Backscatter Dose Research Articles

Measurement and modeling of the effect of support arm backscatter on dosimetry with a Varian EPID

Amorphous silicon EPIDs have been used for planar dose verification in IMRT treatments for many years. The support arm used to attach some types of EPIDs to linear accelerators can introduce inaccuracies to dosimetry measurements due to the presence of metallic parts in their structures. It is demonstrated that this uncertainty may be as large as approximately 6% of maximum image signal for large fields. In this study, a method has been described to quantify, model and correct for the effect of backscattered radiation from the EPID support arm (E-Arm type, Varian Medical Systems). Measurements of a support arm backscatter kernel were made using several 1 x 1 cm2 6 MV pencil beam irradiations at a sample of positions over the sensitive area of the EPID in standard clinical setup and repeated with the EPID removed from the support arm but at the same positions. A curve-fit to the subtraction of EPID response obtained on and off the arm was used to define the backscatter kernel. The measured kernel was compared with a backscatter kernel obtained by Monte Carlo simulations with EGS/BEAM code. A backscatter dose prediction using the measured backscatter kernel was added to an existing EPID dose prediction model. The improvement in the agreement of the modified model predictions with EPID measurements for a number of open fields and IMRT beams were investigated by comparison to the original model results. Considering all functions tested to find the best functional fit to the data points, a broad Gaussian curve proved to be the optimum fit to the backscatter data. The best fit through the Monte Carlo simulated backscatter kernel was also found to be a Gaussian curve. The maximum decrease in normalized root mean squared deviation of the measured and modeled EPID image profiles for open fields was 13.7% for a 15 x 15 cm2 field with no decrease observed for a 3 x 3 cm2 (the smallest) field as it was not affected by the arm backscatter. Gamma evaluation (2%, 2 mm criteria) showed the improvement in agreement between the model and measurement results when the backscatter was incorporated. The average increase in Gamma pass rate was 2% for head and neck and 1.3% for prostate IMRT fields investigated in this study. The application of the backscatter kernel determined in this study improved the accuracy of dosimetry using a Varian EPID with E-arm for open fields of different sizes: Eight head and neck and seven prostate IMRT fields. Further improvement in the agreement between the model predictions and EPID measurements requires more sophisticated modeling of the backscatter.

Dosimetric characterisation of bismuth shields in CT: measurements and Monte Carlo simulations

Although bismuth shields are frequently used in radiology to reduce radiation dose, its mechanism has not been fully investigated. Dosimetric characteristics of bismuth shields in computed tomography (CT) were assessed with ion chamber and Monte Carlo (MC) simulations. Primary attenuation and backscatter effects of paediatric (2-ply) and adult (4-ply) bismuth shields were measured. Simulated CT beams were used for ion chamber measurements. Radiation doses were measured free-in-air and in the tissue-equivalent slabs. MC simulations for the same settings were also performed. Mean dose reductions from primary attenuation were 23% (2-ply) and 40% (4-ply). The dose increase from backscatter was 2% for both shields. MC simulations for primary beam dose reduction were 20% (2-ply) and 38% (4-ply); the backscatter dose increase was around 6% for both shields. In summary, primary attenuation is the major factor that introduces the dose reduction in bismuth and the dose increase from backscatter is negligible.

Effect of dental restorations and prostheses on radiotherapy dose distribution: a Monte Carlo study

Dental restorations, fixed prosthodontics, and implants affect dose distribution in head and neck radiation therapy due to the high atomic number of the materials utilized. The backscatter of electrons from metallic materials due to the impinging treatment x‐ray results in localized dose enhancements. These dose enhancements cause localized mucositis in patients who have dental work, a significant clinical complication. We investigated the backscatter effect of 23 configurations of dental work using the EGS4nrc Monte Carlo (MC) simulation system. We found that all‐metal fixed partial dentures caused the highest amount of dose enhancement – up to 33% – while amalgam restorations did not cause a significant amount. Restorations with a ceramic veneer caused up to 8% enhancement. Between 3 mm and 5 mm of water‐equivalent material almost completely absorbed the backscatter. MC simulations provide an accurate estimate of backscatter dose, and may provide patient‐specific estimates in future.

Radiochromic film dosimetry: Reflection vs transmission scanning

The objective of this study was to compare the transmission and the reflection scanning reading modes of radiochromic MD-55 films of a document scanner. The use of the red channel of the red-green-blue images results in measurement accuracy that did not differ between the two reading modes. On the other hand, the scanner uniformity and temporal stability, the long-term stability of the film readings and the dynamic range is superior using the reflection mode. The dynamic range of dose measurements can be extended up to least 500 Gy using the alternative color channels by both reading techniques. Similar backscattering dose factors close to high Z inhomogeneities were found using both scanning modes. In conclusion, the use of reflection scanning mode of MD55 films was superior to the traditionally used transmission mode.

Calculated effects of backscattering on skin dosimetry for nuclear fuel fragments

The size of hot particles contained in nuclear fallout ranges from 10 nm to 20 microm for the worldwide weapons fallout. Hot particles from nuclear power reactors can be significantly bigger (100 microm to several millimetres). Electron backscattering from such particles is a prominent secondary effect in beta dosimetry for radiological protection purposes, such as skin dosimetry. In this study, the effect of electron backscattering due to hot particles contamination on skin dose is investigated. These include parameters such as detector area, source radius, source energy, scattering material and source density. The Monte-Carlo Neutron Particle code (MCNP4C) was used to calculate the depth dose distribution for 10 different beta sources and various materials. The backscattering dose factors (BSDF) were then calculated. A significant dependence is shown for the BSDF magnitude upon detector area, source radius and scatterers. It is clearly shown that the BSDF increases with increasing detector area. For high Z scatterers, the BSDF can reach as high as 40 and 100% for sources with radii 0.1 and 0.0001 cm, respectively. The variation of BSDF with source radius, source energy and source density is discussed.

TU‐C‐L100J‐03: Radiation Dose Reduction in Interventional Cardiology Using a Fixed Circular Collimator Attached to Cardiovascular Angiography System with Flat Panel Detector

Purpose: Fixed Circular Collimator employed in cardiovascular angiography system with flat panel detector was developed to reduce radiation dose to both patient and cardiologist in coronary intervention. This study was designed to assess the radiation dose reduction using this new collimator. Method and Materials: This new collimator makes the radiation field circular and has tapered aluminum to reduce radiation dose at peripheral field.1. Entrance air kerma with backscatter (patient) and personnel dose equivalent (Hp) to cardiologist (scattered radiation dose) were measured to verify the radiation dose reduction using fixed circular collimator under a fixed geometric arrangement. The PMMA phantom was employed to simulate the patient thickness varying from 1.0 cm to 35.0 cm in increments of 1.0 cm. Upon completion of the data acquisition, entrance air kerma and Hp to cardiologist were plotted against the phantom thickness.2. Entrance skin regions (patient) were imaged to verify the overlap region reduction under ten routine angulations (RAO30, RAO10‐CAU30, RAO30‐CAU30, RAO30‐CRA30, LAO10‐CRA30, LAO20‐CRA30, LAO30‐CRA30, LAO45‐CRA30, LAO45, and LAO45‐CAU30). All tabletop position for every angulations were set up to image the heart center of chest phantom, subsequently, chest phantom was replaced by computed radiography and irradiated 1 sec. Results: Entrance air kerma and Hp to cardiologist were reduced by 8.0 % and 47.6 %, respectively. On the other hand, the overlapping region of skin was also reduced considerably. Conclusions: Due to the use of fixed circular collimator, entrance air kerma, the overlap region of exposed skin and Hp to cardiologist were decreased concurrently. It was considered that the use of fixed circular collimator makes the lower density of backscatter and region of interest optimize. Therefore, rectangular images should be reconsidered for safer coronary intervention. Although this study doesn't include image quality evaluation, cardiologists adopt this new collimator in clinical study currently.

SU‐FF‐T‐107: Assessment of Dental Amalgam Backscatter with a Beacon Transponder Embedded Mouthpiece for Real‐Time Tracking During Head and Neck IMRT

Purpose: A mouthpiece embedded with Beacon® transponders (Calypso Medical) for continuous localization and tracking during IMRT head and neck irradiation was evaluated for backscatter to the oral mucosa. Dental amalgam can generate excessive backscatter dose to the oral mucosa during treatment. Study objectives were to measure backscatter dose from amalgams, transponders and mouthpiece relative to the oral mucosa. Method and Materials: A dental phantom having 16 of 28 teeth containing amalgam, and a prototype acrylic mouthpiece embedded with 3 transponders (1mm depth) were fabricated. To simulate overlying oral mucosa, a 1.5cm bolus was placed over the dental phantom and mouthpiece for baseline measurements. A TN502RD MOSFET dosimeter and TLD chips (3×3×1mm) with a 6 MV photon beam 10×10 cm2 field (Elekta Precise) with and without mouthpiece were used for measurements. Backscatter was measured with and without the mouthpiece at a source‐to‐detector distance of 100 cm in a single field. To assess dosimetric effects of amalgam in the presence of the mouthpiece, a parallel‐opposed field beam setup was used. Results: Standard amalgam causes dose increases of 24–29% at the tissue interface; however, backscatter decreases rapidly to 7–8% at 0.8mm depth. MOSFET and TLD measurements show the mouthpiece reduced backscatter dose by 15% (at 0.4 mm) for single beam and 6–8% for parallel‐opposed beams. The transponders (1.8mm × 8.5mm) in the mouthpiece do not generate measurable additional backscatter. Conclusion: A mouthpiece when used for continuous target localization and tracking, can effectively reduce backscatter dose caused by amalgams, potentially decreasing mucositis. Embedded transponders do not generate measurable backscatter dose. Conflict of Interest: Mate and Zeller have a financial interest in Calyspo Medical.

TH‐E‐224C‐06: The Effect of Dental Restorations and Fixed Prosthodontics On Radiation Therapy Dose Distribution: A Monte Carlo Study

Purpose: To investigate the effect of dental restorations, fixed prosthodontics, and implants on dose distributions in head and neck radiation therapy using Monte Carlo simulations. Specifically, we seek to understand how to prevent localized mucositis caused by backscatter dose. Method and Materials: Simplified models of a range of dental restorations, fixed prosthodontics, and implants were constructed using a representative sample of materials and configurations. These models were irradiated with a simulated 6MV lateral beam. The resulting dose distributions were compared against dose distributions on models without dental work. Results: Exposed dental alloy (Au‐Pd) caused the most significant amount of backscatter, and corresponding hot spots in the dose distribution. Dental alloy which was surrounded by porcelain also caused backscatter hot spots, although lower compared to exposed metal. These backscatter effects do not appear in pencil beam dose calculations. This work showed that backscatter from dental work caused a dose enhancement of up to 40% at a distance of 1mm in the upstream direction for exposed metal surfaces. The dose enhancement from porcelain‐veneered materials was up to 20% 1mm from the surface. The smaller enhancement was attributed to absorbtion within the ceramic veneer. Isodose lines for the backscatter formed a contour roughly conforming to the shape of the dental work. Beyond 3mm from the surface of the prosthodontic device, the dose enhancement had completely decayed. Conclusion: The metal content of dental restorations and fixed prosthodontics create significant enhanced dose to adjacent soft tissue. This is a major cause of morbidity. Since we have shown that the enhancement decays at a distance of about 3mm in tissue‐equivalent material, we may reduce the likelihood and intensity of mucositis by displacing the soft tissue from the teeth with 3mm of tissue‐equivalent material.

Evaluation of scatter dose of dental titanium implants exposed to photon beams of different energies and irradiation angles in head and neck radiotherapy

In this study, quantification of backscatter doses between scattering titanium dental implant and the thermoluminescent dosimeter (TLD(100)) radiation detector at axial beam irradiation-angle range of 0-90 degrees in head and neck radiotherapy is done to evaluate irradiation angle dependency of dose enhancement contributing to osteoradionecrosis. A cylindrical titanium dental implant with diameter of 4 mm and length of 9 mm was implanted into a specially-designed human mandible phantom with a TLD100 chip placed on the buccal site and irradiated with 6 MV X, 25 MV X and Co-60 gamma sources at 19 different irradiation angles. Dose enhancement on a buccal site of the titanium implant depends on the incident beam angle. At angles of 65 degrees, 60 degrees and 40 degrees the maximum detected scatter doses over the titanium implant are 36%, 32% and 23% for Co-60 gamma, 6 MV X-ray and 25 MV X-ray, respectively. The dose enhancement at different beam angles was less pronounced in 25 MV X and more pronounced in Co-60 gamma irradiation. For the different radiation beams studied, the irradiation angle between scattering titanium dental implants and the central axis does not significantly affect the total dose that may lead to osteoradionecrosis of the mandible.

A survey of patient dose and clinical factors in a full-field digital mammography system

In this work, we analyse the dose to 5034 patients (20 137 images) who underwent mammographic examinations with a full-field digital mammography (FFDM) system over a 2-y period. The information relevant to this study has been extracted from the image Digital Imaging and Communications in Medicine (DICOM) headers. Entrance surface air-kerma (ESAK) without backscatter and average glandular dose (AGD) were estimated following the methodology proposed in the European Protocol on Dosimetry in Mammography. Mean values for patient age and compressed breast thickness were 56 +/- 11 y and 52 +/- 13 mm, respectively. The mean ESAK value was 8.1 mGy and the mean AGD was 1.9 mGy. In addition, the dose values from both FFDM and screening-film mammographic (SFM) examinations were compared. The third quartile (TQ) of the ESAK values delivered by the FFDM system was 33% lower and 32% higher than the TQ for SFM with slow and fast screen/film receptors. Differences between dose values for cranio-caudal (CC) and medio-lateral oblique (MLO) images (about 27% for SFM) decreased to 11% for FFDM.

Backscatter and Dose Perturbations for Low- to Medium-Energy Electron Point Sources at the Interface between Materials with Different Atomic Numbers

Electron backscatter at interfaces between dissimilar media can affect dosimetry and should be taken into consideration in radiotherapy and in radiobiology experiments. Backscatter dose perturbations depend upon factors such as electron energy, medium atomic number (Z), and distance from the interface. This study quantifies the backscatter dose factor (BSDF) for electron point sources of energy between 0.1 to 3 MeV in water at the interface with scattering materials ranging in Z from (13)Al to (79)Au. A Monte Carlo code that performs dose calculations for monoenergetic and continuous-spectrum electron sources was developed using EGSnrc transport routines. The BSDF was quantified in a parallel layers geometry (BSDF(1D)) and three-dimensional voxel geometry (BSDF(3D)). The BSDF(1D) near the interface increased up to 52% with decreasing energy from 3 to 0.1 MeV and increasing Z from 13 to 79. The analysis of the BSDF(3D) showed a significant dependence of the scattered electron angular distribution on Z and energy, with a decrease in isotropy going from high to low Z. This effect proves the importance of considering the correct geometry when quantifying the BSDF for electron sources, especially when the dimensions of the relevant dose-collecting volume are comparable with the CSDA range of the source.

A Monte-Carlo Method for Interface Dosimetry of Beta Emitters

Biologically targeted radiotherapy optimization requires accurate dose estimation, from macroscopic to cellular/subcellular dimensions. In particular, dose perturbations produced at interfaces between dissimilar media could affect therapy outcome. The magnitude of these perturbations depends on a complex set of parameters. This study investigates perturbations on electron dose for materials with atomic numbers (Z) up to 79 (79Au) at their interface with water as a function of Z, energy, distance from interface and geometry. A Monte-Carlo method that produces absorbed dose distributions in a voxel geometry has been developed using EGSnrc transport routines. Heterogeneous media and activity distributions can be input into this code. The backscatter dose factor (BSDF), which quantifies interface dose perturbations, was estimated using this code. The BSDF magnitude ranged from approximately 3% to approximately 50%, depending on source energy and Z. The BSDF decreased with increasing energy and showed a logarithmic dependence on Z. Empirical functions were fit to the results, that could be used to correct dose calculations performed using dose-point-kernels estimated in water, to cases involving different scattering materials. The BSDF was found to be highly dependent on interface geometry and scoring volume; thus it is vital that BSDFs are used only in geometry conditions that are similar to those in which they were originally produced.

Dose perturbations at high- Z interfaces in kilovoltage photon beams: comparison with Monte Carlo simulations and measurements

Dose perturbations, backscatter dose perturbation factor (BSDF), and forward dose perturbation factor (FDPF) near high- Z material interfaces have been reported to be intense and significant in kilovoltage photon beams. Validity of estimation of dose perturbations is critical in the evaluation of radiation effects associated with high- Z interfaces. The magnitude of dose perturbations has been debated due to limitations in the measuring devices (mainly window thickness and chamber perturbations). Monte Carlo (MC) simulations have been proposed for the interface effects but poor statistics in small spatial bins (1 μm) near the interface makes MC data questionable even with a well designed code. A moving-window least-square polynomial fit (MLPF) method is proposed to smooth MC simulated data. This method is shown to be useful in achieving reasonable accuracy from statistically poor MC data obtained within a reasonable computation time. The EGS4 and PENELOPE MC simulated data at the interfaces between high- Z materials and a water phantom have been used to calculate the BSDF and FDPF which differ significantly from the measurements suggesting the necessity of further refinements in MC approaches and better measuring devices for interface effects.

Backscatter dose from metallic materials due to obliquely incident high-energy photon beams.

If metallic material is exposed to ionizing radiation of sufficient high energy, an increase in dose due to backscatter radiation occurs in front of this material. Our purpose in this study was to quantify these doses at variable distances between scattering materials and the detector at axial beam angles between 0 degree (zero angle in beams eye view) and 90 degrees. Copper, silver and lead sheets embedded in a phantom of perspex were exposed to 10 MV-bremsstrahlung. The detector we developed is based on the fluorescence property of pyromellitic acid (1,2,4,5 benzenetetracarboxylic acid) after exposure to ionizing radiation. Our results show that the additional doses and the corresponding dose distribution in front of the scattering materials depend quantitatively and qualitatively on the beam angle. The backscatter dose increases with varying beam angle from 0 degree to 90 degrees up to a maximum at 55 degrees for copper and silver. At angles of 0 degree and 55 degrees the integral backscatter doses over a tissue-equivalent depth of 2 mm are 11.2% and 21.6% for copper and 24% and 28% for silver, respectively. In contrast, in front of lead there are no obvious differences of the measured backscatter doses at angles between 0 degree and 55 degrees. With a further increase of the beam angle from 55 degrees to 90 degrees the backscatter dose decreases steeply for all three materials. In front of copper a markedly lower penetrating depth of the backscattered electrons was found for an angle of 0 degree compared to 55 degrees. This dependence from the beam angle was less pronounced in front of silver and not detectable in front of lead. In conclusion, the dependence of the backscatter dose from the angle between axial beam and scattering material must be considered, as higher scattering doses have to be considered than previously expected. This may have a clinical impact since the surface of metallic implants is usually curved.

Mammographic Dosimetry Using MCNP-4B

Three doses involved in the female breast, Adipose, Glandular and Total Doses, were calculated for different adipose tissue ratio and four breast thickness using MCNP-4B code. Glandular Dose is the most important contribution to the breast carcinogenesis risk. As breast composition changes with woman age, a function that relates age with dose has been estimated. For all thickness, the calculated results have shown the dependence of both parameters. A different behaviour of Adipose and Glandular doses for different four breast thickness has been observed, basically due to backscattering dose in the outer adipose tissue region and the thickness-distance X-ray source effect.

Dose Profile Assessment at Gold-Tissue Interfaces Using TSEE

The radiological and clinical significance of dose distributions in the vicinity of media interfaces in radiotherapy and the complex nature of these dose distributions have long been recognised. The dosimetry under these conditions, however, is not simple, and even small dosemeters like TLDs are not adequate for it. A possible dosimetry method for dose profile assessment near interfaces is the use of the so-called thermally stimulated exoelectron emission (TSEE) dosemeter. In this work the possibility of using BeO TSEE dosemeters to assess the forward and back scatter dose profile at the interface soft tissue-gold was investigated for the diagnostic heavily filtered X ray spectrum A-60 of ISO Standard A quality. Dose profiles are presented. Dose enhancement factors as high as 80 can be measured.

TLD Responses Backed by Copper, Silver or Gold for 20-40 keV Monoenergetic Photons

Doses in TLDs backed by copper, silver or gold foils were measured using 20, 30 and 40 keV monoenergetic photons from synchrotron radiation. The TLDs used were LiF TLDs of 0.38 mm thickness and Li 2 B 4 O 7 :Cu of 90 μm thickness, and for the backing foils various thicknesses were used. The backscattering dose factor, which was defined as the ratio of the dose in the TLD with backing to that in free air, was found to become larger with decreasing TLD thickness and increasing foil thickness and photon energy. The dose eliminating the electrons from the foils was also measured by inserting a thin Teflon film between the TLDs and the foils. Compared with EGS4 Monte Carlo calculations, it was confirmed that the influence of the electrons became larger with decreasing TLD and foil thickness and increasing photon energy and atomic number of the foils.

Evaluation of Eye Shields Made of Tungsten and Aluminum in High-Energy Electron Beams

Purpose: To protect the lens and cornea of the eye when treating the eyelid with electrons, we designed a tungsten and aluminum eye shield that protected both the lens and cornea, and also limited the amount of backscatter to the overlying eyelid when using electron beam therapy. Methods and Materials: Custom curved tungsten eye shields, 2 mm and 3 mm thick, were placed on Kodak XV film on 8 cm polystyrene and irradiated to evaluate the transmission through the shields. To simulate the thickness of the eyelid and to hold the micro-TLDs, an aquaplast mold was made to match the curvature of the eye shields. Backscatter was measured by placing the micro-TLDs on the beam entrance side to check the dose to the underside of the eyelid. Measurements were done with no aluminum, 0.5, and 1.0 mm of aluminum on top of the tungsten eye shields. The measurements were repeated with 2- and 3-mm flat pieces of lead to determine both the transmission and the backscatter dose for this material. Results: Tungsten proved to be superior to lead for shielding the underlying structures and for reducing backscatter. At 6 MeV, a 3-mm flat slab of tungsten plus 0.5 mm of aluminum, resulted in .042 Gy under the shield when 1.00 Gy is delivered to d max. At 6 MeV for a 3-mm lead plus 0.5-mm aluminum, .046 Gy was measured beneath the shield, a 9.5% decrease with the tungsten. Backscatter was also decreased from 1.17 to 1.13 Gy, a 4% decrease, when using tungsten plus 0.5 mm of aluminum vs. the same thickness of lead. Measurements using 9 MeV were performed in the same manner. With 3 mm tungsten and 0.5 mm of aluminum, at 3 mm depth the dose was .048 Gy compared to .079 Gy with lead and aluminum (39% decrease). Additionally, the backscatter dose was 3% less using tungsten. Simulating the lens dose 3 mm beyond the shield for the 2-mm and 3-mm custom curved tungsten eye shields plus 0.5 mm of aluminum was .030 and .024 Gy, respectively, using 6 MeV (20% decrease). Using 9-MeV electrons, the dose 3 mm beyond the shield was .048 Gy for the 2-mm shield and .029 Gy for the 3-mm shield (40% decrease). Backscatter was not further decreased using thicker tungsten. With a 6-MeV beam, using the 2-mm or 3-mm custom tungsten eye shields plus 0.5 mm of aluminum, the backscattered doses were 1.03 and 1.02 Gy, respectively. The backscatter dose with 9 MeV was 1.06 Gy using the 2-mm custom shield plus 0.5 mm aluminum and 1.05 Gy with a 3-mm custom shield plus 0.5 mm aluminum. There was very little difference in backscatter dosage under the eyelid using 0.5 vs. 1.0 mm of aluminum. Therefore, for patient comfort, we recommend using 0.5 mm of aluminum. Conclusions: Tungsten is superior to lead as a material for eye shields due to its higher density and lower atomic number (Z). Using 6- and 9-MeV electrons, tungsten provides the necessary protection for the lens and cornea of the eye and decreases the amount of backscatter to the eyelid above the shield.

Forward dose perturbation at high atomic number interfaces in kilovoltage x-ray beams.

High atomic number (Z) materials such as lead, used for field shaping and shielding normal tissues in kilovoltage beams could produce significant dose enhancement in the forward direction contrary to our normal belief with respect to the attenuation of photon beams. Such a dose enhancement has not been studied in kilovoltage beams, which is investigated in this study. Using a Siemens ortho-voltage unit (60-240 kVp) and a thin window (5 microns) parallel plate ion chamber, forward dose perturbation factor (FDPF) was measured at interfaces created by high- and low-Z materials. The FDPF is defined as the ratio of doses with and without an interface (FDPF = Di/Dh; where Di is the dose at an interface and Dh is the dose in a homogeneous medium). Results indicate that dose enhancement (FDPF > 1) as high as 20-fold can be observed for a thin (> or = 0.02 mm) Pb sheet in contact with soft tissue. The magnitude of FDPF is relatively independent of field size and falls off exponentially with Pb thickness. The typical photon beam attenuation takes at a thickness > 1 mm. This intense dose enhancement is localized within 250 microns of the interface. The FDPF is energy dependent but saturates above 140 kVp, unlike the backscatter dose perturbation that peaks around 200 kVp. The FDPF varies inversely with the thickness of high Z and distance between the surface and high-Z medium. The FDPF falls off rapidly to a level of photon transmission usually predicted by exponential attenuation when distance is increased. In conclusion, with kilovoltage beam, a high-Z medium placed in contact with soft tissue may not attenuate radiation dose unless adequate thickness and proper distance between the surface and high-Z medium is used. The localized intense dose enhancement (approximately 20-fold) created by the high-Z interface could be exploited for clinical use.

Measurement of the backscatter coefficient using resist response curves for 20–100 keV electron beam lithography on Si

The effective backscatter coefficient η is a quantity that must be known with precision so that the proximity effect can be adequately compensated to minimize feature size variations in electron beam lithography. A unique technique to measure η that does not require the precise form of the backscatter dose distribution was employed. This method simply compares the resist response near the center of printed features that are much smaller and much larger than the characteristic range of the long range scatter. This technique was already employed to estimate the backscatter coefficient on Si at 100 keV beam energies. We have extended this measurement to determine η at lower beam energies. Results show that η on Si is 0.38, 0.50, 0.55, and 0.46 for 100, 50, 40, and 20 keV beam energies, respectively. Monte Carlo simulations indicate a trend of decreasing η with increasing beam energy, consistent with the experimental results except at 20 keV.