DNA-damage RBE assessment for combined boron and gadolinium neutron capture therapy

PURPOSE

Neutron capture therapy (NCT) by 10B and 157Gd agents is a unique irradiation-based method which can be used to treat brain tumors. Current study aims to quantitatively evaluate the relative biological effectiveness (RBE) and dose distributions during the combined BNCT and GdNCT modalities through a hybrid Monte Carlo (MC) simulation approach.

METHODS

Snyder head phantom as well as a cubic hypothetical tumor was at first modeled by Geant4 MC Code. Then, the energy spectra and dose distribution relevant to the released secondary particles during the combined Gd/BNCT were scored for different concentrations of 157Gd and 10B inside tumor volume. Finally, the scored energy spectra were imported to the MCDS code to estimate both RBESSB and RBEDSB values for different 157Gd concentrations.

RESULTS

The results showed that combined Gd/BNCT increases the fluence-averaged RBESSB values by about 1.7 times when 157Gd concentration increments from 0 to 2000 µg/g for both considered cell oxygen levels (pO2 = 10% and 100%). Besides, a reduction of about 26% was found for fluence-averaged RBEDSB values with an increment of 157Gd concentration in tumor volume.

CONCLUSION

From the results, it can be concluded that combined Gd/BNCT technique can improve tumor coverage with higher dose levels but in the expense of RBEDSB reduction which can affect the clinical efficacy of the NCT technique.

Evaluation of organ dose using size-specific dose estimation (SSDE) and related cancer risk due to chest CT scan during the COVID-19 pandemic

This study aimed to estimate lung and breast doses for individual patients using the size-specific dose estimate (SSDE) method, as well as calculating effective doses, in patients who underwent chest CT scans during the COVID-19 pandemic. Cancer risk incidence was estimated using excess relative risk (ERR), excess absolute risk (EAR), and lifetime attributable risk (LAR) models from the Biological Effects of Ionizing Radiation Report VII (BEIR-VII). Data from about 570 patients who underwent CT scans for COVID-19 screening were utilized for this study. Using the header of the CT images in a Python script, SSDE and effective dose were calculated for each patient. The SSDE obtained by water equivalent effective diameter (wSSDE) was considered as lung and breast dose, and applied in organ-specific cancer risk estimation. The mean wSSDE value for females (13.3 mGy) was slightly higher than that for males (13.1 mGy), but the difference was not statistically significant (P value = 0.41). No significant differences were observed between males and females in terms of calculated EAR and ERR for lung cancer at 5 and 30 years after exposure (P value = 0.47, 0.46, respectively). Similarly, there was no significant difference in lung cancer LAR values between females and males (P value = 0.48). The results also indicated a decrease in LAR values for both lung and breast cancers with increasing exposure age. In accordance with the ALARA (as low as reasonably achievable) principle, it is important for medical staff and the general public to consider the benefits of CT imaging in detecting such infections. Additionally, imaging medical physicists and CT scan experts should optimize imaging protocols and strike a balance between image quality for detecting abnormalities and radiation dose, all while adhering to the ALARA principle.

Impact of gadolinium concentration and cell oxygen levels on radiobiological characteristics of gadolinium neutron capture therapy technique in brain tumor treatment

Neutron capture therapy (NCT) with various concentrations of gadolinium (157Gd) is one of the treatment modalities for glioblastoma (GBM) tumors. Current study aims to evaluate how variations of 157Gd concentration and cell oxygen levels can affect the relative biological effectiveness (RBE) of gadolinium neutron capture therapy (GdNCT) technique through a hybrid Monte Carlo (MC) simulation approach. At first, Snyder phantom including a spherical tumor was simulated by Geant4 MC code and relevant energy electron spectra to different 157Gd concentrations including 100, 250, 500, and 1000 ppm were calculated following the neutron irradiation of simulated phantom. Scored energy electron spectra were then imported to Monte Carlo damage simulation (MCDS) code to estimate RBE values (both RBESSB and RBEDSB) at different gadolinium concentrations and oxygen levels from 10 to 100%. The results indicate that variations of 157Gd can affect the energy spectrum of released secondary electrons including Auger electrons. Variation of gadolinium concentration from 100 to 1000 ppm in tumor region can change RBESSB and RBEDSB values by about 0.1% and 0.5%, respectively. Besides, maximum variations of 4.3% and 2% were calculated for RBEDSB and RBESSB when cell oxygen level changed from 10 to 100%. From the results, variations of considered gadolinium and oxygen concentrations during GdNCT can influence RBE values. Nevertheless, due to the not remarkable changes in the intensity of Auger electrons, a slight difference in RBE values would be expected at various 157Gd concentrations, although considerable RBE changes were calculated relevant to the oxygen alternations inside tumor tissue.

The effect of geometrical parameters of skin brachytherapy patch source on depth dose distribution using Monte Carlo simulation

Brachytherapy of superficial skin tumors using beta-emitting sources is a method that has been investigated by some researchers in both simulation and experimental studies with promising results. In the current study, the effect of geometrical parameters of some relevant radionuclides including Y-90, Re-188, P-32, and Ho-166 on the depth dose distribution in skin tissue has been investigated through Monte Carlo simulation. MCNPX Monte Carlo code was employed to model the above-mentioned patch sources in cylindrical format and then the effect of patch geometrical parameters including the source-to-skin distance (SSD), patch thickness, and patch diameter on depth dose distribution was assessed through modeling and calculation of the dose inside a cubic phantom mimicking the skin tissue. The obtained results demonstrated that increasing the SSD, patch thickness, and patch diameter (with the same activity) will reduce the depth dose distribution. Changing the SSD has a more significant effect on the dose gradient within the depth than other geometrical parameters. It was also observed that the effect of patch diameter on the skin-delivered dose gets less sensible as the patch size goes beyond the range of beta radiation inside tissue. Finally, it can be concluded that the patch source geometrical parameters can affect the depth dose distribution inside the skin tissue. This fact may be of concern regarding the delivery of a high radiation dose in a single treatment session. Therefore, variations of patch source geometrical parameters should be considered during the skin dose calculation plan.

An inter-comparison between radiobiological characteristics of a commercial low-energy IORT system by Geant4-DNA and MCDS Monte Carlo codes

INTRODUCTION

The need for accurate relative biological effectiveness (RBE) estimation for low energy therapeutic X-rays (corresponding to 50 kV nominal energy of a commercial low-energy IORT system (INTRABEAM)) is a crucial issue due to increased radiobiological effects, respect to high energy photons. Modeling of radiation-induced DNA damage through Monte Carlo (MC) simulation approaches can give useful information. Hence, this study aimed to evaluate and compare RBE of low energy therapeutic X-rays using Geant4-DNA toolkit and Monte Carlo damage simulation (MCDS) code.

MATERIALS AND METHODS

RBE calculations were performed considering the emitted secondary electron spectra through interactions of low energy X-rays inside the medium. In Geant4-DNA, the DNA strand breaks were obtained by employing a B-DNA model in physical stage with 10.79 eV energy-threshold and the probability of hydroxyl radical's chemical reactions of about 0.13%. Furthermore, RBE estimations by MCDS code were performed under fully aerobic conditions.

RESULTS

Acquired results by two considered MC codes showed that the same trend is found for RBEDSB and RBESSB variations. Totally, a reasonable agreement between the calculated RBE values (both RBESSB and RBEDSB) existed between the two considered MC codes. The mean differences of 9.2% and 1.8% were obtained between the estimated RBESSB and RBEDSB values by two codes, respectively.

CONCLUSION

Based on the obtained results, it can be concluded that a tolerable accordance is found between the calculated RBEDSB values through MCDS and Geant4-DNA, a fact which appropriates both codes for RBE evaluations of low energy therapeutic X-rays, especially in the case of RBEDSB where lethal damages are regarded.

Evaluating the induced photon contamination by different breast IOERT shields using Monte Carlo simulation

BACKGROUND AND OBJECTIVE

Avoiding the underlying healthy tissue over-exposure during breast intraoperative electron radiotherapy (IOERT) is owing to the use of some dedicated radioprotection disks during patient irradiation. The originated contaminant photons from some widely used double-layered shielding disks including PMMA+Cu, PTFE+steel, and Al+Pb configurations during the breast IOERT have been evaluated through a Monte Carlo (MC) simulation approach.

METHODS

Produced electron beam with energies of 6, 8, 10, and 12 MeV by a validated MC model of Liac12 dedicated IOERT accelerator was used for disk irradiations. Each of above-mentioned radioprotection disks was simulated inside a water phantom, so that the upper disk surface was positioned at R90 depth of each considered electron energy. Simulations were performed by MCNPX (version 2.6.0) MC code. Then, the energy spectra of the contaminant photons at different disk surfaces (upper, middle, and lower one) and relevant contaminant dose beneath the studied disks were determined and compared.

RESULTS

None of studied shielding disks show significant photon contamination up to 10 MeV electron energy, so that the induced photon dose by the contaminant X-rays was lower than those observed in the disk absence under the same conditions. In return, the induced photon dose at a close distance to the lower disk surface exceeded from calculated values in the disk absence at 12 MeV electron energy. The best performance in contaminant dose reduction at the energy range of 6-10 MeV belonged to the Al+Pb disk, while the PMMA+Cu configuration showed the best performance in this regard at 12 MeV energy.

CONCLUSION

Finally, it can be concluded that all studied shielding disks not only don't produce considerable photon contamination but also absorb the originated X-rays from electron interactions with water at the electron energy range of 6-10 MeV. The only concern is related to 12 MeV energy where the induced photon dose exceeds the dose values in the disk absence. Nevertheless, the administered dose by contaminant photons to underlying healthy tissues remains beneath the tolerance dose level by these organs at the entire range of studied electron energies.

Comparing the DNA-damage RBE of intraoperative and conventional electron beams using a hybrid simulation approach

PURPOSE

Employing electron beam for radiotherapy purposes now has been established as one of the standard cancer treatment modalities. Both dedicated intraoperative and conventional electron beams can be employed in patient irradiation. Due to the differences between accelerating structure and electron beam delivery of dedicated intraoperative radiotherapy (IORT) machines and conventional ones, the initial energy spectra of the produced electron beam by these machines may be different. Accordingly, this study aims to evaluate whether these spectral differences can affect the relevant relative biological effectiveness (RBE) values of intraoperative and conventional electron beams.

MATERIALS AND METHODS

A hybrid Monte Carlo simulation approach was considered. At first, the head LIAC12 machine (as an IORT accelerator) and Varian 2100C/D (as a conventional accelerator) were simulated by MCNPX code and electron energy spectra at different depths and off-axis distances were scored for two nominal electron energies of 6 and 12 MeV at the field sizes of 6 and 10 cm. Then, the calculated spectra were imported to MCDS code to estimate the induced DNA-damage RBE values. Finally, the obtained RBE values for intraoperative and conventional electron beams were compared together.

RESULTS

The results showed that the RBE values of the intraoperative electron beam are superior to those obtained for conventional electron beam at the same energy/field size combination. Variations of the depth can regularly affect the RBE value for both conventional and intraoperative electron beams, while no ordered variation trend was observed for RBE with changing the off-axis distance. Variations of electron energy and field size can also influence the RBE value for both types of studied electron beams.

CONCLUSIONS

From the results, it can be concluded the structural differences between the dedicated IORT and conventional Linacs can lead to distinct initial electron energy spectra for intraoperative and conventional electron beams. These physical differences can finally lead to different RBE values for intraoperative and conventional electron beams at the same energy and field size.

Comparison of derived correction factors for effects of ion recombination and photon beam quality index following TG-51 and TRS-398 dosimetry protocols

In this study, ion recombination correction factor (k_S) and beam quality conversion factor ( [Formula: see text] ) values were extracted following the recommendations of the TRS-398 and TG-51 dosimetry protocols for widely used cylindrical ionization chambers for high energy photon beam dosimetry to quantify the agreement between the instructions for these two protocols for absolute dosimetry inside water. Four different types of cylindrical ionization chambers comprising Farmer (TM30013), Semiflex 0.125 cm³ (TM31010), Semiflex 0.3 cm³ (TM31013), and PinPoint (TM31016) were considered, and k_S and [Formula: see text] values were determined at photon energies of 6 MV and 15 MV. The maximum difference between the measured k_S values according to the instructions in the TRS-398 and TG-51 protocols was 0.03%. The k_S data measured with both protocols agreed well with those measured by using the Jaffe-plot approach, where the maximum difference was about 0.33%. The observed differences between the [Formula: see text] factors measured by using the TRS-398 and TG-51 dosimetry protocols at photon energies of 6 MV and 15 MV were 0.37% and 0.55%, respectively. The [Formula: see text] values measured using the TG-51 dosimetry protocols were slightly closer to those measured by a reference ionization chamber dosimeter. We conclude that the maximum differences were about 0.4% and 0.6% in the absorbed dose measurements according to the TRS-398 and TG-51 instructions at photon energies of 6 MV and 15 MV, respectively. The type of ionization chamber employed also affected the differences, where the maximum and minimum dose differences were found using the Farmer and PinPoint chambers, respectively.

On the measurement of scaling factors in the RW3 plastic phantom during high energy electron beam dosimetry

Ionometric electron dosimetry inside water-equivalent plastic phantoms demands special considerations including determination of depth scaling and fluence scaling factors (c_pl and h_pl) to shift from in-phantom measurements to those relevant to water. This study evaluates these scaling factors for RW3 slab phantom and also introduces a new coefficient, k(RW3), for direct conversion from RW3 measurements to water without involving scaling factors. The RW3 solid phantom developed by the PTW Company was used and the corresponding scaling factors including c_pl, h_pl, and k(RW3) were measured for conventional electron energies of 4, 6, 9, 12, and 16 MeV. Separate measurements were performed in water and the RW3 slab phantom using the Advanced Markus chamber. The validity of the reported scaling factors was confirmed by comparing the direct and indirect percentage depth dose (PDD) measurements in water and in the RW3 phantom. The c_pl values for the RW3 phantom were respectively equal to 0.915, 0.927, 0.934, 0.937, and 0.937 for 4, 6, 9, 12, and 16 MeV electron energies. The h_pl and k(RW3) values were dependent on the depth of investigation and electron energy. Application of the c_pl-h_pl factors and k(RW3) coefficients to measured data inside the RW3 can reliably reproduce the measured PDD curves in water. The mean difference between the PDDs measured directly and indirectly in water and in the RW3 phantom was less than 1.2% in both approaches for PDD conversion (c_pl-h_pl coupling and the use of k(RW3)). The measured scaling factors and k(RW3) coefficients are sufficiently relevant to mimic water-based dosimetry results through indirect measurements inside the RW3 slab phantom. Nevertheless, employing k(RW3) is more straightforward than the c_pl-h_pl approach because it does not involve scaling and it is also less time-consuming.

Performance evaluation of buildup bolus during external radiotherapy of mastectomy patients: treatment planning and film dosimetry

A buildup bolus is used during the post-mastectomy radiotherapy (PMRT) to overcome under-dosage issues in the chest wall. The current study is aimed at evaluating the performance of a bolus in dose enhancement through both film dosimetry and treatment planning approaches. Twenty patients were enrolled in current research. The received dose by the skin at the lateral and medial regions of the chest wall in the presence and absence bolus was evaluated. Film dosimetry results showed that the presence of the bolus can averagely increase the skin dose by about 80% (P value < 0.001) and 92% (P value < 0.001) in lateral and medial regions, respectively. No significant difference was observed between the measured and treatment planning system (TPS)-calculated dose values in the presence of bolus. The presence of the bolus can considerably increase the absorbed dose by superficial chest wall regions. The TPS shows a favorable performance in superficial dose calculations in the presence of the buildup bolus. Hosseini et al.: demonstration of implemented research in the current study.

Comparing the inter-seed effect for some iodine-125 brachytherapy sources through a Monte Carlo simulation approach

BACKGROUND AND OBJECTIVE

One of the principal deficits of TG-43(U1) dosimetry protocol is ignoring the attenuation effect of each seed during the multiseed brachytherapy implants. To take into account this shadowing effect, a parameter known as the inter-seed effect (ISE) has been introduced. Current study aims to evaluate and compare the ISE for some I-125 brachytherapy seeds through a Monte Carlo (MC) simulation approach.

METHODS

Five different models of I-125 seeds including 6711, ProstaSeed, STM 1251, 3500, and IAI-125A were simulated by MCNPX MC Code. Validity of simulated seed models was confirmed through comparing the corresponding dosimetric parameters such as radial dose function and anisotropy function with those reported by relevant literature. Then, the ISE for each seed at different distances was determined in three seeds implant configuration. Additionally, the relevant attenuation factors (AF) were also determined for each brachytherapy source.

RESULTS

The obtained results demonstrated that minimum inter-seed attenuation belongs to the ProstaSeed. In return, the maximum inter-seed attenuation was found for 3500 brachytherapy seed. The mean attenuation factor for 6711, ProstaSeed, STM 1251, 3500, and IAI-125A seed models was equal to 0.764, 0.829, 0.785, 0.594, and 0.772, respectively.

CONCLUSIONS

It can be concluded that the considered I-125 brachytherapy seeds show different inter-seed effects and mean AF values for the same multiseed implant arrangement. This finding can be attributed to discrepancies in internal seed design and configuration.

Investigation of radiation dose around C-arm fluoroscopy and relevant cancer risk to operating room staff

This study aimed to evaluate the ambient dose equivalent around a C-arm device during spinal surgeries and determine the optimum locations for the surgeon and staff to keep radiation exposure as low as reasonably achievable. Furthermore, cancer risk incidence was estimated using the excess relative risk (ERR) concept of the biologic effects of ionizing radiation VII report for operating room (OR) staff. A lateral projection of the C-arm setup was considered in the current study. The ambient dose equivalent rate was measured using an electronic dosimeter in 30° steps all around for 1, and 1.6-m heights as well as 1, and 2-m distances away from a water tank (scattering medium). By assuming a typical workload, the annual ambient dose and a maximum number of permissible operations were determined. For a worst-case scenario, the dose was used to estimate the ERR for various organs including prostate, ovary, breast, lung, thyroid, and colon for attained ages of 35, 40, and 50 years. The maximum ambient dose equivalent rate was seen at 330° and 30° (about 600 µSv/h at 1 m height and a distance of 1 m from the scattering medium). The corresponding permissible workload for an OR staff was about 30,660 operations. Based on the obtained results, 60° next to the image intensifier was the optimum position for the surgeon, while 30° next to the tube was the worst position because of backscattered radiation. The ERR results showed that the lung and colon have the highest cancer risk incidence among the considered organs for both males and females, respectively.

Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry

The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol. Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (k_pol), recombination (k_s), and quality conversion factor ( [Formula: see text] ) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (z_ref), and 10 × 10 cm² field size at the phantom surface. The maximum and minimum range of k_pol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum k_s values also belonged to the Markus ionization chamber, while the minimum k_s values were observed for the Advanced Markus chamber. The measured k_s values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R₅₀), while it can have a more considerable impact on [Formula: see text] value, especially in the case of the Markus chamber. From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.

Impact assessment of breast glandularity on relative biological effectiveness of low energy IORT X-rays through Monte Carlo simulation

INTRODUCTION

Intraoperative radiotherapy (IORT) by low energy X-rays is a single fraction treatment modality for tumor bed irradiation after breast-conserving surgery. It has been shown that the variations of breast tissue composition can affect the absorbed dose in this method. Apart from physical quantities such as absorbed dose value, radiobiological quantities including relative biological effectiveness (RBE) may also change with the variations of breast tissue composition. Accordingly, the current study aims to quantify both single and double-strand break RBE values (RBE_SSB and RBE_DSB) of low energy X-rays at different breast glandular fractions using a hybrid Monte Carlo (MC) simulation approach.

MATERIALS AND METHODS

Produced low-energy X-rays by a validated MC model of INTRABEAM machine with 50 kV nominal voltage were considered as the radiation source. The secondary electron energy spectra at various depths inside the breast tissue with different glandular fractions were scored through GEANT4 MC Toolkit. Calculated spectra were then imported to MCDS MC code for DNA strand break calculation and RBE assessment. Both RBE_SSB and RBE_DSB were calculated for various breast glandular fractions.

RESULTS

Changing the breast glandularity can affect both the trend of secondary electron spectra and relevant RBE values at different depths inside the breast volume. In this regard, RBE_SSB increments by about 1% with increasing the breast glandular fraction from 0% to 100%. On the other hand, RBE_DSB decrements by about 3.3% with increasing the glandular fraction in the range of 0% to 100%. Variations of the depth within the breast tissue can also influence the RBE value so that RBE_SSB reduces by about 1% with increasing the depth from 2 mm to 10 mm one, while RBE_DSB increases about 3.4%. The relevant RBE_SSB and RBE_DSB values to the entire target volume (breast PTV) respectively increment and decrement by about 0.8% and 3.2% with increasing the breast glandularity from 0% to 100%.

CONCLUSION

From the results, it can be concluded that the breast tissue composition has a measurable effect on RBE values of employed low energy X-rays during breast IORT which can cause variations of prescribed dose for patients with distinct breast glandularity fractions.

Monte Carlo-based calculation of nano-scale dose enhancement factor and relative biological effectiveness in using different nanoparticles as a radiosensitizer

INTRODUCTION

Nowadays, some nanoparticles (NPs) are known and used as radiosensitizers in radiotherapy and radiobiology, due to their desired biological, physical, and chemical effects on cells. This study aimed to evaluate and compare the dose enhancement factor (DEF) and the biological effectiveness of some common NPs through EGSnrc and MCDS Monte Carlo (MC) simulation codes.

MATERIALS AND METHODS

To evaluate considered NPs' DEF, a single NP with 50 nm diameter was simulated at the center of concentric spheres. NP irradiations were done with 30, 60, and 100 keV photon energies. The secondary electron spectra were scored at the surface of considered NPs, and the dose values were scored at surrounding water-filled spherical shells which were distributed up to 4000 nm from the NP surface. The electron spectra were used in the MCDS code to obtain different initial DNA damages for the calculation of enhanced relative biological effectiveness (eRBE).

RESULTS

By decreasing the photon energy, an increment of DEF was seen for all studied NPs. The maximum DEF at 30, 60, and 100 keV photon energies were respectively related to silver (Ag), gadolinium (Gd), and bismuth (Bi) NPs. The maximum double-strand break (DSB) related (eRBE_DSB) values for the 30 keV photon belonged to Ag, while BiNPs showed the maximum values at other photon energies. The minimum eRBE_DSB values were also related to iron (Fe) NPs at the entire range of studied photon energies.

CONCLUSIONS

The compared nanoscale physical and biological results of our study can be helpful in the selection of optimum NP as a radiosensitizer in future radiobiological studies. Bi, gold (Au), Ag, and platinum (Pt) NPs had great potential, respectively, as radiosensitizers relative to the other studied NPs.

Dosimetric comparison between different tangential field arrangements during left-sided breast cancer radiotherapy

This study aimed to evaluate variations in dose distribution within the target volume and dose received by the organs at risk (OARs) for different tangential field arrangements during three-dimensional (3D) conformal treatment planning for left-sided breast cancer. Computed tomography (CT) images of 25 breast cancer patients were included, and three different mono-isocentric half-block (MIHB) treatment plans-parallel central axis technique (PCAXT), posterior border parallel technique (PBPT), and parallel quadrant technique (PQUDT)-were considered for each patient. The dosimetric and geometric parameters related to each followed plan were then extracted for the planning target volume (PTV) and the OARs, and compared. The results showed no significant differences among the extracted dosimetric and geometric parameters of the OARs for the different plans, while the D_max, V_95%, homogeneity index (HI), and conformity index (CI) values related to the PTV were significantly different (P < 0.05). The lowest D_max and V_95% values inside the PTV were related to the PCAXT plan. The best HI was achieved with the PBPT plan, whereas the best CI was observed for the PCAXT plan. The best correlation between the geometric and dosimetric parameters of the OARs was between V_5Gy-central lung distance for the ipsilateral lung and the V_5Gy-maximum heart distance for the heart in all plans. These results demonstrate that variations in the tangential field arrangement at the posterior border for optimal coverage of the PTV may not considerably affect the dose received by the OARs.

Effective energy assessment during breast cancer intraoperative radiotherapy by low-energy X-rays: A Monte Carlo study

The study reported in the present paper aimed to evaluate the effective energy (E_eff) of X-rays emitted from the surface of a bare X-ray probe and from different spherical applicators with various diameters, which are widely employed for low kV intraoperative radiotherapy (IORT) of breast cancer. A previously validated Monte Carlo model of the INTRABEAM system along with applicator diameters of 1.5-5 cm (with 0.5 cm increments) was employed for this purpose. The results show that the presence of the applicator can considerably harden the X-rays produced by the bare probe so that E_eff increases by a factor of about 2.6. Variations of applicator diameter also affects the X-ray effective energy. Specifically, increasing the applicator diameter from 1.5 to 3 cm and 3.5-5 cm resulted in an increase in the E_eff by 8.8% and 14.6%, respectively. The validity of the calculated E_eff values was confirmed by a reasonable agreement between the obtained probability density distributions (PDDs) for the full X-ray energy spectrum and those for the corresponding single effective energies, for different applicator diameters. The E_eff values obtained for different applicator diameters and the bare probe alone can be used as an alternative for the corresponding full energy spectra, in Monte Carlo-based dosimetry simulations of low-energy therapeutic X-rays, as well as for determining quality conversion factors of any ion chambers employed for low kV-IORT absolute dosimetry.

An inter-comparison between accuracy of EGSnrc and MCNPX Monte Carlo codes in dosimetric characterization of intraoperative electron beam

BACKGROUND

Ionometric dosimetry in IOERT is a complicated process, due to the sophisticated beam setup and the necessity for dedicated protocols for ion chamber response correction. On the other hand, the Monte Carlo (MC) technique can easily overcome such limitations and be considered as an alternative dosimetry approach. This paper presents a comparative analysis of two widely used MC codes, EGSnrc and MCNPX, for intraoperative electron beam dosimetry.

METHOD

The head of LIAC12, a dedicated IOERT accelerator, was modeled by both mentioned MC codes. Then, the percentage depth dose (PDD) curves, transverse dose profiles (TDPs), and output factor (OF) values were accordingly calculated within the water phantom. To realize the accuracy of MC codes in dosimetric characterization of intraoperative electron beam, their results were finally compared with those measured by corresponding ionometric dosimetry for all forms of electron energy/applicator size.

RESULTS

A good agreement was observed between the simulated and measured PDDs/TDPs for both considered MC codes, such that the calculated gamma index values were always lower than unity for both considered MC codes. Nevertheless, the lower gamma index values were found in the case of the EGSnrc code. The maximum difference between the measured and calculated OF was obtained as 2.3% and 3.1% for EGSnrc and MCNPX code, respectively.

CONCLUSIONS

Although both studied MC codes showed compatible results with the measured ones, EGSnrc code has superior accuracy in this regard and can be considered as a more reliable toolkit in Monte Carlo-based commissioning of dedicated IOERT accelerators.

Comparing the performance of some dedicated radioprotection disks in breast intraoperative electron radiotherapy: a Monte Carlo study

Radiation-shielding of healthy tissue is mandatory in breast intraoperative electron radiotherapy (IOERT). In this regard, dedicated radioprotection disks have been introduced. The aim of this study was to evaluate and compare the performance of three radioprotection disks widely used for breast IOERT. A Monte Carlo simulation approach was used for this purpose. The considered disks included Al + Pb, PMMA + Copper, and PTFE + Steel. They were stimulated by means of the MCNPX Monte Carlo code at depths around R₁₀₀ and R₉₀ of different electron energies in a water phantom, and their impact on the dosimetric properties of the therapeutic beam was evaluated in both correct and upside down disk placements. The electron energy spectrum immediately above and below each disk was calculated and analyzed. Furthermore, performance characteristics of the studied disks such as backscatter factors (BSFs) and transmission factors (TFs) at different electron energies were determined and compared. The results show that the Al + Pb disk most effectively attenuates the beam, while at the same time exhibits maximum BSF values. Employing the PMMA + Copper disk can minimize the BSF value but at the expense of an increased TF. The Al + Pb disk showed the best performance from the radiation protection viewpoint, while its highest BSF values could lead to perturbation of dose homogeneity within the target volume. PTFE + Steel disk showed an intermediate performance regarding the electron backscattering and transmission among the studied disks. The reverse placement of each disk can substantially increase the BSF value as compared to the correct situation but had less impact on the TF value.

Monte Carlo-based determination of radiation leakage dose around a dedicated IOERT accelerator

Evaluating the stray radiation around medical electron accelerators is a mandatory issue. Surveying the radiation leakage dose is important for patients, technicians, and health physicists, due to radiation protection aspects. Consequently, radiation leakage dose around the head of a mobile-dedicated intraoperative radiotherapy accelerator (LIAC), at different electron energies and field sizes have been evaluated in this study. More specifically, the MCNPX Monte Carlo code was used to model the LIAC head, connected applicator, and employed water phantom. Radiation leakage dose around the LIAC head was calculated for different energy and field sizes through tuning the Monte Carlo results to the practically measured doses. These measurements were performed using an Advance Markus ionization chamber inside an automated MP3-XS water phantom. The good agreement between the calculated dose distributions within the water tank and corresponding dose measurements show that the simulation model of the LIAC head is appropriate for radiation leakage assessment. The obtained radiation leakage dose distribution highly depends on the electron energy and applicator diameter. With increasing the electron energy, the leakage dose decreased, while increasing the field size increased the leakage dose. It is concluded that the rate of stray radiation and leakage dose around the LIAC head in both vertical and horizontal planes were acceptable according to the recommended radiation protection criteria. To meet the recommended dose limit (100 µSv/week for controlled areas), the maximum number of patients should be kept to four patients per week inside a standard and unshielded operating room.

Breast intraoperative electron radiotherapy: Image-based setup verification and in-vivo dosimetry

INTRODUCTION

Single fraction nature of intraoperative radiotherapy highly demands a quality assurance procedure to qualify both beam setup and treatment delivery. The aim of this study is to evaluate the treatment setup during breast intraoperative electron radiotherapy (IOERT) and in-vivo dose delivery verification.

MATERIALS AND METHODS

Twenty-five breast cancer patients were enrolled and setup verification for each case was performed using C-arm imaging. The received dose by surface and distal end of target was measured by EBT2 film. The significance level of difference between obtained dosimetry results and predicted ones was evaluated by the T statistical test.

RESULTS

Acquired C-arm images in two different oblique views revealed any misalignment between the applicator and shielding disk. The mean difference between the measured surface dose and expected one was 1.8% ± 1.2 (p = 0.983) while a great disagreement, 11.1% ± 1.5 (p < 0.001), was observed between the measured distal end dose and expected one. This discrepancy is mainly correlated to the backscattering effect from the shielding disk. Target depth nonuniformities can also contribute to this remarkable difference.

CONCLUSION

Employing the intraoperative imaging for IOERT setup verification can considerably improve the treatment quality. Therefore, it is suggested to implement this imaging procedure as a part of treatment quality assurance. Favorable agreement between the predicted and measured surface doses demonstrates the applicability of EBT2 film for dose delivery verification. The results of in-vivo dosimetry showed that the electron backscattering from employed shielding disk can affect the received dose by the distal end of tumor bed.

Monte Carlo Simulation of Electron Beams produced by LIAC Intraoperative Radiation Therapy Accelerator

Background

One of the main problems of dedicated IORT accelerators is to determine dosimetric characteristics of the electron beams. Monte Carlo simulation of IORT accelerator head and produced beam will be useful to improve the accuracy of beam dosimetry.

Materials and Methods

Liac accelerator head was modeled using the BEAMnrcMonte Carlo simulation system. Phase-space files were generated at the bottom of the applicators. These phase-space files were used as an input source in DOSXYZnrc and BEAMDP codes for dose calculation and analysis of the characteristic of the electron beams in all applicators and energies.

Results

The results of Monte Carlo calculations are in very close agreement with the measurements. There is a decrease in the peak of the initial spectrum when electrons come from the end of accelerator wave guide to the end of applicator. By decreasing the applicator diameter, the mean energy of electron beam decreased. Using applicators and increasing their size, X-ray contamination will increase. The percentage of X-ray contamination increases by applicator diameter. This is related to the increase of the mean energy of electron beams.

Conclusion

Application of PMMA collimator leads to, although well below accepted level, the production of bremsstrahlung. The results of this study showed that special design of LIAC head accompanying by PMMA collimator system cause to produce an electron beam with an individual dosimetric characteristic making it a useful tool for intraoperative radiotherapy purposes.

Monte Carlo Determination of Dosimetric Parameters of a New (125)I Brachytherapy Source According to AAPM TG-43 (U1) Protocol

BACKGROUND

(125)I is one of the important sources frequently used in brachytherapy. Up to now, several different commercial models of this source type have been introduced to the clinical radiation oncology applications. Recently, a new source model, IrSeed-125, has been added to this list. The aim of the present study is to determine the dosimetric parameters of this new source model based on the recommendations of TG-43 (U1) protocol using Monte Carlo simulation.

METHODS

The dosimetric characteristics of Ir-125 including dose rate constant, radial dose function, 2D anisotropy function and 1D anisotropy function were determined inside liquid water using MCNPX code and compared to those of other commercially available iodine sources.

RESULTS

The dose rate constant of this new source was found to be 0.983+0.015 cGyh-1U-1 that was in good agreement with the TLD measured data (0.965 cGyh-1U-1). The 1D anisotropy function at 3, 5, and 7 cm radial distances were obtained as 0.954, 0.953 and 0.959, respectively.

CONCLUISON

The results of this study showed that the dosimetric characteristics of this new brachytherapy source are comparable with those of other commercially available sources. Furthermore, the simulated parameters were in accordance with the previously measured ones. Therefore, the Monte Carlo calculated dosimetric parameters could be employed to obtain the dose distribution around this new brachytherapy source based on TG-43 (U1) protocol.

Comparing the dosimetric characteristics of the electron beam from dedicated intraoperative and conventional radiotherapy accelerators

The specific design of the mobile dedicated intraoperative radiotherapy (IORT) accelerators and different electron beam collimation system can change the dosimetric characteristics of electron beam with respect to the conventional accelerators. The aim of this study is to measure and compare the dosimetric characteristics of electron beam produced by intraoperative and conventional radiotherapy accelerators. To this end, percentage depth dose along clinical axis (PDD), transverse dose profile (TDP), and output factor of LIAC IORT and Varian 2100C/D conventional radiotherapy accelerators were measured and compared. TDPs were recorded at depth of maximum dose. The results of this work showed that depths of maximum dose, R90,R50, and RP for LIAC beam are lower than those of Varian beam. Furthermore, for all energies, surface doses related to the LIAC beam are substantially higher than those of Varian beam. The symmetry and flatness of LIAC beam profiles are more desirable compared to the Varian ones. Contrary to Varian accelerator, output factor of LIAC beam substantially increases with a decrease in the size of the applicator. Dosimetric characteristics of beveled IORT applicators along clinical axis were different from those of the flat ones. From these results, it can be concluded that dosimetric characteristics of intraoperative electron beam are substantially different from those of conventional clinical electron beam. The dosimetric characteristics of the LIAC electron beam make it a useful tool for intraoperative radiotherapy purposes.

PACS number: 87.56.‐v, 87.56.bd

Application of Gafchromic EBT2 film for intraoperative radiation therapy quality assurance

PURPOSE

Intraoperative radiation therapy (IORT) using electron beam is commonly done by mobile dedicated linacs that have a variable range of electron energies. This paper focuses on the evaluation of the EBT2 film response in the green and red colour channels for IORT quality assurance (QA).

METHODS

The calibration of the EBT2 films was done in two ranges; 0-8 Gy for machine QA by red channel and 8-24 Gy for patient-specific QA by green channel analysis. Irradiation of calibration films and relative dosimetries were performed in a water phantom. To evaluate the accuracy of the film response in relative dosimetry, gamma analysis was used to compare the results of the Monte Carlo simulation and ionometric dosimetry. Ten patients with early stage breast cancer were selected for in-vivo dosimetry using the green channel of the EBT2 film.

RESULTS

The calibration curves were obtained by linear fitting of the green channel and a third-order polynomial function in the red channel (R2=0.99). The total dose uncertainty was up to 4.2% and 4.7% for the red and green channels, respectively. There was a good agreement between the relative dosimetries of films by the red channel, Monte Carlo simulations and ionometric values. The mean dose difference of the in-vivo dosimetry by green channel of this film and the expected values was about 1.98% ± 0.75.

CONCLUSION

The results of this study showed that EBT2 film can be considered as an appropriate tool for machine and patient-specific QA in IORT.

Dosimetric evaluation of Gafchromic EBT2 film for breast intraoperative electron radiotherapy verification

PURPOSE

Quality assurance (QA) is one of the most important issues that should be addressed for intraoperative electron radiotherapy (IOERT), which is not benefiting from image-based treatment planning system. The aim of this study is to evaluate the dosimetric characteristics of Gafchromic EBT2 film for breast IOERT QA procedure.

METHODS

Due to the fact that some dedicated accelerators are being used for IOERT, dependence of the film response to energy, field size, dose rate and incidence angle of electron beam from the LIAC IOERT accelerator was studied. Then, film response curve to breast IOERT doses was obtained and its accuracy was evaluated and justified through comparison to the results of ionometric dosimetry.

RESULTS

The results of this study indicated that there are no significant differences between the film responses at different energies of 6, 8, 10 and 12 MeV (P-value = 0.99). Similarly, no field size dependency was found when evaluating the response of the film to different field sizes ranging from 4 to 10 cm (P-value = 0.94). Film response was found to be independent of the dose rate of intraoperative electron beam (P-value = 0.12). Film response variations with changing the beam incidence angle were not significant (P-value > 0.8). Calibration curve at the dose range of 8-24 Gy had an acceptable accuracy. The difference between the results of film dosimetry and ionometric dosimetry was around 5% which was in agreement with the results of dose uncertainty estimation.

CONCLUSION

The EBT2 film was found to be a potentially appropriate tool for breast IOERT verification.

Effectiveness Evaluation of Skin Covers against Intravascular Brachytherapy Sources Using VARSKIN3 Code

BACKGROUND AND OBJECTIVE

The most common intravascular brachytherapy sources include (32)P, (188)Re, (106)Rh and (90)Sr/(90)Y. In this research, skin absorbed dose for different covering materials in dealing with these sources were evaluated and the best covering material for skin protection and reduction of absorbed dose by radiation staff was recognized and recommended.

METHOD

Four materials including polyethylene, cotton and two different kinds of plastic were proposed as skin covers and skin absorbed dose at different depths for each kind of the materials was calculated separately using the VARSKIN3 code.

RESULTS

The results suggested that for all sources, skin absorbed dose was minimized when using polyethylene. Considering this material as skin cover, maximum and minimum doses at skin surface were related to (90)Sr/(90)Y and (106)Rh, respectively.

CONCLUSION

polyethylene was found the most effective cover in reducing skin dose and protecting the skin. Furthermore, proper agreement between the results of VARSKIN3 and other experimental measurements indicated that VRASKIN3 is a powerful tool for skin dose calculations when working with beta emitter sources. Therefore, it can be utilized in dealing with the issue of radiation protection.