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.

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.

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.

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.

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.

Impact of lung density on isolated lung tumor dose in VMAT using inline MR-Linac.

PURPOSE: This study investigated the impact of lung density on the isolated lung tumor dose for volumetric modulated arc therapy (VMAT) in an inline magnetic resonance linear accelerator (MR-Linac) using the Monte Carlo (MC) simulation. METHODS: CT images of the thorax phantoms with lung tumors of 1, 2, and 3 cm diameters were converted into voxel-base phantoms with lung densities of 0.1, 0.2, and 0.3 g/cm3, respectively. The dose distributions were calculated for partial-arc VMAT. The dose distributions were compared using dose differences, dose volume histograms, and dose volume indices. RESULTS: In all cases, the inline magnetic field significantly enhanced the lung tumor dose compared to that at 0 T. For the 1 cm lung tumor, the inline magnetic field of 1 T increased the minimum dose of 95% of the Planning target volume (PTV D95) by 14.0% in 0.1 g/cm3 lung density as compared to that in 0.3 g/cm3 at 0 T. In contrast, at 0 and 0.5 T, the PTV D95 in 0.3 g/cm3 lung density was larger than that in lung density of 0.1 g/cm3. For the 2 cm lung tumor, a similar tendency to 1 cm was observed, whereas the dose impact of lung density was smaller than that for 1 cm. For the 3 cm lung tumor, the lung tumor dose was independent of lung density at 0.5 T and 1.0 T. CONCLUSION: The inline MR-Linac with the magnetic field over 1 T can enhance the PTV D95 for VMAT regardless of the lung density.

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.

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.

Impact of transverse magnetic fields on dose response of a nanoDot OSLD in megavoltage photon beams.

PURPOSE: We investigated the impact of transverse magnetic fields on the dose response of a nanoDot optically stimulated luminescence dosimetry (OSLD) in megavoltage photon beams. METHODS: The nanoDot OSLD response was calculated via Monte Carlo (MC) simulations. The responses RQ and RQ,B without and with the transverse magnetic fields of 0.35-3 T were analyzed as a function of depth at a 10 cm × 10 cm field for 4-18 MV photons in a solid water phantom. All responses were determined based on comparisons with the response under the reference conditions (depth of 10 cm and a 10 cm × 10 cm field) for 6 MV without the magnetic field. In addition, the influence of air-gaps on the nanoDot response in the magnetic field was estimated according to Burlin's general cavity theory. RESULTS: The RQ as a function of depth for 4-18 MV ranged from 1.013 to 0.993, excepting the buildup region. The RQ,B increased from 2.8% to 1.5% at 1.5 T and decreased from 3.0% to 1.1% at 3 T in comparison with RQ as the photon energy increased. The depth dependence of RQ,B was less than 1%, excepting the buildup region. The top air-gap and the bottom air- gap were responsible for the response reduction and the response increase, respectively. CONCLUSIONS: The response RQ,B varied depending on the magnetic field intensity, and the variation of RQ,B reduced as the photon beam energy increased. The air-gaps affected the dose deposition in the magnetic fields.

Impact of transverse magnetic fields on dose response of a radiophotoluminescent glass dosimeter in megavoltage photon beams.

PURPOSE: The purpose of this study was to investigate the impact of transverse magnetic fields on the dose response of a radiophotoluminescent glass dosimeter (RGD) in megavoltage photon beams. METHODS: The RGD relative response (i.e., RGD dose per absorbed dose to water at the midpoint of the detector in the absence of the detector) was calculated using Monte Carlo (MC) simulations. Note that the Monte Carlo calculations do not account for changes of the signal production per unit dose to the RGD caused by the magnetic field strength. The relative energy response RQ , the relative magnetic response RB , and the relative overall response RQ , B with the transverse magnetic fields of 0-3 T were analyzed as a function of depth, for a 10 cm × 10 cm field in a solid water phantom, for 4-18 MV photons. Although magnetic resonance (MR) linacs with flattening filter free beams are commercially available, flattening filter beams were used to investigate the RGD response in this study. RQ is the response in beam quality Q relative to that in the reference beam with quality 6 MV, RB is the response in beam quality Q with the magnetic field relative to that in beam quality Q without the magnetic field, and the RQ,B is the response in beam quality Q with the magnetic field relative to that in the reference beam with quality 6 MV without the magnetic field. Two RGD orientations were considered: RGD long axis is parallel (direction A) and perpendicular (direction B) to the magnetic field. The reference irradiation conditions were at the depth of 10 cm for a 10 cm × 10 cm field for 6 MV, without the magnetic field. In addition, the influence of a small air-gap between the holder inner wall and the RGD on the dose response in the magnetic field, Rgap , was analyzed in detail. Rgap is the response in beam quality Q without/with the air-gap. RESULTS: RQ decreased by up to 2.7% as the energy increased in the range of 4-18 MV, except in the buildup region. In direction A, the variation of RB owing to the magnetic field strength was below 1.0%, regardless of the photon energy. In contrast, in direction B, RB decreased with increasing magnetic field strength and decreased up to 4.0% at 3 T for 10 MV. The Rgap for 0.03 and 0.05 cm air-gap models in direction A decreased up to 2.3% and up to 4.0%, respectively. CONCLUSIONS: The variation of RQ,B changed with the direction of the RGD relative to the magnetic field. For dose measurements, RGDs should be positioned with the long axis parallel to the magnetic field, without air-gaps.

Comparison of dose distributions between transverse magnetic fields of 0.35 T and 1.5 T for radiotherapy in lung tumor using Monte Carlo calculation.

We investigated the impact of the transverse magnetic fields of 0.35 T and 1.5 T on the dose distributions for a 6 MV beam, by using a thorax phantom with a lung tumor. First, the dose distributions in the magnetic flux densities of 0 T, 0.35 T, and 1.5 T were compared by increasing the number of irradiation fields. Next, the dose distributions for stereotactic body radiotherapy (SBRT) with 5-fields for an isolated lung tumor was compared in transverse magnetic fields. All dose distributions were calculated by the Monte Carlo method. The prescription doses for SBRT with 5-fields was 48 Gy for D95 (dose covering 95% volume) in the planning target volume (PTV). The dose distributions were analyzed by the dose difference map (DD map), dose volume histogram (DVH), and dose indices. For the 1-field, the dose distributions were more affected at 1.5 T rather than 0.35 T. The DVHs for PTV at 1.5 T almost agreed with those at 0 T for more than 5-fields. In contrast, the D98 in the PTV at 0.35 T reduced constantly by 6.0% with more than 5-fields. The D95 in PTV for SBRT with 5-fields was 9.0% lower at 0.35 T and 2.5% higher at 1.5 T, in comparison with that at 0 T. For dispersed irradiation angles of more than 5-fields, it is more desirable to use the magnetic flux density of 1.5 T than 0.35 T for the radiotherapy in the lung tumor.

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

PURPOSE: 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. METHODS: 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 6 cm 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. RESULTS: The MGD in a standard heterogeneous phantom (CBT: 4 cm, 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. CONCLUSION: 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.

Geant4 Monte Carlo investigation of the magnetic field effect on dose distributions in low-density regions in magnetic resonance image-guided radiation therapy.

It is necessary to evaluate the effect of the presence of a magnetic field when treating lung tumors with MRIgRT. In this study, the effect of transverse and longitudinal magnetic fields on dose distributions in low-density regions was quantitatively investigated. The dose distributions in a virtual lung phantom under the influence of magnetic fields were calculated using the Geant4 Monte Carlo code. The phantom size was 30 × 20 × 30 cm3, and it was composed of three layers: water (3 cm thickness), lung (12 cm thickness), and water (5 cm thickness). The density of the lung layer was set to 0.1, 0.3 and 0.6 g/cm3. The uniform magnetic flux densities of 0.35 T and 1.5 T were used for 2 × 2, 5 × 5, and 10 × 10 cm2 fields at a source-to-surface distance of 100 cm, using a 6 MV photon spectrum. The dose at the water-lung interface in a lung phantom increased by 58%, 51%, and 22% for the lung densities of 0.1, 0.3, and 0.6 g/cm3, respectively. In the 1.5 T longitudinal magnetic field, the penumbra at the lung center (at the depth of 9 cm) decreased by 5.14, 1.50 and 0.35 mm for a 5 × 5 cm2 field, respectively. The dose distributions in lowdensity regions are more affected by the magnetic field. In conclusion, the doses at the water-lung interface increased in the transverse magnetic field. The depth dose increased, and the penumbra decreased in the longitudinal magnetic field.

Response of a nanoDot OSLD system in megavoltage photon beams.

PURPOSE: The aim of this study was to investigate the response of a nanoDot optically stimulated luminescence dosimeter (OSLD) system in megavoltage photon beams. METHODS: The nanoDot response was compared with the ionization chamber measurements for 4-18-MV photons in a plastic water phantom. The response was also calculated by the Monte Carlo method. In addition, the perturbation correction factor, PQ, in the nanoDot cavity was calculated according to the Burlin's cavity theory. The angular dependence of the nanoDot was evaluated using a spherical phantom. RESULTS: The calculated and measured nanoDot responses at a 10-cm depth and 10 × 10-cm2 field were in agreement within 1% for 4-18-MV. The response increased by 3% at a 20 × 20-cm2 field for the lower energy of 4 MV; however, it was constant within ±1% for 6-18 MV. The response was in a range from 1.0 to 0.99 for mean photon energy of more than 1.0 MeV but it increased with less than the 1.0 MeV. PQ for the nanoDot cavity was approximately constant at 0.96-0.97 for greater than and equal to 10 MV. The angular dependence decreased by 5% and 3% for 6 and 15 MV, respectively. CONCLUSIONS: The nanoDot was energy-independent in megavoltage photon beams.

Impact of inline magnetic fields on dose distributions for VMAT in lung tumor.

The purpose of this study is to investigate the impact of inline magnetic field on dose distribution for volumetric modulated arc therapy (VMAT) in lung tumors located at the chest wall and mediastinum. Two VMAT plans for a thorax phantom with lung tumors of 1 cm and 2 cm in diameter located at the chest wall were created by a treatment planning system. Next, five clinical VMAT plans for a non-small cell lung cancer (NSCLC) at early stages I and II of 5 cm or less in diameter were also used. The planning target volume (PTV) sizes were in the range from 11.1 to 82.7 cm3. The prescription dose was 60 Gy for D95 in the PTV. The VMAT dose distributions without and with uniform inline magnetic field of 0.5 T and 1.0 T were calculated using the Monte Carlo method. The dose distributions were analyzed by dose volume histograms, dose differences, and dose indices. In all VMAT plans, the PTV dose was enhanced by inline magnetic field. The dose enhancement was larger with 1.0 T than with 0.5 T. In phantom plans, D98 in the PTV with 0.5 T and 1.0 T increased by 2.9-6.6 Gy and 3.9-9.8 Gy, respectively, in comparison with that at 0 T. Similarly, in clinical plans, it increased by 2.2-6.0 Gy and 3.9-10.7 Gy, respectively. Thus, the VMAT with the inline magnetic field was proved useful for the dose enhancement in the lung tumor located at the chest wall and mediastinum.

Perturbation effect of parallel-plate ionization chambers on buildup dose measurements in transverse magnetic fields.

The aim of this study is to investigate the perturbation effect of parallel-plate ionization chambers on the buildup dose measurement in transverse magnetic fields, using Monte Carlo (MC) simulation. The NACP-02 and ROOS parallel-plate chambers and a PTW31010 cylindrical chamber were modeled for buildup dose measurement in magnetic fields, using the EGSnrc/cavity code. The irradiation condition was set to a 10 × 10 cm2 field in a water phantom at a source-to-surface distance (SSD) of 100 cm, using 6-MV photon spectrum. Magnetic fields of 0 0.35, 1.0, 1.5, and 3.0 T were applied perpendicularly to the direction of the photon beam. The overall perturbation factor PQ,B for the ionization chambers in the magnetic fields was also calculated. The dose to water was enhanced with increasing the magnetic field strength at a depth of less than 1 cm. Over a depth of 1.5 cm, there was no significant difference in the depth doses with and without magnetic field in water. The maximum depth dose (%) for the NACP-02 and ROOS chambers at 1.5 T was higher up to 12% and 14% than the maximum depth dose at 0 T, respectively. The depth dose curves of a PTW31010 chamber have a similar tendency to those of water. The PQ,B values for each chamber were the largest at the phantom surface. The transverse magnetic field has a greater effect on the dose response of the NACP and ROOS chambers than that of the PTW31010 chamber in the buildup region.

Image-guidance technique comparison on respiratory reproducibility and dose indexes for stereotactic body radiotherapy in lung tumor.

We investigated respiratory reproducibility from position errors of gold internal fiducial markers for breath-hold (BH) and real-time tumor tracking (RTT) techniques for stereotactic body radiotherapy in lung tumors. The relationship between position errors and dose indexes was checked for both techniques. The stereotactic body radiotherapy plan in lung tumors was planned for 29 patients. The tumor positioning was arranged using 1.5 mm diameter gold internal fiducial markers. First, CT images were acquired to analyze position errors of gold markers for BH and RTT techniques. The offset plans for both techniques were calculated by displacing the mean position errors. The dose indexes (D98, D95, D2, mean dose) in a planning target volume were evaluated from dose volume histograms for the original plan, BH, and RTT offset plans. The relationship between position errors and dose indexes was analyzed using the root mean square (RMS) for both techniques. For the BH, the RMS was 3.29 mm at the lower lobe. Similarly, it was 1.34 mm for the RTT. The difference for D98 by position error for BH was -7.0 ± 10.8% at the lower lobe and the difference of all dose indexes for the RTT was less than 1%. The D2 and mean dose for both techniques were nearly the same as those of the original plan. In conclusion, the adaptation of the BH technique should be ≤2 mm RMS. If the position error is >2 mm RMS, the RTT technique should be used instead of the BH technique.

Monte Carlo dose verification for a single-isocenter VMAT plan in multiple brain metastases.

The purpose of this study was to verify the accuracy of dose calculation algorithms of a treatment planning system for a single-isocenter volumetric modulated arc therapy (VMAT) plan in multiple brain metastases, by comparing the dose distributions of treatment planning system with those of Monte Carlo (MC) simulations. We used a multitarget phantom containing 9 acrylic balls with a diameter of 15.9 mm inside a Lucy phantom measuring 17 × 17 × 17 cm3. Seven VMAT plans were created using the multitarget phantom: 1 multitarget plan (MTP) and 6 single target plans (STP). Three of the STP plans had a large jaw field setting, almost equivalent to that of the MTP, while the other plans had a jaw field setting fitted to each planning target volume. The isocenter for all VMAT plans was set to the center of the phantom. The VMAT dose distributions were calculated using the analytical anisotropic algorithm (AAA) and were also recalculated through Acuros XB (AXB) and MC simulations under the same irradiation conditions. The AAA and AXB methods tended to overestimate dosage compared with the MC method in the MTP and in STPs with large jaw field settings. The dose distribution in single-isocenter VMAT plans for multiple brain metastases was influenced by jaw field settings. Finally, we concluded that MC-VMAT dose calculations are useful for 3D dose verification of single-isocenter VMAT plans for multiple brain metastases.