations and experimental data, are described. Below 20 MeV, the evaluated cross sections from the ENDF/B-VI library are adopted. Comparisons are shown between the evaluated charged-particle production cross sections and measured data. Kerma coefficients are derived from the neutron cross sections, for major isotopes of H, C, N, O, Al, Si, P, Ca, Fe, Cu, W, Pb, and for ICRU-muscle, A-150 tissue-equivalent plastic, and other compounds important for treatment planning and dosimetry. Numerous comparisons are made between our kerma coefficients and experimental kerma coefficient data, to validate our results, and agreement is found to be good. An important quantity in neutron dosimetry is the kerma coefficient ratio of ICRU-muscle to A-150 plastic. When this ratio is calculated from our kerma coefficient data, and averaged over the neutron energy spectra for higher-energy clinical therapy beams [three p (68) + Be beams, and a d (48.5) + Be beam], a value of 0.94 +/- 0.03 is obtained. Kerma ratios for water to A-150 plastic, and carbon to oxygen, are also compared with measurements where available.

The effect of delta rays on the ionometric dosimetry of proton beams

The interface effects arising in the measurement of absorbed dose by ionization chambers, owing to the inhomogeneity between the walls and the gas, have been evaluated by an analytical model. The geometrical situation considered here is appropriate for representing the behaviour of a plane-parallel ionization chamber exposed to a radiotherapeutic beam of protons. Two gases, dry air and tissue equivalent gas (methane based), as well as six materials commonly used in ionization chamber walls, i.e. graphite, A-150 tissue equivalent plastic, C-522 air equivalent plastic, nylon type 6, polymethyl methacrylate and polystyrene, have been examined. The analysis of the results shows that, within the limits of the detector dimensions and proton energies commonly used in the dosimetry of radiotherapeutic beams, these effects, if not taken into account in the measurement interpretation, can entail deviations of up to about 2% with respect to the correct absorbed dose in gas.