Ratio Of Kerma Research Articles

Analytical and fine Monte Carlo Kerma coefficients evaluation for a wide range photon energy relevant to some chemotherapy agents

Simultaneously combination of chemotherapy drugs and ionizing radiations has become a common treatment method for advanced cancers. Tumor irradiation in presence of chemotherapy drugs can affect deposited energy within tissue. Hence, to evaluate how chemotherapy drugs affect the absorbed dose within tissues during chemoradiotherapy, Kerma coefficient values for 26 anti-cancer agents were estimated for photon energies ranging from 1 keV to 20 MeV through analytical analysis and Monte Carlo (MC) simulation approaches.Estimation of Kerma coefficients through MC simulation relevant to different chemotherapy drugs was performed with MCNPX V.2.6.0 MC code. Besides, Kerma coefficient values were calculated using the reported mass energy absorption coefficients, as the theoretical method. Significant statistical test was also employed to evaluate the accordance of obtained Kerma coefficients by two considered methods.Simulation results were consistent with evaluated Kerma coefficients by analytical method which minimum and maximum mean relative differences are found for Bevacizumab and Methotrexate, respectively. Furthermore, a significant correlation is found with chemical composition of selected chemotherapy drugs at considered photon energy range. Fluorouracil showed the closest Kerma ratio to unity among the studied anti-cancer drugs. Kerma coefficient ratios for most of selected drugs were close to unity at higher energy regions (100 keV–20 MeV) while greater values were found at lower photon energies (below 100 keV).From the results, presence of these anti-cancer agents can impact Kerma coefficient and subsequently alter the absorbed dose within tissue during the cancer treatment with chemoradiotherapy technique.

A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT).

Despite the number of treatments performed with electronic brachytherapy there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator. 
The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Two ionization chambers were calibrated at the PSDL in terms of air kerma. The chambers were used to measure reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water, Dw, at the surface to Dw at 1 cm depth in water. These were compared with Monte Carlo calculations. Main results. Calculations and measurements were combined to estimate the Dw at the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k = 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k = 2).
The feasibility of the formalism and methodology for the Esteya system was demonstrated. Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.

Characterization of a new radiochromic film (LD-V1) using mammographic beam qualities

PurposeRadiochromic film (RCF) is a detector that can obtain a two-dimensional dose distribution with high resolution; it is widely used in medical and industrial fields. Several types of RCFs exist based on their application. The type of RCF mainly used for mammography dose assessment has been discontinued; however, a new type of RCF (LD-V1) has been distributed as a successor. Since the medical use of LD-V1 has rarely been studied, we investigated the response characteristics of LD-V1 in mammography. MethodsMeasurements were performed using Mo/Mo and Rh/Ag on a Senographe Pristina mammography device (GE, Fairfield, CT, USA). The reference air kerma was measured using a parallel-plate ionization chamber (PPIC) (C-MA, Applied Engineering Inc, Tokyo, Japan). Pieces of LD-V1 film model were irradiated at the same position where the reference air kerma in air was measured by the PPIC. Irradiation was performed using the time scale method based on the load on the equipment. Two methods of irradiation were considered: placing the detector in air and on the phantom. The LD-V1 was scanned five times at 72 dpi in RGB (48 bit) mode using a flatbed scanner (ES-G11000, Seiko Epson Corp, Nagano, Japan) 24 h following irradiation. The response ratio of the reference air kerma and the air kerma obtained from the LD-V1 were compared and examined for each beam quality and air kerma range. Results and discussionWhen the beam quality was altered, the response ratio varied from 0.8 to 1.2 with respect to the measurement value of the PPIC; however, some outliers were observed. Response ratios were highly variable in the low-dose range; however, as the air kerma increased, the ratios approached 1. Thus, LD-V1 does not need calibration for each beam quality used in mammography. LD-V1 enables air kerma evaluation by creating air kerma response curves under certain X-ray conditions used in mammography. ConclusionWe suggest that the dose range be limited to 12 mGy or more to keep the response variation with beam qualities below ±20%. If further measurement is required for reducing the response variation, the dose range should be shifted to a higher dose range.

Can we reduce dose to ureters as avoidance organs for MRI based brachytherapy for cervical cancer? A dosimetric feasibility study

BACKGROUND AND PURPOSEUreteral stenosis (US) is an underreported complication of brachytherapy (BT) for cervical cancer (CC), with limited data on toxicity risk reduction. A previous study demonstrated ureter EQD2 D0.1cc > 77 Gy correlated with US development. We sought to assess feasibility of this constraint while maintaining similar HR-CTV coverage. MATERIALS AND METHODSPatients with locally advanced CC treated with EBRT plus HDR MRI-based brachytherapy boost without hydronephrosis at diagnosis and with ureter dose EQD2 D0.1cc > 77 Gy were included. Replan was attempted to achieve HR-CTV D90 ≥ 80–85 Gy and ureter dose reduction. Ureter distance from lateral margin of HR-CTV and tandem was recorded. t-test was performed to compare ureteral dose and HR-CTV D90. RESULTsOf 25 patients were identified. Hundred percent received 45 Gy in 25 fractions to the pelvis ± paraaortic lymph nodes and 80% receiving median additional parametrial dose of 5.4 Gy. Replan meeting ureteral dose of ≤77 Gy was feasible in 18 of 25 patients, with a reduction in median ureter D0.1cc from 82.3 to 76.8 Gy (p < 0.001). Median HR-CTV D90 was similar (84.7 vs. 85.0 Gy). Replan achieved D0.1cc ≤77 Gy in 56% of patients who experienced US. All unilateral US cases occurred in the ureter closest to HR-CTV. CONCLUSIONSOptimization to reduce ureter dose to ≤77 Gy is feasible when ureters are visible and contoured. Ureters may be considered as potential OAR during MRI-based brachytherapy treatment. Reduced ring to tandem total reference air kerma (TRAK) ratio may provide an additional metric by which to lower US risk.

A PRELIMINARY STUDY OF RADIATION QUALITY CORRECTION FACTOR (KQ) DETERMINATION FOR WELL-TYPE IONIZATION CHAMBER

This paper deals with the determination of the radiation quality correction factor (kQ) well-type ionization chamber for the measurement of Co-60 brachytherapy. The measurement of the brachytherapy source has been done in the hospital brachytherapy facilities. The measurement of air Kerma has been done using three different ionization chambers 0.6 cm3, 30 cm3, 80 cm3, which have calibration factor traceability for Co-60 and one well-type ionization chamber which calibrated for Ir-192. The determination of the radiation quality correction factor (kQ) was determined based on the results of the air Kerma ratio between measurements using ionization chambers, which have traceability to Co-60 and Ir-192 sources. The results of the measurement of the reference air Kerma rate (RAKR) obtained from the three chambers were 21.36, 19.87, 19.34 mGy.m2.h-1, while the results of measurements with ionization chambers get a value of 19.01 mGy.m2.h-1. The kQCal results from 0.6 cm3 ionization chamber get a value of 1.07. The kQCal value was compared with the value of Andreas Schuller’s et al. kQreff of 1.05 and get a deviation of 2.2%. Implementation of the booth kQ value on the results of the RAKR from the HDR1000Plus well-type ionization chamber in the measurement of brachytherapy in two different facilities gets maximal deviation 1.7% with dose value from Treatment Planning System (TPS). The deviation was in the acceptable range of ±5%. Based on this, the use of radiation quality correction factor (kQ) value can be implemented as one method if it does not have the traceability factor calibration of the Co-60 brachytherapy source.

Determination of backscatter factors based on the quality index for diagnostic kilovoltage x-ray beams.

This study aims to investigate the relationship between backscatter factors and Al-half-value-layers (Al-HVL) by making the quality index (QI) a parameter for diagnostic kilovoltage x-ray beams. Backscatter factors, Bw, for x-ray fluence spectra were calculated from the weighted average of Bw for monoenergetic photons of between 8 and 140keV with field sizes of 10cm×10cm to 40cm×40cm. The value of Bw for monoenergetic photons was calculated from the ratio of the water kerma at the surface of a water phantom and that at the same point free-in-air using the EGSnrc/cavity code. The weighted averaged backscatter factors were validated by comparing them with those of direct Monte Carlo calculations for the x-ray fluence spectra. The Bw for the x-ray fluence spectra were classified by a QI of 0.35, 0.4, 0.5, 0.6, and 0.7 specified by the ratio of the effective energy and maximum energy. The relationship between Bw and Al-HVL was evaluated for the given QI values. The x-ray fluence spectra were generated for tube voltages of 40-140 kVp with Al-HVLs of 0.5-13.2mm using the SpekCalc program. The weighted averaged backscatter factors for x-ray fluence spectra agreed within 0.7% with those of the direct Monte Carlo calculations. The backscatter factors were represented by the fitting curves of R2>0.99 with Al-HVL for the given QI values. It is possible to obtain Bw more accurately by using QI specified by the measured Al-HVL.

Assessment of Kerma coefficients for OSL dosimeters by analytical and Monte Carlo approaches

In optically stimulated luminescent dosimeters (OSLD), the intensity of luminescent is a function of the dose of radiation absorbed and then it may be utilized as the basis of a radiation dosimetry method. For photons in the energy range from 1 keV to 20 MeV, the Kerma coefficients for fifteen OSLD, air, and ICRU standard tissue were calculated respectively from the analytical method and Monte Carlo simulation (MCNPX code). The results of MCNPX are consistent within estimated uncertainties with values evaluated by analytical method. The Kerma ratios of the OSLD relative to tissue were also obtained. The BeO has the closer Kerma to the tissue and alkaline OSL materials have relatively more values so that in 50 keV these ratios reach about 100.

The impact of a magnetic field on the dose distribution using the Bebig60Co HDR sources

Objective To evaluate the impact of an external magnetic field on the dose distribution and electronic disequilibrium region around a Bebig type 60Co HDR brachytherapy source and to judge the feasibility of applying MRI scanner during brachytherapy. Methods The source model was established based on the Monte Carlo package Geant4 software. The simulated geometries consisted of the 60Co source inside a water phantom of 10.0cm×10.0cm×10.0cm in size. The magnetic field strength of the 0T, 1.5T and 3.0T was considered, respectively. The voxels with a size of 0.2 mm, 0.5 mm and 0.5 mm were established along the x-, y-and z-axis. The influence of the magnetic field on the Kerma (kinetic energy released to matter) distribution and dose distribution within the 10.0mm region from the source center was evaluated. Furthermore, the ratio of the Kerma to dose as a function of the distance to the center source was acquired. Results The 1.5T magnetic field exerted no effect on the dose distribution adjacent to 60Co HDR brachytherapy source, whereas3.0T magnetic field caused significant increase in the dose distribution within r<6 mm from the source center. The dose distribution was increased by 40% at r=5.4 mm from the source center. The ratio of Kerma to dose was less than 1 within the region of 1.2 mm<r<6.0 mm, suggesting that 3.0T magnetic field can lead to electronic disequilibrium within a larger region from the source center. Conclusions For Bebig 60Co HDR brachytherapy source, it is safe and reliable to apply1.5T external magnetic field. Nevertheless, 3.0T magnetic field can cause high dose risk. Consequently, safety assessment and verification should be delivered prior to clinical application. Key words: Magnetic field; Brachytherapy; Dose; Kerma

Glandular dose indices using a glandular dose to air kerma volume histogram in mammography.

To develop a dose evaluation for heterogeneous glandular dose distributions by using glandular dose indices obtained from a glandular dose to air kerma (GD/K) volume histogram in addition to the existing mean glandular dose (MGD) in mammography. The mammographic x-ray fluence spectra were created using the EGSnrc/BEAMnrc Monte Carlo code. The combinations of the target and filter were Mo-Mo and W-Rh. The breast phantom was a half-ellipsoidal form, composed of adipose and glandular tissues, and a 1.5-mm-thick outer skin. The compressed breast thicknesses (CBT) were 2, 4, and 6cm with a glandularity of 15%. Two types of breast voxel phantoms with homogeneous and realistic heterogeneous glandular distributions were modeled. The glandular dose distributions in both breast voxel phantoms were calculated using the simulated x-ray fluence spectra. The GD/K volume histograms for the glandular dose were presented by a relative glandular volume (%) as a function of the ratio of glandular dose (GD) and air kerma (K). Finally, the dose indices of MGD, GD2% /K (GD2% the dose covered by a 2% glandular volume), and VMGD (glandular volume percentage (%) receiving at least MGD), homogeneity index (HI) were obtained from the glandular dose distributions and GD/K volume histograms. The HI indicates the uniformity of GD distributions. The MGD in a standard heterogeneous phantom (CBT: 4cm, glandularity: 15%) was 25%-37% lower than that in a standard homogeneous phantom. The lower the tube voltage, the larger was the difference between GD2% and MGD. The larger the CBT, the greater is the nonuniformity of the glandular dose distribution. The HI values for heterogeneous phantoms were higher than those for homogeneous phantoms. Values of VMGD were 35%-47% and increased slightly with the tube voltage. Values of VMGD for heterogeneous phantoms were smaller than those for homogenous phantoms at the same tube voltage. The GD distribution differs depending on the CBT and tube voltage, even if the MGD has the same value. The new GD indices proposed in this study are useful for the dose evaluation of the heterogeneous GD distributions.

Соотношение между поглощенной дозой, кермой и ионизационной кермой для полей малых размеров

Purpose: Research of the relationships between spatial distributions in water of the main dosimetric values, namely the absorbed dose, kerma and ionization kerma, for small-size fields with a circular cross section created by divergent beams of bremsstrahlung spectrum with a maximum energy of 6 MeV.&#x0D; Material and methods: Using the Monte-Carlo method with the codes EGSnrc and MCNP4C2, calculations were carried out for these distributions in a water phantom for beam radii on the phantom surface from 0.1 to 3.0 cm and for depths up to 40 cm. The ratio at depths up to 5 cm, where there is a so-called build-up area, is studied in particular detail.&#x0D; Results: The results of calculations show that the ratio of ionization kerma to kerma for such beams at depths up to 40 cm is practically constant and equal to 0.9930 ± 0.0005. The ratio of the absorbed dose to the ionization kerma, in contrast to conventional square beams with a cross-sectional area  20 cm2, is much less than unity at radii of 1 cm at all the depths considered.&#x0D; Conclusion: The data obtained show that the relationship between absorbed dose, kerma and ionization kerma for photon fields produced by beams of small cross sections is very different from that for traditional beams. This circumstance should be taken into account when conducting dosimetry of small fields.

Assessment of scatter radiation dose and absorbed doses in eye lens and thyroid gland during digital breast tomosynthesis.

Digital breast tomosynthesis (DBT) is an alternative tool for breast cancer screening; however, the magnitude of peripheral organs dose is not well known. This study aimed to measure scattered dose and estimate organ dose during mammography under conventional (CM) and Tomo (TM) modes in a specific DBT system. Optically stimulated luminescence dosimeters (OSLDs), whose responses were corrected using a parallel‐plate ionization chamber, were pasted on the surface of custom‐made polymethyl methacrylate (PMMA) and RANDO phantoms to measure entrance surface air kerma (ESAK). ESAK measurements were also acquired with a 4.5‐cm thick breast phantom for a standard mammogram. Organ dose conversion factors (CFD) were determined as ratio of air kerma at a specific depth to that at the surface for the PMMA phantom and multiplied by the ratio of mass energy absorption coefficients of tissue to air. Normalized eye lens and thyroid gland doses were calculated using the RANDO phantom by multiplying CFD and ESAK values. Maximum variability in OSLD response to scatter radiation from the DBT system was 33% in the W/Rh spectrum and variations in scattered dose distribution were observed between CM and TM. The CFD values for eye lens and thyroid gland ranged between 0.58 to 0.66 and 0.29 to 0.33, respectively. Mean organ doses for two‐view unilateral imaging were 0.24 (CM) and 0.18 (TM) μGy/mAs for the eye lens and 0.24 (CM) and 0.25 (TM) μGy/mAs for the thyroid gland. Higher organ doses were observed during TM compared to CM as the automatic exposure control (AEC) system resulted in greater total mAs values in TM.

Оценка неопределенности ОБЭ рентгеновского излучения, связанной с определением поглощенной дозы в радиобиологических экспериментах

Purpose: Determining the absorbed dose produced by photons, it is often assumed that it is equal to the radiation kerma. This assumption is valid only in the presence of an electronic equilibrium, which in turn is never ensured in practice. It leads to some uncertainty in determining the absorbed dose in the irradiated sample during radiobiological experiments. Therefore, it is necessary to estimate the uncertainty in determining the relative biological effectiveness of X-rays associated with uncertainty in the determination of the absorbed dose. Material and methods: The monochromatic X-ray photon emission is simulated through a standard 25 cm2 plastic flask containing 5 ml of the model culture medium (biological tissue with elemental composition C5H40O18N). The calculation of the absorbed dose in a culture medium is carried out in two ways: 1) the standard method, according to which the ratio of the absorbed dose in the medium and the ionization chamber is equal to the ratio of kerma in the medium and air; 2) determination of the absorbed dose in the medium and in the sensitive volume of the ionization chamber by computer simulation and calculating the ratio of these doses. For each primary photon energies, 108 histories are modeled, which makes it possible to achieve a statistical uncertainty not worse than 0.1 %. The energy step was 1 keV. The spectral distribution of X-ray energy is modeled separately for each set of anode materials, thickness and materials of the primary and secondary filters. The specification of the X-ray beams modeled in this work corresponds to the standards ISO 4037 and IEC 61267. Within the linear-quadratic model, the uncertainty of determining the RBEmax values is directly proportional to the uncertainty in the determination of the dose absorbed by the sample under study. Results: At energy of more than 60 keV, the ratios for water and biological tissue practically do not differ. At lower energies, up to about 20 keV, the ratio of the coefficients of air and water is slightly less than that of air and biological tissue. The maximum difference is ~ 1 % than usual and the equality of absorbed doses in the ionization chamber and sample is justified. At photon energy of 60 keV for the geometry in question, the uncertainty in determining the dose is about 50 %. For non-monochromatic radiation, the magnitude of the uncertainty is determined by the spectral composition of the radiation, since the curves vary greatly in the energy range 10–100 keV. It is shown that, depending on the spectral composition of X-ray radiation, uncertainty in the determination of the absorbed dose can reach 40–60 %. Such large uncertainty is due to the lack of electronic equilibrium in the radiation geometry used in practice. The spread of RBE values determined from the data of radiobiological experiments carried out by different authors can be determined both by differences in the experimental conditions and by uncertainty in the determination of the absorbed dose. Using Fricke dosimeters instead of ionization chambers in the same geometry allows you to reduce the uncertainty approximately 2 times, up to 10–30 %. Conclusion: The computer simulation of radiobiological experiments to determine the relative biological effectiveness of X-ray radiation is performed. The geometry of the experiments corresponds to the conditions for the use of standard bottles placed in the side holders. It is shown that the ratio of absorbed doses and kerma in the layers of biological tissue and air differ among themselves with an uncertainty up to 60 %. Depending on the quality of the beam, the true absorbed dose may differ from the one calculated on the assumption of kerma and dose equivalence by 50 %. Uncertainty in determining the RBE in these experiments is of the same order. The results are presented for X-ray beams with negligible fraction of photons with energies less than 10 keV. For beams of a different quality, the uncertainty in determination can significantly increase. For the correct evaluation of RBE, it is necessary to develop a uniform standard for carrying out radiobiological experiments. This standard should regulate both the geometry of the experiments and the conduct of dosimetric measurements.

Monte Carlo Study of 3D Stray Radiation during Interventional Procedures

In interventional medical procedures, other than the highly important issue of optimizing image quality and patient exposure using the primary beam, there remains a continuing need for the study of staff exposure from the scattered radiation. Herein, investigation is made of the 3D stray-radiation distribution, the simulation being made of a realistic interventional scenario through use of the Monte Carlo code Geant4 (version 10.3). The simulation is conducted based on the high definition reference Korean-man (HDRK-man) computational phantom and a GE Infinia 3/8” C-arm machine, focusing on the effect of variation of kVp and field of view (FoV) on the scattered particles’ spatial distribution. With direct measurement of the absorbed dose remaining challenging, not least in respect of the organs at risk, we computed the scatter fractions, defined as the ratio of the air kerma free-in-air to the entrance surface air kerma (ESAK), which are both easily quantifiable. Scatter fraction distributions were simulated for X-ray tube outputs (and half-value layers, HVL) of 60 kVp (2.3 mm Al), 80 kVp (3.2 mm Al) and 120 kVp (4.3 mm Al) and FoV of 15, 20, 25 and 30 cm. The distributions are obtained for different height levels, corresponding to the lens of the eye, and the lung and prostate, all radiosensitive organs. Investigations are made for eight likely locations around the patient. At fixed FoV results reveal an inverse relationship between ESAK and kVp, also that change in kVp from 60 to 80 has a greater effect than from 80 to 120. For change in FoV at fixed kVp, the scatter fraction remains constant. The particular staff locations are found to be optimal in seeking mitigation of dose. Moreover, the combined usage of numerical human model and Monte Carlo simulation can be considered as an added value to the radiation safety research field, especially to the interventional radiology staff and to the patient.

Introduction of optically stimulated luminescence based postal audits for radiation protection level dosimetry at Secondary Standard Dosimetry Laboratories

The IAEA has introduced an Optically Stimulated Luminescence Dosimetry (OSLD) system to conduct postal audits to Secondary Standard Dosimetry Laboratories (SSDLs) for radiation protection level. The audit independently checks 137Cs radiation protection air kerma calibrations provided by the SSLDs. The OSLD system has replaced the previously used TLD based audit for radiation protection.The OSLD system used in the audit consists of nanoDot dosimeters and MicroSTARii reader (Landauer, IL, USA). Prior to introducing the audit service, an extensive commissioning process quantified the dosimeter reading reproducibility, signal depletion and fading, linearity of response to air kerma, individual sensitivities of OSLDs and optical annealing procedure for the dosimeters. In an audit run, the participating SSLDs are supplied with two dosimeter sets consisting of two lightweight holders each preloaded with two OSLDs. The participants are asked to irradiate them with 5 mGy air kerma free in air, following their standard laboratory procedures. The OSLDs returned to the IAEA are evaluated by the IAEA Dosimetry Laboratory.The OSLDs demonstrated acceptable reading reproducibility of 1.75% and a signal depletion of 0.25% per readout. The OSLDs signal fading resulted in a signal decrease of 2.1% in the period from eight days to over 160 days post irradiation. A relative expanded (k = 2) uncertainty of the OSLD determined air kerma was estimated to 3.08%. In two audit runs in 2015–2016, 29 SSDLs participated. The average ratio of the air kerma measured by the IAEA to the air kerma stated by the SSDL was 0.998 and the standard deviation of the distribution of results was 1.8%. All participants except one had results within the acceptance limits of ±7%. One laboratory with the result outside the acceptance limit was invited to a follow-up procedure with the aim to resolve the discrepancy.

Triple ionization chamber method for clinical dose monitoring with a Be-covered Li BNCT field.

Fast neutron, gamma-ray, and boron doses have different relative biological effectiveness (RBE). In boron neutron capture therapy (BNCT), the clinical dose is the total of these dose components multiplied by their RBE. Clinical dose monitoring is necessary for quality assurance of the irradiation profile; therefore, the fast neutron, gamma-ray, and boron doses should be separately monitored. To estimate these doses separately, and to monitor the boron dose without monitoring the thermal neutron fluence, the authors propose a triple ionization chamber method using graphite-walled carbon dioxide gas (C-CO2), tissue-equivalent plastic-walled tissue-equivalent gas (TE-TE), and boron-loaded tissue-equivalent plastic-walled tissue-equivalent gas [TE(B)-TE] chambers. To use this method for dose monitoring for a neutron and gamma-ray field moderated by D2O from a Be-covered Li target (Be-covered Li BNCT field), the relative sensitivities of these ionization chambers are required. The relative sensitivities of the TE-TE, C-CO2, and TE(B)-TE chambers to fast neutron, gamma-ray, and boron doses are calculated with the particle and heavy-ion transport code system (PHITS). The relative sensitivity of the TE(B)-TE chamber is calculated with the same method as for the TE-TE and C-CO2 chambers in the paired chamber method. In the Be-covered Li BNCT field, the relative sensitivities of the ionization chambers to fast neutron, gamma-ray, and boron doses are calculated from the kerma ratios, mass attenuation coefficient tissue-to-wall ratios, and W-values. The Be-covered Li BNCT field consists of neutrons and gamma-rays which are emitted from a Be-covered Li target, and this resultant field is simulated by using PHITS with the cross section library of ENDF-VII. The kerma ratios and mass attenuation coefficient tissue-to-wall ratios are determined from the energy spectra of neutrons and gamma-rays in the Be-covered Li BNCT field. The W-value is calculated from recoil charged particle spectra by the collision of neutrons and gamma-rays with the wall and gas materials of the ionization chambers in the gas cavities of TE-TE, C-CO2, and TE(B)-TE chambers (10B concentrations of 10, 50, and 100 ppm in the TE-wall). The calculated relative sensitivity of the C-CO2 chamber to the fast neutron dose in the Be-covered Li BNCT field is 0.029, and those of the TE-TE and TE(B)-TE chambers are both equal to 0.965. The relative sensitivities of the C-CO2, TE-TE, and TE(B)-TE chambers to the gamma-ray dose in the Be-covered Li BNCT field are all 1 within the 1% calculation uncertainty. The relative sensitivities of TE(B)-TE to boron dose with concentrations of 10, 50, and 100 ppm 10B are calculated to be 0.865 times the ratio of the in-tumor to in-chamber wall boron concentration. The fast neutron, gamma-ray, and boron doses of a tumor in-air can be separately monitored by the triple ionization chamber method in the Be-covered Li BNCT field. The results show that these doses can be easily converted to the clinical dose with the depth correction factor in the body and the RBE.

SU‐F‐T‐13: Transit Dose Comparisons for Co‐60 and Ir‐192 HDR Sources

Purpose:The purpose of this study is to compare the transit dose due to the movement of high dose rate (HDR) Ir‐192 and Co‐60 sources along the transfer tube. This is performed by evaluating air‐kerma differences in the vicinity of the transfer tube when both sources are moved with the same velocity from a HDR brachytherapy afterloader into a patient.Methods:Monte Carlo simulations have been performed using PENELOPE2014. mHDR‐v2 and Flexisource sources have been considered. Collisional kerma has been scored. The sources were simulated within a plastic catheter located in an infinite air phantom. The movement of the seed was included by displacing their positions along the connecting catheter from z=−75 cm to z=+75 cm and combining them. Backscatter from the afterloader and the patient was not considered. Since modern afterloaders like Flexitron (Elekta) or Saginova (Bebig) are able to use equally Ir‐192 and Co‐60 sources it was assumed that both sources are displaced with equal speed. Typical content activity values were provided by the manufacturer (460 GBq for Ir‐192 and 75 GBq for Co‐60).Results:2D distributions were obtained with type‐A uncertainties (k=2) less than 0.01%. From those, the air kerma ratio Co‐60/Ir‐192 was evaluated weighted by their corresponding activities. It was found that it varies slowly with distance (less than 10% variation) but strongly in time due to the shorter half‐life of the Ir‐192 (73.83 days). The maximum ratio is located close to the catheter with a value of 0.57 when both sources are installed by the manufacturer, while increasing up to 1.25 at the end of the recommended working life (90 days) of the Ir‐192 source.Conclusion:Air‐kerma ratios are almost constant (0.51–0.57) in the vicinity of the source. Nevertheless, air‐kerma ratios increase rapidly whenever the Ir‐192 is approaching the end of its life.

SU‐E‐T‐90: Accuracy of Calibration of Lithium‐6 and ‐7 Enriched LiF TLDs for Neutron Measurements in High Energy Radiotherapy

Purpose:To determine the potential error involved in the interpretation of neutron measurements from medical linear accelerators (linacs) using TLD‐600H and TLD‐700H if standard AmBe and 2⁵2Cf neutron sources are used for calibration without proper inclusion of neutron energy spectrum information.Methods:The Kerma due to neutrons can be calculated from the energy released by various nuclear interactions (elastic and inelastic scatter, (n,α), (n,p), (n,d), (n,t), (n,2n), etc.). The response of each TLD can be considered the sum of the neutron and gamma components; each proportional to the Kerma. Using the difference between the measured TLD responses and the ratio of the calculated Kerma for each material, the neutron component of the response can be calculated. The Monte Carlo code MCNP6 has been used to calculate the neutron energy spectra resulting from photonuclear interactions in a Varian 21EX linac. TLDs have been exposed to the mixed (γ‐n) field produced by a linac and AmBe and 2⁵2Cf standard neutron sources.Results:For dosimetry of neutrons from AmBe or 2⁵2Cf sources, assuming TLD‐700H insensitivity to neutrons will Result in 10% or 20% overestimation of neutron doses respectively.For dosimetry of neutrons produced in a Varian 21EX, applying a calibration factor derived from a standard AmBe or 2⁵2Cf source will Result in an overestimation of neutron fluence, by as much as a factor of 47.The assumption of TLD‐700H insensitivity to neutrons produced by linacs leads to a negligible error due to the extremely high Kerma ratio (600H/700H) of 3000 for the assumed neutron spectrum.Conclusion:Lithium‐enriched TLDs calibrated with AmBe and/or 2⁵2Cf neutron sources are not accurate for use under the neutron energy spectrum produced by a medical linear accelerator.

Neutron kerma factors and water equivalence of some tissue substitutes

The Kerma factors and Kerma relative to the air and water of 24 compounds that are used as tissue substitutes were calculated for neutron energies ranging from 2.53×10−8 to 29MeV. The Kerma ratios of the tissue substitutes relative to air and water were calculated. The water equivalence of the selected tissue substitutes was observed above neutron energies of 100eV. The Kerma ratio relative to the air for poly-vinylidene fluoride and Teflon were nearest to unity at very low energy (up to 1eV) and above 63eV, respectively. It was found that the natural rubber was a water-equivalent tissue substitute compound. The results of the Kerma factors in our investigation show good agreement with those published in ICRU-44. We found that at higher neutron energies, the Kerma factors and Kerma ratios of the selected tissue substitute compounds were approximately the same, but though the differences were large for energies below 100eV.

Radiation doses in volume-of-interest breast computed tomography--A Monte Carlo simulation study.

Cone beam breast computed tomography (breast CT) with true three-dimensional, nearly isotropic spatial resolution has been developed and investigated over the past decade to overcome the problem of lesions overlapping with breast anatomical structures on two-dimensional mammographic images. However, the ability of breast CT to detect small objects, such as tissue structure edges and small calcifications, is limited. To resolve this problem, the authors proposed and developed a volume-of-interest (VOI) breast CT technique to image a small VOI using a higher radiation dose to improve that region's visibility. In this study, the authors performed Monte Carlo simulations to estimate average breast dose and average glandular dose (AGD) for the VOI breast CT technique. Electron-Gamma-Shower system code-based Monte Carlo codes were used to simulate breast CT. The Monte Carlo codes estimated were validated using physical measurements of air kerma ratios and point doses in phantoms with an ion chamber and optically stimulated luminescence dosimeters. The validated full cone x-ray source was then collimated to simulate half cone beam x-rays to image digital pendant-geometry, hemi-ellipsoidal, homogeneous breast phantoms and to estimate breast doses with full field scans. 13-cm in diameter, 10-cm long hemi-ellipsoidal homogeneous phantoms were used to simulate median breasts. Breast compositions of 25% and 50% volumetric glandular fractions (VGFs) were used to investigate the influence on breast dose. The simulated half cone beam x-rays were then collimated to a narrow x-ray beam with an area of 2.5 × 2.5 cm(2) field of view at the isocenter plane and to perform VOI field scans. The Monte Carlo results for the full field scans and the VOI field scans were then used to estimate the AGD for the VOI breast CT technique. The ratios of air kerma ratios and dose measurement results from the Monte Carlo simulation to those from the physical measurements were 0.97 ± 0.03 and 1.10 ± 0.13, respectively, indicating that the accuracy of the Monte Carlo simulation was adequate. The normalized AGD with VOI field scans was substantially reduced by a factor of about 2 over the VOI region and by a factor of 18 over the entire breast for both 25% and 50% VGF simulated breasts compared with the normalized AGD with full field scans. The normalized AGD for the VOI breast CT technique can be kept the same as or lower than that for a full field scan with the exposure level for the VOI field scan increased by a factor of as much as 12. The authors' Monte Carlo estimates of normalized AGDs for the VOI breast CT technique show that this technique can be used to markedly increase the dose to the breast and thus the visibility of the VOI region without increasing the dose to the breast. The results of this investigation should be helpful for those interested in using VOI breast CT technique to image small calcifications with dose concern.

Properties and usage of radiophotoluminescent glass dosimeters for computed tomography dosimetry

There are two types of radiophotoluminescent glass dosimeters (RPLDs). One has a tin filter in the capsule (GD-352M) and the other has no filter (GD-302M). The purpose of our study was to evaluate the properties of these RPLDs for computed tomography (CT) dosimetry: energy dependence, variation, angular dependence, and dose distribution in a single slice. Energy dependence and variation were investigated for ratio of the air kerma measured by RPLDs to that by ion chamber. Angular dependence was investigated for RPLDs and ion chamber. RPLDs were irradiated at 90°, 60°, 30°, 0°, -30°, -60°, and -90°: 0° was vertical to long axial direction of the RPLD, and plus and minus meant clockwise and anti-clockwise, respectively. Dose distribution in a single slice of an anthropomorphic phantom was acquired using 46 RPLDs. The dose responses of GD-302M and GD-352M depended on beam energy and irradiation angle of X-ray, respectively. The dose variation among dosimeters was large with GD-352M. The dose obtained from GD-352M was lower than that from GD-302M. It was expected that the dose distribution of GD-352M was formed by the primary X-ray, so RPLD without tin filter should be used for CT dosimetry.