Kilovoltage Photon Beams Research Articles

An algorithm for kilovoltage x-ray dose calculations with applications in kV-CBCT scans and 2D planar projected radiographs

A new model-based dose calculation algorithm is presented for kilovoltage x-rays and is tested for the cases of calculating the radiation dose from kilovoltage cone-beam CT (kV-CBCT) and 2D planar projected radiographs. This algorithm calculates the radiation dose to water-like media as the sum of primary and scattered dose components. The scatter dose is calculated by convolution of a newly introduced, empirically parameterized scatter dose kernel with the primary photon fluence. Several approximations are introduced to increase the scatter dose calculation efficiency: (1) the photon energy spectrum is approximated as monoenergetic; (2) density inhomogeneities are accounted for by implementing a global distance scaling factor in the scatter kernel; (3) kernel tilting is ignored. These approximations allow for efficient calculation of the scatter dose convolution with the fast Fourier transform. Monte Carlo simulations were used to obtain the model parameters. The accuracy of using this model-based algorithm was validated by comparing with the Monte Carlo method for calculating dose distributions for real patients resulting from radiotherapy image guidance procedures including volumetric kV-CBCT scans and 2D planar projected radiographs. For all patients studied, mean dose-to-water errors for kV-CBCT are within 0.3% with a maximum standard deviation error of 4.1%. Using a medium-dependent correction method to account for the effects of photoabsorption in bone on the dose distribution, mean dose-to-medium errors for kV-CBCT are within 3.6% for bone and 2.4% for soft tissues. This algorithm offers acceptable accuracy and has the potential to extend the applicability of model-based dose calculation algorithms from megavoltage to kilovoltage photon beams.

Seeing the invisible: Direct visualization of therapeutic radiation beams using air scintillation

To assess whether air scintillation produced during standard radiation treatments can be visualized and used to monitor a beam in a nonperturbing manner. Air scintillation is caused by the excitation of nitrogen gas by ionizing radiation. This weak emission occurs predominantly in the 300-430 nm range. An electron-multiplication charge-coupled device camera, outfitted with an f/0.95 lens, was used to capture air scintillation produced by kilovoltage photon beams and megavoltage electron beams used in radiation therapy. The treatment rooms were prepared to block background light and a short-pass filter was utilized to block light above 440 nm. Air scintillation from an orthovoltage unit (50 kVp, 30 mA) was visualized with a relatively short exposure time (10 s) and showed an inverse falloff (r(2) = 0.89). Electron beams were also imaged. For a fixed exposure time (100 s), air scintillation was proportional to dose rate (r(2) = 0.9998). As energy increased, the divergence of the electron beam decreased and the penumbra improved. By irradiating a transparent phantom, the authors also showed that Cherenkov luminescence did not interfere with the detection of air scintillation. In a final illustration of the capabilities of this new technique, the authors visualized air scintillation produced during a total skin irradiation treatment. Air scintillation can be measured to monitor a radiation beam in an inexpensive and nonperturbing manner. This physical phenomenon could be useful for dosimetry of therapeutic radiation beams or for online detection of gross errors during fractionated treatments.

SU-E-T-555: A Model-Based Dose Calculation Algorithm for Kilovoltage X-Rays

Purpose: Current model-based dose calculation algorithms implemented in commercial radiotherapy treatment planning systems are inaccurate for kilovoltage photon beams. This work presents a new model-based dose calculation algorithm designed to provide accurate, patient-specific dose distributions for kV x-rays. Methods: This algorithm calculates the radiation dose as the sum of primary and scatter dose components. The scatter dose is calculated by convolving an empirically parameterized scatter kernel with the primary photon fluence. Several approximations are introduced to increase calculation speed: (1) the photon energy spectrum is approximated as monoenergetic; (2) density inhomogeneities are accounted for by implementing a global distance scaling factor in the scatter kernel; (3) kernel tilting is ignored. These approximations allow for efficient calculation of the scatter dose convolution with the fast Fourier transform. Medium atomic number dependent effects are accounted for by implementing a Medium-Dependent Correction, which correlates the effects of photoabsorption in bone with a surrogate quantity that is determined for an x-ray source associated with an imaging procedure. Results: The accuracy of this algorithm was tested by comparing dose distributions resulting from kV-CBCT scans and radiographs of real patients with those calculated by the Monte Carlo method. Imaging sites studied include the head-and-neck, thorax, and pelvis. For all patients studied, the differences of mean dose between the Monte Carlo Method and the model-based algorithm for kV-CBCT were within 4% for bone, and 3% for soft tissues. This is in contrast to current model-based methods used in commercial treatment planning systems that Result in underestimations exceeding 100% for bone and overestimations of 8% for soft tissues. Conclusion: This new algorithm overcomes the limitations of current model-based photon beam calculation algorithms and can be implemented in commercial treatment planning systems to extend dose calculations to kV photon beams resulting from imaging guidance procedures or therapeutic orthovoltage beams.

Photon dosimetry methods outside the target volume in radiation therapy: Optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry

Dosimetry methods outside the target volume are still not well established in radiotherapy. Luminescence detectors due to their small dimensions, very good sensitivity, well known dose and energy response are considered as an interesting approach in verification of doses outside the treated region. The physical processes of thermoluminescence (TL), radiophotoluminescence (RPL) and optically stimulated luminescence (OSL) are very similar and can be described in terms of the energy band model of electron-hole production following irradiation.This work is a review of the main dosimetric characteristics of luminescence detectors which were used in experiments performed by EURADOS Working Group 9 for in-phantom measurements of secondary radiation (scattered and leakage photons). TL LiF:Mg,Ti detectors type MTS-7 (IFJ PAN, Poland), types TLD-100 and TLD-700 (Harshaw), OSL Al2O3:C detectors type nanoDot™ (Landauer Inc.) and RPL rod glass elements type GD-352M (Asahi Techno Glass Coorporation) are described. The main characteristics are discussed, together with the readout and calibration procedures which lead to a determination of absorbed dose to water.All dosimeter types used show very good uniformity, batch reproducibility and homogeneity. For improved accuracy, individual sensitivity correction factors should be applied for TL and OSL dosimeters while for RPL dosimeters there is no need for individual sensitivity corrections.The dose response of all dosimeters is linear for a wide range of doses.The energy response of GD-352M type dosimeters (with Sn filter) used for out-of-field measurements is flat for medium and low energy X-rays.The energy dependence for TLDs is low across the range of photon energies used and the energy correction was neglected. A significant over response of Al2O3:C OSLDs irradiated in kilovoltage photon beams was taken into account. The energy correction factor fen was calculated by using the 2006 PENELOPE Monte Carlo code.With suitable calibration, all dosimeter types are appropriate for out-of-field dose measurements as well as for the in-phantom measurements of radiotherapy MV X-rays beams.

Cost-effective Radiation Therapy With Kilovoltage Photon Beams: A Monte Carlo Study

Due to their maximum dose deposition on the skin, kilovoltage (kV) photon beams have been considered unfeasible for treatment of deep-seated tumors. The purpose of this work was to design and optimize a cost-effective kV x-ray source for treatments of deep targets by means of Monte Carlo (MC) simulations with the objective of matching kV treatment plans to ones generated with commonly used megavoltage beams.

Monte Carlo simulation on pre-clinical irradiation: A heterogeneous phantom study on monoenergetic kilovoltage photon beams

This study investigated radiation dose variations in pre-clinical irradiation due to the photon beam energy and presence of tissue heterogeneity. Based on the same mouse computed tomography image dataset, three phantoms namely, heterogeneous, homogeneous and bone homogeneous were used. These phantoms were generated by overriding the relative electron density of no voxel (heterogeneous), all voxel (homogeneous) and the bone voxel (bone homogeneous) to one. 360° photon arcs with beam energies of 50 – 1250 keV were used in mouse irradiations. Doses in the above phantoms were calculated using the EGSnrc-based DOSXYZnrc code through the DOSCTP. Monte Carlo simulations were carried out in parallel using multiple nodes in a high-performance computing cluster. It was found that the dose conformity increased with the increase of the photon beam energy from the keV to MeV range. For the heterogeneous mouse phantom, increasing the photon beam energy from 50 keV to 1250 keV increased seven times the dose deposited at the isocenter. For the bone dose enhancement, the mean dose was 2.7 times higher when the bone heterogeneity was not neglected using the 50 keV photon beams in the mouse irradiation. Bone dose enhancement affecting the mean dose was found in the photon beams with energy range of 50 – 200 keV and the dose enhancement decreased with an increase of the beam energy. Moreover, the MeV photon beam had a higher dose at the isocenter, and a better dose conformity compared to the keV beam.

Erratum: "The energy dependence and dose response of a commercial optically stimulated luminescent detector for kilovoltage photon, megavoltage photon, and electron, proton, and carbon beams" [Med. Phys. 36(5), 1690-1699 (2009)

Erratum: "The energy dependence and dose response of a commercial optically stimulated luminescent detector for kilovoltage photon, megavoltage photon, and electron, proton, and carbon beams" [Med. Phys. 36(5), 1690-1699 (2009)

Bone and mucosal dosimetry in skin radiation therapy: a Monte Carlo study using kilovoltage photon and megavoltage electron beams

This study examines variations of bone and mucosal doses with variable soft tissue and bone thicknesses, mimicking the oral or nasal cavity in skin radiation therapy. Monte Carlo simulations (EGSnrc-based codes) using the clinical kilovoltage (kVp) photon and megavoltage (MeV) electron beams, and the pencil-beam algorithm (Pinnacle3 treatment planning system) using the MeV electron beams were performed in dose calculations. Phase-space files for the 105 and 220 kVp beams (Gulmay D3225 x-ray machine), and the 4 and 6 MeV electron beams (Varian 21 EX linear accelerator) with a field size of 5 cm diameter were generated using the BEAMnrc code, and verified using measurements. Inhomogeneous phantoms containing uniform water, bone and air layers were irradiated by the kVp photon and MeV electron beams. Relative depth, bone and mucosal doses were calculated for the uniform water and bone layers which were varied in thickness in the ranges of 0.5–2 cm and 0.2–1 cm. A uniform water layer of bolus with thickness equal to the depth of maximum dose (dmax) of the electron beams (0.7 cm for 4 MeV and 1.5 cm for 6 MeV) was added on top of the phantom to ensure that the maximum dose was at the phantom surface. From our Monte Carlo results, the 4 and 6 MeV electron beams were found to produce insignificant bone and mucosal dose (<1%), when the uniform water layer at the phantom surface was thicker than 1.5 cm. When considering the 0.5 cm thin uniform water and bone layers, the 4 MeV electron beam deposited less bone and mucosal dose than the 6 MeV beam. Moreover, it was found that the 105 kVp beam produced more than twice the dose to bone than the 220 kVp beam when the uniform water thickness at the phantom surface was small (0.5 cm). However, the difference in bone dose enhancement between the 105 and 220 kVp beams became smaller when the thicknesses of the uniform water and bone layers in the phantom increased. Dose in the second bone layer interfacing with air was found to be higher for the 220 kVp beam than that of the 105 kVp beam, when the bone thickness was 1 cm. In this study, dose deviations of bone and mucosal layers of 18% and 17% were found between our results from Monte Carlo simulation and the pencil-beam algorithm, which overestimated the doses. Relative depth, bone and mucosal doses were studied by varying the beam nature, beam energy and thicknesses of the bone and uniform water using an inhomogeneous phantom to model the oral or nasal cavity. While the dose distribution in the pharynx region is unavailable due to the lack of a commercial treatment planning system commissioned for kVp beam planning in skin radiation therapy, our study provided an essential insight into the radiation staff to justify and estimate bone and mucosal dose.

TH-E-BRC-10: Accuracy of Monte Carlo Dose Calculations with Kilovoltage Photon Beams

Purpose: To assess the importance of tissue segmentation for Monte Carlo(MC)dose calculations with kilovoltage photonbeams and to suggest a new segmentation method based on effective atomic number differences. Methods: MCdose to 34 ICRU‐44 tissues was calculated in a phantom irradiated by a number of kilovoltage beams using the EGSnrc/BEAMnrc and DOSXYZnrc codes. Photonbeams currently used in small animal radiotherapy were studied: a microCT 120 kV beam, two 225 kV beams filtered with either 4 mm of Al or 0.5 mm of Cu, a heavily filtered 320 kV beam, and a 192Ir beam.MCdose was calculated for a five 120 kV beam treatment plan for a mouse with lungcancer. The microCT images were segmented using three different segmentation schemes — with 4 (the DOSXYZnrc default), 8, and 39 tissue types. The 4‐tissue and 8‐tissue segmentation schemes were based only on mass density differences. The new advanced 39‐tissue segmentation scheme is based on the combination of both mass density differences and atomic number differences. Results: Using our model we showed that mis‐assignment of adipose to cartilage caused dose calculation differences of 36%, 24%, and 14% for the 120 kV beam and the 225 kV beams filtered with 4 mm Al and 0.5 mm Cu, respectively. A second order polynomial approximated well the absorbed dose to tissue as a function of the effective number for all beams. The small animal study showed that the doses to adipose and bone were overestimated by 30–50% when the 4‐tissue segmentation was used compared to the 8‐ tissue and 39‐tissue segmentation schemes. The dose to the lungs and bone were underestimated by 5–10% with the 39‐tissue segmentation compared to the 8‐tissue segmentation. Conclusions: A new tissue segmentation method was proposed for a more accurate MCdose calculations with kilovoltage beams.

Comment on “Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition” [Med. Phys. 37, 1083–1092 (2010)

Comment on “Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition” [Med. Phys. 37, 1083–1092 (2010)

SU‐GG‐T‐382: Evaluation of Genipin Gel Dosimeter for Depth Dose Measurement of X‐Ray Beams

Purpose: To evaluate genipin gel dosimeter for depth dose measurement of x‐ray beams. Method and Materials: Differences in depth doses between the genipin gel dosimeter and water for clinical megavoltage and kilovoltage x‐ray beams was determined using Monte Carlo modeling. All the Monte Carlo calculations were performed using the EGSnrc/BEAMnrc package (V4 r2‐2‐5). The clinical x‐ray beams were generated from a Varian 21iXs linear accelerator generating 6MV x‐rays and a Pantak Therapax DXT300 kilovoltage x‐ray unit with a tube potentials from 50 to 280kVp. A higher energy kilovoltage x‐ray beam was also calculated by increasing the maximum energy of incident electron energy hitting the target to 350keV. The BEAMnrc user code was used to calculate the phase space files for each of the x‐ray beams. The generated phase space files by BEAMnrc were then used to calculate depth dose curves for the genipin gel dosimeter and water using the DOSXYZnrc user code. The dose in each phantom was defined as 60×60cm2 for 6MV and 20×20cm2 for kilovoltage energies in the lateral planes while along the depth direction, smaller voxel sizes (Minimum voxel thickness of 0.25cm for 6MV and 0.5cm for kilovoltage energies) were used in the dose buildup region. Results: For the calculated depth dose using the 6MV x‐ray beam, there were differences of less than 1% between the genipin gel dosimeter and water. For the kilovoltage energies, the relative difference in the depth doses of the genipin gel dosimeter and water is approximately 2% for the 50 and 75kVp photon beams, 1% for the 100 and 125kVp photon beams, 0.5% for 180 and 200kVp photon beams and less than 0.1% for 250–350kVp photon beams. Conclusion: According to this result, genipin gel dosimeter is a water equivalent dosimeter for depth dose measurement of the clinical 6MV and kilovoltage photon beams.

SU‐GG‐J‐161: Dosimetric Variations Due to the Photon Beam Energy in Monte Carlo Dose Calculation Using Sub‐Millimeter Voxels

Purpose: This study investigates dosimetric variations in Monte Carlo calculation using sub‐millimeter voxels due to the photon beam energy and presence of inhomogeneity. The bone dose enhancement due to the photoelectric effect for the kilovoltage (kV) photon beams was evaluated using Monte Carlo simulation. Methods and Materials: Three Monte Carlo phantoms namely, inhomogeneous, homogeneous and bone homogeneous using the same mouse CT image set were used. These phantoms were generated by overriding the relative electron density of no voxel (inhomogeneous), all voxel (homogeneous) and the bone voxel (bone homogeneous) to one. The 360 deg photon arcs with beam energies of 50, 100, 150, 200, 250, 300 and 1250 keV were used in the dose calculations. Doses in the above phantoms were calculated using the EGSnrc‐based DOSXYZnrc code through DOSCTP. Results: For the inhomogeneous mouse phantom, increasing the photon beam energy from 50 keV to 1250 keV increased seven times the dose deposited at the isocenter based on the same number of history in Monte Carlo simulations. For the bone dose enhancement, the mean dose was 2.7 times higher when the bone inhomogeneity was not neglected using the 50 keV photon beams. However, the effect of the presence of the lung inhomogeneity on the variation of the mean dose was only within ±5% for the photon beams with energies in the range of 50 – 1250 keV. Conclusions: Bone dose enhancement affecting the mean dose in the mouse irradiation can be found in the photon beams with energy range of 50 – 200 keV, and the dose enhancement decreases with an increase of the beam energy. When keV photon beam is used in the dose calculation, the increase of the mean dose due to the bone dose enhancement may be a dosimetric concern.

Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition

A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions. The authors describe a two-dimensional reference dosimetry technique for measuring dose from CBCT scans using the on-board imaging system on a Varian Clinac-iX linear accelerator that employs the XR-QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom. The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR-QA radiochromic film dosimetry system. Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one-sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of "air kerma in air," which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR-QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass-energy absorption coefficient ratios water-to-air.

Depth dose dependence of the mouse bone using kilovoltage photon beams: A Monte Carlo study for small-animal irradiation

This study investigated the dose enhancement due to the presence of mouse bone irradiated by the kilovoltage (kV) photon beams. Dosimetry of the bone associated with soft and lung tissue was determined by Monte Carlo simulations using the EGSnrc-based code in millimeter scale. Two inhomogeneous phantoms with 2 mm of bone layer sandwiched by: (1) water and lung (bone–lung phantom); and (2) water (bone–water phantom), were used. Relative depth doses along the central beam axes in the phantoms and dose enhancement ratios (bone dose in the above inhomogeneous phantoms to the dose at the same point in the water phantom) were determined using the 100 and 225 kVp photon beams. For the 100 kVp photon beams, the depth dose gradient in the bone was significantly larger compared to that in a water phantom without the bone. This is due to the beam hardening effect that some low-energy photons were filtered out in the deeper depth, resulting in less photoelectric interactions and hence energy depositions in the bone. Moreover, dose differences between the top and downstream (bottom) bone edges at depths of 1–5 mm were 168–192% and 149–166% for the bone–lung and bone–water phantom, respectively. These differences were larger than 21–27% (bone–lung) and 12–23% (bone–water) for the 225 kVp photon beams. The maximum dose enhancement ratio in the bone for the bone–lung and bone–water phantoms in various depths was about 5.7 using the 100 kVp photon beams. This ratio was larger than two times of that (2.4) for the 225 kVp photon beams. It is concluded that, apart from the basic beam characteristics such as attenuation and penumbra, which are related to the photon beam energy in the mouse irradiation, the bone dose is another important factor to consider when selecting the beam energy in the small-animal treatment planning, provided that the bone dose enhancement is a concern in the preclinical model.

The energy dependence and dose response of a commercial optically stimulated luminescent detector for kilovoltage photon, megavoltage photon, and electron, proton, and carbon beams

Optically stimulated luminescent detectors, which are widely used in radiation protection, offer a number of potential advantages for application in radiation therapy dosimetry. Their introduction into this field has been somewhat hampered by the lack of information on their radiation response in megavoltage beams. Here the response of a commercially available optically stimulated luminescent detector (OSLD) is determined as a function of energy, absorbed dose to water, and linear energy transfer (LET). The detector response was measured as a function of energy for absorbed doses from 0.5 to 4.0 Gy over the following ranges: 125 kVp to 18 MV for photons, 6-20 MeV for electrons, 50-250 MeV for protons, and 290 MeV/u for the carbon ions. For the low LET beams, the response of the detector was linear up to 2 Gy with supralinearity occurring at higher absorbed doses. For the kilovoltage photons, the detector response relative to 6 MV increased with decreasing energy due to the higher atomic number of aluminum oxide (11.2) relative to water (7.4). For the megavoltage photons and electrons, the response was independent of energy. The response for protons was also independent of energy, but it was about 6% higher than its response to 6 MV photons. For the carbon ions, the dose response was linear for a given LET from 0.5 to 4.0 Gy, and no supralinearity was observed. However, it did exhibit LET dependence on the response relative to 6 MV photons decreasing from 1.02 at 1.3 keV/microm to 0.41 at 78 keV/microm. These results provide additional information on the dosimetric properties for this particular OSL detector and also demonstrate the potential for their use in photon, electron, and proton radiotherapy dosimetry with a more limited use in high LET radiotherapy dosimetry.

Experimental Evidence of Gold Nanoparticle Radio-sensitization in vitro

Radiotherapy is a prominent tool in oncology but it can be held responsible for important biological damages as ionizing radiations also induce degradation of healthy tissues. It is the reason why since more than five decades, great efforts have been devoted to increase its efficiency and tolerance. Different approaches have been developed to increase the dose to cancerous cells specifically. Herold and co-workers first showed that gold microspheres, suspended in cell cultures or distributed in tumour tissues, can produce an increased biologically effective dose when exposed to kilovoltage photon beams (1). Later, Hainfeld et al demonstrated that gold nanoparticles (GNPs) injected intravenously to mice bearing subcutaneous EMT-6 mammary carcinomas enhance 250 kVp X-rays radiotherapy (2), an experiment which was confirmed by theoretical studies (3). However, to optimize this treatment, a more complete understanding of the mechanisms responsible for such a radio-sensitization is necessary. For this reason, we studied the effect of X-rays on biologically relevant molecules (DNA, proteins) in the presence of GNPs. Here, we present results obtained with plasmid DNA irradiation in solution. We confirmed the radio-sensitizing effect of GNPs and showed that it is amplified as the ratio gold: DNA grows. As the emergence of resistant bacteria to traditional antibiotics motivates new antibacterial systems development, we also studied the potential application of such an effect towards bacteria. GNPs proved to be non-toxic to E. coli K-12 MG1655 strain and were shown to increase the efficiency of ionizing radiation to induce bacteria cell death. So GNPs submitted to X-Ray radiation appear to be a potential tool for anti-microbial proliferation or pathogenic bacteria killing systems. 1. Herold, D. M., Das, I. J., Stobbe, C. C., Iyer, R. V., and Chapman, J. D. (2000) Int. J. Radiat. Biol. 76, 1357-1364. 2. Hainfeld, J. J., Slatkin, D. N., and Smilowitz, H. M. (2004) Phys. Med. Biol. 49, N309-N315. 3. Cho, S. H. (2005) Phys. Med. Biol. 50, N163-N173.

Comparison of the energy-response factor of LiF and Al2O3 in radiotherapy beams

A Monte Carlo study of the energy-response factor of aluminium oxide (Al2O3) and lithium fluoride (LiF) TLDs in kilovoltage and megavoltage photon beams relative to 60Co gamma rays has been performed using EGSnrc Monte Carlo simulations. The sensitive volume of the detector was simulated as a disc of diameter 2.85 mm and thickness 1 mm. The phantom material was water and the irradiation depth was 2.0 cm in kilovoltage photon beams and 5.0 cm for megavoltage photon beams. The results show that the energy-response of the Al2O3 and LiF-TLDs is constant within 3% for photon beam energies in the energy range of 60Co gamma rays to 25 MV X rays. However, both detectors show an enhanced response for kilovoltage photon beams, which in the case of 50 kV X rays is 3.2 times higher than that for 60Co gamma rays. The energy-response factor was 1.46 for LiF irradiated in 50 kV X rays. The Al2O3 detector has an energy-response that is 2.2 times higher than that of LiF in 50 kV X rays decreasing to 1.19 for 250 kV X rays. The results show that the addition of 0.1 or 1% of carbon by weight (as dopant) into the Al2O3 does not change the Monte Carlo determined energy-response factor by more than 1%.

Energy response of an aluminium oxide detector in kilovoltage and megavoltage photon beams: an EGSnrc Monte Carlo simulation study

A Monte Carlo study of the energy response of an aluminium oxide (Al(2)O(3)) detector in kilovoltage and megavoltage photon beams relative to (60)Co gamma rays has been performed using EGSnrc Monte Carlo simulations. The sensitive volume of the Al(2)O(3) detector was simulated as a disc of diameter 2.85 mm and thickness 1 mm. The phantom material was water and the irradiation depth chosen was 2.0 cm in kilovoltage photon beams and 5.0 cm in megavoltage photon beams. The results show that the energy response of the Al(2)O(3) detector is constant within 3% for photon beam energies in the energy range of (60)Co gamma rays to 25 MV X rays. However, the Al(2)O(3) detector shows an enhanced energy response for kilovoltage photon beams, which in the case of 50 kV X rays is 3.2 times higher than that for (60)Co gamma rays. There is essentially no difference in the energy responses of LiF and Al(2)O(3) detectors irradiated in megavoltage photon beams when these Al(2)O(3) results are compared with literature data for LiF thermoluminescence detectors. However, the Al(2)O(3) detector has a much higher enhanced response compared with LiF detectors in kilovoltage X-ray beams, more than twice as much for the case of 50 kV X rays.

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.

Gold microspheres: a selective technique for producing biologically effective dose enhancement

Purpose : To investigate dose enhancement and radiosensitization associated with electrons produced and scattered from gold particles suspended in cells in vitro and with tumour cells growing in vivo irradiated with low-energy photons. Materials and methods : CHO-K1, EMT-6 and DU-145 cells were irradiated with kilovoltage X-ray and Cs-137 beams in slowly stirred suspensions in the presence of various concentrations of gold particles (1.5-3.0 μ m); cell survival was measured by clonogenic assay. Gold particles were injected directly into EMT-6 tumours growing in scid mice prior to their irradiation. Tumour cell killing was assayed by an in vivo-in vitro technique. Results : Dose enhancement was confirmed by both Fricke dosimetry and cell killing for 100, 140, 200 and 240kVp X-rays, but not for Cs-137 γ-rays. For the chemical dosimeter, a dose enhancement (DMF) of 1.42 was measured for 1% gold particle solutions irradiated with 200 kVp X-rays. When rodent and human cells were irradiated in the presence of 1% gold particles, DMF values at the 10% survival level ranged from 1.36 to 1.54, with an overall average value of 1.43. Preliminary attempts to deliver these gold particles to tumour cells in vivo by intra-tumour injection resulted in modest radiosensitization but extremely heterogeneous distribution. Conclusions : An increased biologically effective dose can be produced by gold microspheres suspended in cell culture or distributed in tumour tissue exposed to kilovoltage photon beams. With the increasing use of interstitial brachytherapy with isotopes that produce low-energy photons, high-Z particles might find a role for significantly improving the therapeutic ratio.