Indices for the evaluation of glandular dose heterogeneity in full-field digital mammography.

This study aims to evaluate the indices of glandular dose heterogeneity in full-field digital mammography. The distributions of GD in a breast phantom with a skin layer of 4 mm were determined using the Monte Carlo method with simulated x-ray fluence spectra. First, the GD to air kerma (GD/Kair) volume histogram was obtained from the GD distributions, which were indicated by the glandular volume (%) as a function of GD/Kair. The GD indices, namely, the maximum glandular dose (GD2%) and glandular volume percentage (%) receiving at least the mean glandular dose (MGD) (VMGD) were calculated from the GD/Kairvolume histogram. Next, the scatter plots of GD2%/MGD andVMGDwere drawn as functions of the normalised mean glandular dose (DgN). Finally, (GD2%)iand (VMGD)iwere obtained from the relationship between the GD indices and DgN for 596 clinical irradiation cases based on individual irradiation conditions. The values of GD2%/MGD were more affected by breast thickness than glandularity and tube voltage, and they decreased according to the power law of DgN for all the target/filter combinations. The values ofVMGDwere proportional to DgN and decreased with increase in the compressed breast thickness. The values of (MGD)iand (GD2%)ifor 596 clinical irradiation cases were estimated to range from 0.6-3.0 mGy to 1.1-7.0 mGy, respectively, and (VMGD)iwas in the range 32%-48%. (GD2%)iand (VMGD)iare mainly affected by breast thickness. These indices are useful for the evaluation of glandular dose heterogeneity in mammography.

Evaluation of Absorbed Dose for CBCT in Image-guided Radiation Therapy: Comparison of Each Devices and Facilities.

Recently, intensity-modulated radiation therapy (IMRT) is used worldwide, highly accurate verification of the location using image-guided radiation therapy (IGRT) has become critical. However, the use of cone-beam computed tomography (CBCT) to ascertain the location each time raises concerns about its influence on radiotherapy dosage and increased radiation exposure. The purpose of this study was to measure the absorbed dose using nine kilovoltage (kV) devices and two megavoltage (MV) devices (total 11 devices) at eight facilities, compare the absorbed dose among the devices, and assess the characteristics of the respective devices to ensure optimal clinical operation. For the measurement of the absorbed dose, a farmer-type ionization chamber dosimeter, calibrated using a 60Co and an IMRT dose verification phantom manufactured from water-equivalent material RW3, was used to measure the absorbed dose at nine points in the phantom for two regions, the pelvic and cephalic region. The average absorbed dose of the pelvic region was 3.09±0.21 cGy in kV-CBCT (OBI), 1.16±0.16 cGy in kV-CBCT (XVI), 5.64±1.48 cGy in MV-CBCT (4 MV), and 6.33±1.54 cGy in MV-CBCT (6 MV). The average absorbed dose of the cephalic region was 0.38±0.03 cGy in kV-CBCT (OBI), 0.23±0.06 cGy in kV-CBCT (XVI), 4.02±0.72 cGy in MV-CBCT (4 MV), and 4.46±0.77 cGy in MV-CBCT (6 MV). There was a difference in the absorbed dose at the measured points as well as in the dose distribution in the phantom cross section. No major difference was observed in the absorbed dose among identical devices, but a difference was identified among the devices installed at multiple facilities. Therefore, the angle of rotation should be paid attention to when CBCT is taken, and the image-taking conditions should be determined. In addition, it is important to handle the devices only after ascertaining the absorbed dose of each device.

Evaluation of a single-scan protocol for radiochromic film dosimetry.

The purpose of this study was to evaluate a single-scan protocol using Gafchromic EBT3 film (EBT3) by comparing it with the commonly used 24-hr measurement protocol for radiochromic film dosimetry. Radiochromic film is generally scanned 24 hr after film exposure (24-hr protocol). The single-scan protocol enables measurement results within a short time using only the verification film, one calibration film, and unirradiated film. The single-scan protocol was scanned 30 min after film irradiation. The EBT3 calibration curves were obtained with the multichannel film dosimetry method. The dose verifications for each protocol were performed with the step pattern, pyramid pattern, and clinical treatment plans for intensity-modulated radiation therapy (IMRT). The absolute dose distributions for each protocol were compared with those calculated by the treatment planning system (TPS) using gamma evaluation at 3% and 3 mm. The dose distribution for the single-scan protocol was within 2% of the 24-hr protocol dose distribution. For the step pattern, the absolute dose discrepancies between the TPS for the single-scan and 24-hr protocols were 2.0 ± 1.8 cGy and 1.4 ± 1.2 cGy at the dose plateau, respectively. The pass rates were 96.0% for the single-scan protocol and 95.9% for the 24-hr protocol. Similarly, the dose discrepancies for the pyramid pattern were 3.6 ± 3.5cGy and 2.9 ± 3.3 cGy, respectively, while the pass rates for the pyramid pattern were 95.3% and 96.4%, respectively. The average pass rates for the four IMRT plans were 96.7% ± 1.8% for the single-scan protocol and 97.3% ± 1.4% for the 24-hr protocol. Thus, the single-scan protocol measurement is useful for dose verification of IMRT, based on its accuracy and efficiency.

[Dose distribution from kV-cone beam computed tomography in image-guided radiotherapy].

Image-guided radiotherapy (IGRT) is an increasingly commonly adopted technique. As a result, however, total patient dose is increasing rapidly, especially when kV-cone beam computed tomography (CBCT) is applied. This study investigated the dosimetry of kV-CBCT using a Farmer ionization chamber with a (60)Co absorbed-dose calibration factor. The absorbed-dose measurements were performed using an I'mRT phantom (RW3, IBA) which is employed for dose verification of intensity-modulated radiotherapy (IMRT). The I'mRT phantom was used as a substitute for head and pelvis phantoms. The kV-CBCT absorbed dose was evaluated from a beam quality conversion factor of kV to (60)Co and the ionization ratio of the I'mRT phantom and water, calculated using the Monte Carlo method. The dose distribution in the I'mRT phantom was also measured using a radiophotoluminescent glass dosimeter (RGD). The absorbed doses for the pelvis phantom (full scan) ranged from 2.5-4 cGy for kV-CBCT and 4-8 cGy for MV-CBCT. TomoTherapy resulted in a lower dose of approximately 1.3 cGy due to fan-beam. For the head phantom (half scan), the doses ranged from 0.1-0.7 cGy for kV-CBCT and 3-5 cGy for MVCBCT. The results for RGD were similar to ion chamber measurements. It is necessary to decrease the absorbed dose of the organs at risk every time IGRT is applied.

[Physical characterizations for an integrated 160-leaf multi-leaf collimator with a new concept design].

In this article, we present a physical characterization of the agility(™) (Elekta). agility(™) is composed of 160 interdigitating multileaf collimators (MLCs) with a width of 5 mm at the isocenter. The physical characterizations that include leaf position accuracy, leakage, field penumbra and the tongue-and-groove (T&G) effect were evaluated using well-commissioned 4, 6 and 10-MV photon beams. The leaf position accuracy was within 0.5 mm for all gantry angles and each MLC. The leakage was 0.44% on average and reached 0.47% at 10 MV: remarkably low due to a new design with tilted leaves. However, the T&G effect occurred due to tilt. It was approximately 20.8% on average and reached 22.3% at 6 MV. The penumbra width increased up to 8.5 mm at a field size of 20×20 cm at 4 MV. High position designed MLCs create a wider penumbra but show lower leakage and large head clearance. Head clearance is an important factor in stereotactic radiotherapy with multiple non-coplanar beams.

[Comparison of dose calculation algorithms in stereotactic radiation therapy in lung].

Dose calculation algorithms in radiation treatment planning systems (RTPSs) play a crucial role in stereotactic body radiation therapy (SBRT) in the lung with heterogeneous media. This study investigated the performance and accuracy of dose calculation for three algorithms: analytical anisotropic algorithm (AAA), pencil beam convolution (PBC) and Acuros XB (AXB) in Eclipse (Varian Medical Systems), by comparison against the Voxel Monte Carlo algorithm (VMC) in iPlan (BrainLab). The dose calculations were performed with clinical lung treatments under identical planning conditions, and the dose distributions and the dose volume histogram (DVH) were compared among algorithms. AAA underestimated the dose in the planning target volume (PTV) compared to VMC and AXB in most clinical plans. In contrast, PBC overestimated the PTV dose. AXB tended to slightly overestimate the PTV dose compared to VMC but the discrepancy was within 3%. The discrepancy in the PTV dose between VMC and AXB appears to be due to differences in physical material assignments, material voxelization methods, and an energy cut-off for electron interactions. The dose distributions in lung treatments varied significantly according to the calculation accuracy of the algorithms. VMC and AXB are better algorithms than AAA for SBRT.

[Dose impact of a carbon fiber couch for stereotactic body radiation therapy of lung tumors].

The aim of this study was to measure the dose attenuation caused by a carbon fiber radiation therapy table (Imaging Couch Top; ICT, BrainLab) and to evaluate the dosimetric impact of ICT during stereotactic body radiation therapy (SBRT) in lung tumors. The dose attenuation of ICT was measured using an ionization chamber and modeled by means of a treatment planning system (TPS). SBRT was planned with and without ICT in a lung tumor phantom and ten cases of clinical lung tumors. The results were analyzed from isocenter doses and a dose-volume histogram (DVH): D95, Dmean, V20, V5, homogeneity index (HI), and conformity index (CI). The dose attenuation of the ICT modeled with TPS agreed to within ±1% of the actually measured values. The isocenter doses, D95 and Dmean with and without ICT showed differences of 4.1-5% for posterior single field and three fields in the phantom study, and differences of 0.6-2.4% for five fields and rotation in the phantom study and six fields in ten clinical cases. The dose impact of ICT was not significant for five or more fields in SBRT. It is thus possible to reduce the dose effect of ICT by modifying the beam angle and beam weight in the treatment plan.

Development of a phantom to evaluate the positioning accuracy of patient immobilization systems using thermoplastic mask and polyurethane cradle.

PURPOSE: The purpose of this study was to develop a new phantom to evaluate the positioning accuracy of patient immobilization systems. METHODS: The phantom was made of papers formed into a human shape, paper clay, and filling rigid polyester. Acrylonitrile butadiene styrene (ABS) pipes were inserted at anterior-posterior (A-P) and right-left (R-L) directions in the phantom to give static load by pulling ropes through the pipes. First, the positioning precision of the phantom utilizing a target locating system (TLS) was evaluated by moving the phantom on a couch along inferior-superior (I-S), A-P, and R-L directions in a range from -5 mm to +5 mm. The phantom's positions detected with the TLS were compared with values measured by a vernier caliper. Second, the phantom movements in a tensile test were chosen from patient movements determined from 15 patients treated for intracranial lesions and immobilized with a thermoplastic mask and polyurethane cradle. The phantom movement was given by minimum or maximum values of patient movements in each direction. Finally, the relationship between phantom movements and the static load in the tensile test was characterized from measurements using the new phantom and the TLS. RESULTS: The differences in all positions between the vernier caliper measurement and the TLS detected values were within 0.2 mm with frequencies of 100%, 95%, and 90% in I-S, A-P, and R-L directions, respectively. The phantom movements according to patient movements in clinical application in I-S, A-P, and R-L directions were within 0.58 mm, 0.94 mm, and 0.93 mm from the mean value plus standard deviation, respectively. The regression lines between the phantom movements and static load were given by y = 0.359x, y = 0.241x, and y = 0.451x in I-S, A-P, and R-L directions, respectively, where x is the phantom movement (mm) and y is the static load (kgf). The relationship between the phantom movements and static load may represent the performance of inhibiting patient movements, so the accuracy of the immobilization system in the intracranial lesion will be estimated in advance by basic tensile test on the new phantom. CONCLUSIONS: The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.

Angular dependence correction of MatriXX and its application to composite dose verification.

We measured the angular dependence of central and off-axis detectors in a 2D ionization chamber array, MatriXX, and applied correction factors (CFs) to improve the accuracy of composite dose verification of IMRT and VMAT. The MatriXX doses were measured with a 10° step for gantry angles (θ) of 0°-180°, and a 1° step for lateral angles of 90°-110° in a phantom, with a 30 × 10 cm2 field for 6 MV and 10 MV photons. The MatriXX doses were also calculated under the same conditions by the Monte Carlo (MC) algorithm. The CFs for the angular dependence of MatriXX were obtained as a function of θ from the ratios of MatriXX-measured doses to MC-calculated doses, and normalized at θ = 0°. The corrected MatriXX were validated with different fields, various simple plans, and clinical treatment plans. The dose distributions were compared with those of MC calculations and film. The absolute doses were also compared with ionization chamber and MC-calculated doses. The angular dependence of MatriXX showed over-responses of up to 6% and 4% at θ = 90° and under-responses of up to 15% and 11% at 92°, and 8% and 5% at 180° for 6 MV and 10 MV photons, respectively. At 92°, the CFs for the off-axis detectors were larger by up to 7% and 6% than those for the central detectors for 6 MV and 10 MV photons, respectively, and were within 2.5% at other gantry angles. For simple plans, MatriXX doses with angular correction were within 2% of those measured with the ionization chamber at the central axis and off-axis. For clinical treatment plans, MatriXX with angular correction agreed well with dose distributions calculated by the treatment planning system (TPS) for gamma evaluation at 3% and 3 mm. The angular dependence corrections of MatriXX were useful in improving the measurement accuracy of composite dose verification of IMRT and VMAT.

Possibility of using cylindrical ionization chambers for percent depth-dose measurements in clinical electron beams.

PURPOSE: This study investigated the possibility of using cylindrical ionization chambers for percent depth-dose (PDD) measurements in high-energy clinical electron beams. METHODS: The cavity correction factor, P(cav), for cylindrical chambers with various diameters was calculated as a function of depth from the surface to R50, in the energy range of 6-18 MeV electrons with the EGSnrc C(++) -based user-code CAVITY. The results were compared with those for IBA NACP-02 and PTW Roos parallel-plate ionization chambers. The effective point of measurement (EPOM) for the cylindrical chamber and the parallel-plate chamber was positioned according to the IAEA TRS-398 code of practice. The overall correction factor, P(Q), and the percent depth-ionization (PDI) curve for a PTW30013 Farmer-type chamber were also compared with those of NACP-02 and Roos chambers. RESULTS: The P(cav) values at depths between the surface and R50 for cylindrical chambers were all lower than those with parallel-plate chambers. However, the variation in depth for cylindrical chambers equal to or less than 4 mm in diameter was equivalent to or smaller than that for parallel-plate chambers. The P(Q) values for the PTW30013 chamber mainly depended on P(cav), and for parallel-plate chambers depended on the wall correction factor, P(waII), rather than P(cav). P(Q) at depths from the surface to R50 for the PTW30013 chamber was consequently a lower value than that with parallel-plate chambers. However, the variation in depth was equivalent to that of parallel-plate chambers at electron energies equal to or greater than 9 MeV. The shift to match calculated PDI curves for the PTW30013 chamber and water (perturbation free) varied from 0.65 to 0 mm between 6 and 18 MeV beams. Similarly, the shifts for NACP-02 and Roos chambers were 0.5-0.6 mm and 0.2-0.3 mm, respectively, and were nearly independent of electron energy. CONCLUSIONS: Calculated PDI curves for PTW30013, NACP-02, and Roos chambers agreed well with that of water by using the optimal EPOM. Therefore, the possibility of using cylindrical ionization chambers can be expected for PDD measurements in clinical electron beams.

Monte Carlo determination of a nanoDot OSLD response using quality index for diagnostic kilovoltage X-ray beams.

PURPOSE: This study aims to investigate the energy response of an optically stimulated luminescent dosimeter known as nanoDot for diagnostic kilovoltage X-ray beams via Monte Carlo calculations. METHODS: The nanoDot response is calculated as a function of X-ray beam quality in free air and on a water phantom surface using Monte Carlo simulations. The X-ray fluence spectra are classified using the quality index (QI), which is defined as the ratio of the effective energy to the maximum energy of the photons. The response is calculated for X-ray fluence spectra with QIs of 0.4, 0.5, and 0.6 with tube voltages of 50-137.6 kVp and monoenergetic photon beams. The surface dose estimated using the calculated response is verified by comparing it with that measured using an ionization chamber. RESULTS: The nanoDot response in free air for monoenergetic photon beams (QI = 1.0) varies significantly at photon energies below 100 keV and reaches a factor of 3.6 at 25-30 keV. The response differs by up to approximately 6% between QIs of 0.4 and 0.6 for the same half-value layer (HVL). The response at the phantom surface decreases slightly owing to the backscatter effect, and it is almost independent of the field size. The agreement between the surface dose estimated using the nanoDot and that measured using the ionization chamber for assessing X-ray beam qualities is less than 2%. CONCLUSIONS: The nanoDot response is indicated as a function of HVL for the specified QIs, and it enables the direct surface dose measurement.

Determination of the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams.

PURPOSE: To determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams. METHODS: An AGMS-DM+ semiconductor detector was calibrated in terms of air kerma measured with an ionization chamber. Air kerma was measured for 20 x-ray beams with tube voltages of 50-140 kVp and a half-value layer (HVL) of 2.2-9.7 mm Al for given quality index (QI) values of 0.4, 0.5, and 0.6, and converted to the surface dose. Finally, the air kerma and HVL measured by the AGMS-DM+ detector were expressed as a ratio of the surface dose for 10 × 10 and 20 × 20 cm2 fields. The ratio of both was represented as a function of HVL for the given QI values and verified by comparing it with that calculated using the Monte Carlo method. RESULTS: The air kerma calibration factor, CF, for the AGMS-DM+ detector ranged from 0.986 to 1.016 (0.9% in k = 1). The CF values were almost independent of the x-ray fluence spectra for the given QI values. The ratio of the surface dose to the air kerma determined by the PTW 30,013 chamber and the AGMS-DM+ detector was less than 1.8% for the values calculated using the Monte Carlo method, and showed a good correlation with the HVL for the given QI values. CONCLUSION: It is possible to determine the surface dose of a water phantom from the air kerma and HVL measured by a semiconductor detector for given QI values.

Real-time estimation of surface dose based on incident air kerma in diagnostic radiology.

PURPOSE: To estimate the surface dose in diagnostic radiology in real time based on the relationship between the incident air kerma and the surface dose. METHODS: The air kerma for 20 X-ray beams with tube voltages of 50-140 kV and a half-value layer (HVL) of 2.27-9.65 mm Al was measured using an ionization chamber. The beam quality was classified based on the quality indexes (QIs) of 0.4, 0.5, and 0.6, which are defined as the ratio of the effective energy to the maximum energy corresponding to the tube potential. The surface dose for 20 X-ray beams was evaluated based on the measured air kerma, backscatter factor, and ratio of the mass-energy absorption coefficients of water to air, which were calculated using the Monte Carlo method. Finally, the relationship between the air kerma and the surface dose was investigated for X-ray beams with the specific QI values. RESULTS: The surface dose at a water phantom was represented by a linear approximation of R2 > 0.98, with the air kerma, regardless of the X-ray beam quality. The surface dose estimated based on a linear approximation with the air kerma indicated an agreement within 8% with that evaluated by the chamber measurements at HVL > 3.4 mm Al. CONCLUSION: It is possible to estimate the surface dose in real time using the linear relationship between the incident air kerma and the surface dose regardless of the X-ray beam quality by accepting ±10% uncertainty in the surface dose estimation.

Energy response of radiophotoluminescent glass dosimeter for diagnostic kilovoltage x-ray beams.

PURPOSE: This study aimed to investigate the energy response of a radiophotoluminescent glass dosimeter (RGD) for diagnostic kilovoltage x-ray beams by Monte Carlo (MC) calculations and measurements. METHODS: The uniformity and reproducibility of GD-352M (with Sn filter) and GD-302M (no filter) were tested with 45 RGDs in free air. Subsequently, the RGD response was obtained as a function of an Al-HVL using the parameter, quality index (QI), which is defined as the ratio of the effective energy (keV) to the maximum energy (keV) of the photons. The x-ray fluence spectra with QI of 0.4, 0.5, and 0.6 were set for tube voltages of 50 ~ 137.6 kVp. The RGD response was calculated in free air using the MC method and verified by the air kerma, Kair, measured using an ionization chamber. RESULTS: The uniformity and reproducibility of the 45 RGDs were ± 2.3% and ± 2.7% for GD-352M and ± 0.7% and ± 1.6% for GD-302M at the one standard deviation level, respectively. The calculated RGD response was 0.965 to 1.062 at Al-HVL 2.73 mm or more for GD-352M and varied from 3.9 to 2.8 for GD-302M. Both RGD responses exhibited a good correlation with the Al-HVL for the given QI. Kair measured by RGDs for each beam quality with a QI of 0.5 was in the range of -5%~0.8% for GD-352M and -1.8%~3% for GD-302M, relative to the chamber measurements. CONCLUSIONS: The RGD response was indicated as a function of the Al-HVL for the given QI, and it presented a good correlation with the Al-HVL.

Monte Carlo study of dosimetric impact of gadolinium contrast medium in transverse field MR-Linac system.

The aim of this study is to evaluate the dosimetric impact of gadolinium contrast medium (Gadovist) in a transverse MR-Linac system using Monte Carlo methods. The dose distributions were calculated using two heterogeneous multi-layer phantoms consisting of Gadovist, water, bone, and lung. The photon beam was irradiated with a filed size of 5 × 5 cm2, and a transverse magnetic field of 0-3.0 T was applied perpendicular to the incident photon beam. Next, dose distributions for brain, head and neck (H&N), and lung cancer patients were calculated using a patient voxel-based phantom with and without replacing the patient's GTV with Gadovist. The dose at the water-Gadovist interface increased by 8% without a magnetic field. A similar dose increment was observed at 0.35 T. In contrast, the dose increment at the water-Gadovist interface was small at 1.5 T and a dose decrement of 5% was observed at 3.0 T. The dose variation at the lung-Gadovist interface was larger than that at the water-Gadovist interface. The mass collision stopping power ratio for Gadovist was 7% lower than that for water, whereas, the electron fluence spectra at the water-Gadovist interface increased by 17.5%. In a patient study, Gadovist increased the Dmean for brain, H&N, and lung cancer patients by 0.65-8.9%. The dose variation due to Gadovist grew large in the low-dose region in H&N and lung cancer. The GTV dose variation due to Gadovist in all treatment site was below 2% at 0-3 T if the Gadovist concentration was lower than 0.2 mmol/ml-1.

Comparison of RTPS and Monte Carlo dose distributions in heterogeneous phantoms for photon beams.

The purpose of this study was to compare dose distributions from three different RTPS with those from Monte Carlo (MC) calculations and measurements, in heterogeneous phantoms for photon beams. This study used four algorithms for RTPS: AAA (analytical anisotropic algorithm) implemented in the Eclipse (Varian Medical Systems) treatment planning system, CC (collapsed cone) superposition from the Pinnacle (Philips), and MGS (multigrid superposition) and FFT (fast Fourier transform) convolution from XiO (CMS). The dose distributions from these algorithms were compared with those from MC and measurements in a set of heterogeneous phantoms. Eclipse/AAA underestimated the dose inside the lung region for low energies of 4 and 6 MV. This is because Eclipse/AAA do not adequately account for a scaling of the spread of the pencil (lateral electron transport) based on changes in the electron density at low photon energies. The dose distributions from Pinnacle/CC and XiO/MGS almost agree with those of MC and measurements at low photon energies, but increase errors at high energy of 15 MV, especially for a small field of 3x3 cm(2). The FFT convolution extremely overestimated the dose inside the lung slab compared to MC. The dose distributions from the superposition algorithms almost agree with those from MC as well as measured values at 4 and 6 MV. The dose errors for Eclipse/AAA are lager in lung model phantoms for 4 and 6 MV. It is necessary to use the algorithms comparable to superposition for accuracy of dose calculations in heterogeneous regions.

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

PURPOSE: 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. METHODS: Backscatter factors, Bw, for x-ray fluence spectra were calculated from the weighted average of Bw for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm × 10 cm to 40 cm × 40 cm. 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 0.5-13.2 mm using the SpekCalc program. RESULTS: 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. CONCLUSIONS: It is possible to obtain Bw more accurately by using QI specified by the measured Al-HVL.

Impact of the cavity on sinus wall dose in magnetic resonance image-guided radiation therapy.

PURPOSE: This study aims to investigate the impact of the cavity on the sinus wall dose by comparing dose distributions with and without the sinus under magnetic fields using Monte Carlo calculations. METHODS: A water phantom containing a sinus cavity (Empty) was created, and dose distributions were calculated for 1, 2, and 4 irradiation fields with 6 MV photons. The sinus in the phantom was then filled with water (Full), and the dose distributions were calculated again. The sinus was set to cubes of 2 cm and 4 cm. The magnetic field was applied to the transverse and inline direction under the magnetic flux densities of 0 T, 0.35 T, 0.5 T, 1.0 T, and 1.5 T. The dose distributions were analyzed by the dose difference, dose volume histogram, and D2 with sinus wall thicknesses of 1 and 5 mm. RESULTS: D2 in the "Empty" sinus wall under transverse magnetic fields for the 1-field and 4-field cases was 51.9% higher and 3.7% lower than that in the "Full" sinus wall at 1.5 T, respectively. Meanwhile, D2 in the Empty sinus wall under inline magnetic fields for 1-field and 4-fields was 2.3% and 2.6% lower than that in the "Full" sinus at B = 0 T, respectively, whereas D2 was 0.9% and 0.7% larger at 1.0 T, respectively. CONCLUSIONS: The impact of the cavity on the sinus wall dose depends on the magnetic flux density, direction of the magnetic field and irradiation beam, and number of irradiation fields.