Effect of tissue inhomogeneity on beta dose distribution of 32P

In a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft-tissue-bone planar interface by a polystyrene (PST)-aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of 32P using 7LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0-31, 125-156, and 283-314 mg/cm2 separations from the interface were increased by (12 +/- 3)%, (8 +/- 2)%, and (3 +/- 2)%, respectively, compared with homogeneous PST. With the point source at a PST-air planar interface to simulate a soft-tissue-air junction, the dose rates at 0-31, 139-170, and 283-314 mg/cm2 from the interface were decreased by (25 +/- 4)%, (11 +/- 7)%, and (5 +/- 2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of 32P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two-group method of calculation for a plane source is also presented.     

Interface perturbation effects in high-energy electron beams

Near interfaces between two different media exposed to high-energy electron beams substantial dose and fluence perturbations due to backscatter can be observed. In this work, dose and fluence perturbations were studied for 4-19 MeV electron beams at backscatter interfaces of polystyrene, graphite, water, aluminium and lead. Measurements of relative dose using an NPL-designed thin-window plane-parallel ion chamber and a Markus ion chamber were performed to determine the effect of different interface materials and thicknesses. Results of Monte Carlo simulations with the EGSnrc code, including models of the ion chambers, were found to be in excellent agreement with the measurements. The well-documented increasing dose perturbation with increasing effective atomic number of the backscatter material and decreasing electron beam energy was confirmed. Simulations in a simplified slab geometry showed that, despite the decrease of average electron energy with depth in water, the dose perturbations decrease with increasing depth of the interface in water for all the materials in the study. This was ascribed to the change of the electron angular distribution with depth which has a different effect in water and in the presence of a high-Z interface. Electron fluence perturbations near a lead/water interface were found to cause small differences in unrestricted mass collision stopping power ratios, water to air. Effects of bremsstrahlung photons, characteristic photons and positrons from the backscattering material were found to be insignificant for electron interface dosimetry. When comparing simulations using EGSnrc and the older version of the same code, EGS4, underestimations of the dose perturbation effects of up to 7% were found when using the latter code to simulate 4 MeV electrons irradiating a lead/water interface. It is concluded that EGSnrc is a highly suitable tool for electron interface dosimetry studies.     

Dosimetric characteristics of a linear array of gamma or beta-emitting seeds in intravascular irradiation: Monte Carlo studies for the AAPM TG-43/60 formalism

In principle, the AAPM TG-43/60 formalism for intravascular brachytherapy (IVBT) dosimetry of catheter-based sources is fully valid with a single seed of cylindrical symmetry and in the region comparable to or larger than the mean-free path of emitting radiation. However, for the geometry of a linear array of seeds within the few millimeter range of interest in IVBT, the suitability of the AAPM TG-43/60 formalism has not been fully addressed yet. We have meticulously investigated the dosimetric characteristics of catheter-based gamma (192Ir) and beta (90Sr/Y) sources using Monte Carlo methods before applying the AAPM TG-43/60 formalism. The dosimetric perturbation due to radiation interactions with neighboring seeds is at most 2% over the entire region of interest for the 192Ir source, while it increases to about 5% for the 90Sr/Y source. As the transaxial distance (y) increases beyond 3 mm, the sum of the dose contributions from neighboring seeds exceeds the dose contribution from the center seed for both sources. However, it continues to increase with the increasing y for 192Ir but is saturated beyond y = 5 mm for 9Sr/Y. Even within a few millimeters from the seeds, the dose from the low-energy betas of 192Ir is still less than 1% of the total dose. The radial dose and anisotropy functions are reformulated in reduced cylindrical coordinate with the reference point at y = 2 mm. The dose rate constant of 192Ir and the dose rate of 90Sr/Y at the reference point showed a fairly good agreement (within +/- 2%) with earlier studies and the NIST-traceable value, respectively. We conclude that the dosimetric perturbation caused by close proximity of neighboring seeds is nearly negligible so that the AAPM TG-43/60 formalism can be applied to a linear array of seeds.     

Parallel-plate ionization chamber response in cobalt-60 irradiated transition zones

The authors study the acceptance of a Capintec parallel-plate ionization chamber. The Capintec chamber is used for dose measurements in a lead and polystyrene slab phantom irradiated with cobalt-60 gamma rays. The authors define an enhancement ratio to quantify the dose measurements. The enhancement ratio equals the ratio of dose measured with the lead slab present to dose measured under equilibrium conditions in polystyrene at equal primary beam attenuation. The measured enhancement ratio at the exit side of the lead/polystyrene interface is 25% lower than the Monte Carlo predicted enhancement ratio. The authors propose that geometric acceptance limitations of the Capintec chamber to large-angle, low-energy electrons are the cause for this difference. A Monte Carlo simulation of the Capintec chamber acceptance confirms the hypothesis.     

Dose perturbations due to contrast medium and air in mammosite treatment: an experimental and Monte Carlo study

In the management of early breast cancer, a partial breast irradiation technique called MammoSite (Proxima Therapeutic Inc., Alpharetta, GA) has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system (Nucletron PLATO system). For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution (in the range 5%-25% by volume) with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances <1 mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances > or = 10 mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface (<1 mm) due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5% per cm3 of air. Monte Carlo simulation suggests that the interface effect (enhanced dose near surface) is primarily due to Compton electrons of short range (<0.5 mm). For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system.     

Monte Carlo studies of the exit photon spectra and dose to a metal/phosphor portal imaging screen

The energy spectra and the dose to a Cu plate/Gd2O2S phosphor portal imaging detector were investigated for monoenergetic incident beams of photons (1.25, 2, and 5 MeV). The Monte Carlo method was used to characterize the influence of the patient/detector geometry, detector material and design, and incident beam energy on the spectral distribution and the dose, at the imaging detector plane, of a photon beam scattered from a water phantom. The results show that radiation equilibrium is lost in the air gap and that, for the geometries studied, this effect led to a reduction in the exit dose of up to 40%. The finding that the effects of the air gap and field size are roughly complementary has led to the hypothesis that an equivalent field size concept may be used to account for intensity and spectral changes arising from air gap variations. The copper plate preferentially attenuates the low-energy scattered photons incident on it, while producing additional annihilation, bremsstrahlung, and scattered photons. As a result, the scatter spectra at the copper surface entrance of the detector differs significantly from that at the Cu/phosphor interface. In addition, the mean scattered photon energy at the interface was observed to be roughly 0.4 MeV higher than the corresponding effective energy for 2 MeV incident beams. A comparison of the dose to various detector materials showed that exit dosimetry errors of up to 24% will occur if it is assumed that the Cu plate/Gd2O2S phosphor detector is water equivalent.     

Secondary electron fluence perturbation by high-Z interfaces in clinical proton beams: a Monte Carlo study.

Fluence perturbation of secondary electrons from clinical proton beams (50-250 MeV) by thin high-Z planar interfaces was studied with Monte Carlo simulations. Starting from monoenergetic proton pencil beams, proton depth doses and proton fluence spectra were calculated, both in homogeneous water and near thin high-Z interfaces by using the proton transport Monte Carlo code PTRAN. This code was modified extensively to enable modelling of proton transport in non-homogeneous geometries. From the proton fluence spectra in water and in the interface materials, electron generation spectra were calculated analytically and were then used as input for an electron transport calculation with the Monte Carlo code EGS4/PRESTAII to obtain electron doses and electron fluence spectra. The interface materials used in the study were graphite, Al, Ti, Cu, Sn and Au. We found significant electron fluence perturbations on both sides of the planar interfaces, resulting in an electron dose increase upstream and a decrease downstream from the interfaces, with the magnitude of the effect depending strongly on the atomic number of the interface. For the most extreme case studied, 250 MeV protons and a gold interface, we obtained an electron dose increase of 41% upstream of the interface and a decrease of 15% downstream with both perturbations having a spatial extent of about 700 microm. The total dose perturbation due to this effect amounts to a 5% increase upstream and a 2% decrease downstream. A detailed analysis of dose and fluence perturbation is presented for a wide range of materials and proton energies.     

Monte carlo simulation of the Leksell Gamma Knife: II. Effects of heterogeneous versus homogeneous media for stereotactic radiosurgery

The absence of electronic equilibrium in the vicinity of bone-tissue or air-tissue heterogeneity in the head can misrepresent deposited dose with treatment planning algorithms that assume all treatment volume as homogeneous media. In this paper, Monte Carlo simulation (PENELOPE) and measurements with a specially designed heterogeneous phantom were applied to investigate the effect of air-tissue and bone-tissue heterogeneity on dose perturbation with the Leksell Gamma Knife. The dose fall-off near the air-tissue interface caused by secondary electron disequilibrium leads to overestimation of dose by the vendor supplied treatment planning software (GammaPlan) at up to 4 mm from an interface. The dose delivered to the target area away from an air-tissue interface may be underestimated by up to 7% by GammaPlan due to overestimation of attenuation of photon beams passing through air cavities. While the underdosing near the air-tissue interface cannot be eliminated with any plug pattern, the overdosage due to under-attenuation of the photon beams in air cavities can be eliminated by plugging the sources whose beams intersect the air cavity. Little perturbation was observed next to bone-tissue interfaces. Monte Carlo results were confirmed by measurements. This study shows that the employed Monte Carlo treatment planning is more accurate for precise dosimetry of stereotactic radiosurgery with the Leksell Gamma Knife for targets in the vicinity of air-filled cavities.     

A differential method for inhomogeneity correction on dose in a photon beam

For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in 60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0 x 2.0 cm2 in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.     

Comparison of beam characteristics in intensity modulated radiation therapy (IMRT) and those under normal treatment condition

In the step-and-shoot delivery of an IMRT plan with a Siemens Primus accelerator, radiation is turned off by desynchronizing the injector while the field parameters are being changed. When the machine is ready again a trigger pulse is sent to the injector to start the beam instantaneously. The objective of this study is to investigate the beam characteristics of the machine operating in the IMRT mode and to study the effect of the Initial Pulse Forming Network (IPEN) on the dark current. The central axis (CAX) output for a 10 x 10 cm2 field over the range 1-100 MU was measured with an ion chamber in a polystyrene phantom for both 6 and 15 MV x rays. Beam profiles were also measured over the range of 2-40 MU with the machine operating in the IMRT mode and compared with those in the normal mode. By adjusting the IPFN value, dark current radiation (DCR) was measured using ion chamber measurements. For both the normal and IMRT modes, dose versus MU is nonlinear in the range 1-5 MUs. Above 5 MU, dose varies linearly with MU for both 6 and 15 MV x rays. For stability of dose profiles, the 2 MU-IM group exhibit 20% variation from one subfield to another. The variation is about 5% for the 8 MU-IM group and <5% for 10 MU and higher. The results are similar in the normal treatment mode. With the IPFN at >80% of the PFN value, a spurious radiation associated with dark current at approximately 0.7% of the dose at isocenter for a 10 x 10 cm2 field is detected during the "PAUSE" state of the accelerator for 15 MV x rays. When the IPFN is lowered to <80% of the PFN value, no DCR is detected. For 6 MV x rays, no measurable DCR was detected regardless of the IPFN setting.     

Dose errors due to charge storage in electron irradiated plastic phantoms

Commercial plastics used for radiation dosimetry are good electrical insulators . Used in electron beams, these insulators store charge and produce internal electric fields large enough to measurably alter the electron dose distribution in the plastic. The reading per monitor unit from a cylindrical ion chamber imbedded in a polymethylmethacrylate (PMMA) or polystyrene phantom will increase with accumulated electron dose, the increase being detectable after about 20 Gy of 6-MeV electrons. The magnitude of the effect also depends on the type of the plastic, the thickness of the plastic, the wall thickness of the detector, the diameter and depth of the hole in the plastic, the energy of the electron beam, and the dose rate used. Effects of charge buildup have been documented elsewhere for very low energy electrons at extremely high doses and dose rates. Here we draw attention to the charging effects in plastics at the dose levels encountered in therapy dosimetry where ion chamber or other dosimeter readings may easily increase by 5% to 10% and where a phantom, once charged, will also affect subsequent readings taken in 60Co beams and high-energy electron and x-ray beams for periods of several days to many months. It is recommended that conducting plastic phantoms replace PMMA and polystyrene phantoms in radiation dosimetry.     

Tissue inhomogeneity correction for brachytherapy sources in a heterogeneous phantom with cylindrical symmetry.

In brachytherapy it is customary to perform dose calculations for an implant assuming that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In this work, the validity of the above assumption is studied quantitatively for points along the transverse axis of 103Pd, 125I, and 241Am brachytherapy sources, using measured and Monte Carlo calculated dose rates in homogeneous and heterogeneous media with cylindrical symmetry. The irradiation geometry chosen was a single source implanted in a Solid Water phantom which had a 1- or 2-cm-thick cylindrical Solid Water shell replaced by a polystyrene shell. The Monte Carlo simulations were performed using the integrated tiger series CYLTRAN Code. Experimental data were obtained for the same geometry to test the validity of the Monte Carlo calculations for a heterogeneous phantom. Measured dose rates just beyond a 2-cm-thick polystyrene heterogeneity were observed to be greater than those in a homogeneous Solid Water phantom by about 130%, 55%, and 10% for 103Pd, 125I, and 241Am, respectively. Thus the effect of a relatively small polystyrene heterogeneity in Solid Water can be substantial for lower energy photons. This perturbation of dose was found to increase steeply with decreasing energy and increasing size (thickness) of inhomogeneity. A simple dose calculation formalism has been developed to predict dose rate in a heterogeneous phantom with cylindrical symmetry, which uses as input the radial dose functions of the uniform media comprising the heterogeneous phantom. Dose rate predictions using this formalism are in reasonable agreement with the experimental data and the Monte Carlo calculated values.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo diode dosimetry for routine quality assurance in IMRT

Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within +/- 10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved +/- 5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process.     

A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams

The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.     

Separation of dose-gradient effect from beam-hardening effect on wedge factors in photon fields

It is known experimentally that a wedge transmission factor depends upon the field size and depth of measurement in particular. Dependence of the transmission upon depth has been attributed to a hardening of the incident beam through the filter, which preferentially absorbs the low-energy photon of the bremsstrahlung component of that beam. We have attempted to separate this hardening effect from that of increased phantom scatter due to dose gradient induced by the wedge filter. Using an experimental wedge machined from cerrobend, the filter transmission at depth is measured and redefined relative to an "equally hardened" beam, obtained by filtering through a flat slab of equal thickness at the center of the wedge. Results of the Co-60, 4-, and 8-MV wedged beams indicate that nearly half of the increase in the transmission at depth is due to the effect of dose-gradient scatter in polystyrene phantom. Based on a simple relationship between primary and scattering radiation, an algebraic presentation is indeed in support of the dose gradient resulting in apparent increase in the wedge factors, at depth.     

Influence of electrodes on the photon energy deposition in CVD-diamond dosimeters studied with the Monte Carlo code PENELOPE

A new dosimeter, based on chemical vapour deposited (CVD) diamond as the active detector material, is being developed for dosimetry in radiotherapeutic beams. CVD-diamond is a very interesting material, since its atomic composition is close to that of human tissue and in principle it can be designed to introduce negligible perturbations to the radiation field and the dose distribution in the phantom due to its small size. However, non-tissue-equivalent structural components, such as electrodes, wires and encapsulation, need to be carefully selected as they may induce severe fluence perturbation and angular dependence, resulting in erroneous dose readings. By introducing metallic electrodes on the diamond crystals, interface phenomena between high- and low-atomic-number materials are created. Depending on the direction of the radiation field, an increased or decreased detector signal may be obtained. The small dimensions of the CVD-diamond layer and electrodes (around 100 microm and smaller) imply a higher sensitivity to the lack of charged-particle equilibrium and may cause severe interface phenomena. In the present study, we investigate the variation of energy deposition in the diamond detector for different photon-beam qualities, electrode materials and geometric configurations using the Monte Carlo code PENELOPE. The prototype detector was produced from a 50 microm thick CVD-diamond layer with 0.2 microm thick silver electrodes on both sides. The mean absorbed dose to the detector's active volume was modified in the presence of the electrodes by 1.7%, 2.1%, 1.5%, 0.6% and 0.9% for 1.25 MeV monoenergetic photons, a complete (i.e. shielded) (60)Co photon source spectrum and 6, 18 and 50 MV bremsstrahlung spectra, respectively. The shift in mean absorbed dose increases with increasing atomic number and thickness of the electrodes, and diminishes with increasing thickness of the diamond layer. From a dosimetric point of view, graphite would be an almost perfect electrode material. This study shows that, for the considered therapeutic beam qualities, the perturbation of the detector signal due to charge-collecting graphite electrodes of thicknesses between 0.1 and 700 microm is negligible within the calculation uncertainty of 0.2%.     

Pencil beam approach for correcting the energy dependence artifact in film dosimetry for IMRT verification

The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50 x 50 cm2 and 25 x 25 cm2 polystyrene and 30 x 30 cm2 solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to, predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%.     

Determination of contamination-free build-up for 60Co

Experimental verification of the difference between absorbed dose in tissue and the collision fraction of kerma requires precise knowledge of the absorbed dose curve, particularly in the build-up and build-down regions. A simple method for direct measurement of contamination-free build-up for 60Co, which should also be applicable for most of the photon energies commonly employed for treatment, is presented. It is shown that the contribution from air-scattered electrons to the surface dose may be removed by extrapolating measurements of build-up to zero field size. The remaining contribution to contamination from the collimators and other source-related hardware may be minimised by measuring these build-up curves sufficiently far from the source. These results were tested by measuring the build-up using a magnet to sweep scattered electrons from the primary photon beam and by measuring the surface dose in the limit of an evacuated beam path. The relative dose at zero depth in polystyrene was found to be approximately 8.9 +/- 0.3% of the dose at the depth of maximum build-up.     

Dose delivered by secondary electrons radiated by metallic objects implanted in human tissue during radiation therapy using high energy photons

This abstract summarizes the results of a research project undertaken during 1983 to 1985 to evaluate the dose to the surrounding tissue contributed by secondary electrons originating from metallic surgical sutures and total hip prosthesis implanted in human tissue, upon interaction with high energy photons during radiation therapy. To date, no such work, has been undertaken on metallic prostheses or sutures but the effect of breast prosthesis made of silicone gel, during radiation therapy with high energy photons and electrons has been reported in two research papers. In this investigation, film and TLD methods of dosimetry are used to evaluate the dose due to these secondary electrons in a polystyrene phantom. Two types of films are used: they are Dupont 7L and Dupont 6 PLUS. Calibrated beams of X-ray photons of 4 MeV and electrons of 8.6 MeV energy are used as the sources of X-ray photons and electrons. The difference in optical density with film and the difference in response in the case of TLDs with and without the metallic objects is a measure of the dose contributes by the secondary electrons. This dose is taken off from the corresponding calibration curves for film and TLDs and this dose varies from 2.5% to 6.72% to soft tissue and the dose to bone varies from 4% to 12% approximately. In certain clinical situations, this amount of dose could be quite significant. Knowing such contribution, a more effective course of radiation therapy can be planned.     

Beam range estimation by measuring bremsstrahlung

We describe a new method for estimating the beam range in heavy-ion radiation therapy by measuring the ion beam bremsstrahlung. We experimentally confirm that the secondary electron bremsstrahlung process provides the dominant bremsstrahlung contribution. A Monte Carlo simulation shows that the number of background photons from annihilation gamma rays is about 1% of the bremsstrahlung strength in the low-energy region used in our estimation (63-68 keV). Agreement between the experimental results and the theoretical prediction for the characteristic shape of the bremsstrahlung spectrum validates the effectiveness of our new method in estimating the ion beam range.