Hidetoshi Saitoh

Dose distribution of narrow beam irradiation for small lung tumor.

PURPOSEnTo aid in the selection of incident X-ray energy for stereotactic irradiation (STI) of lung tumor, dose distribution was investigated in a model of a thorax embedded with a tumor.nnnMETHODS AND MATERIALSnThe dose distribution in a thorax model was calculated using the EGS4 Monte Carlo simulation; it was also measured with dosimetric film of a tentative thorax phantom. Uniformity of dose distribution in a tumor region was compared among the results of irradiation for several X-ray energies, and optimal X-ray energy for STI of a lung tumor was discussed.nnnRESULTSnDose distributions in the thorax were obtained. An increase in X-ray energy led not only to an increased dose delivered to the tumor, but also to an increased dose to surrounding normal lung tissue.nnnCONCLUSIONSnThe flat range in dose distribution along the beam axis and in the beam profiles of the tumor increases with decreasing X-ray energy. Consequently, lower energy, rather than higher energy, is recommended for STI of a lung tumor in terms of higher uniformity in the target volume.

Reference dosimetry condition and beam quality correction factor for CyberKnife beam

This article is intended to improve the certainty of the absorbed dose determination for reference dosimetry in CyberKnife beams. The CyberKnife beams do not satisfy some conditions of the standard reference dosimetry protocols because of its unique treatment head structure and beam collimating system. Under the present state of affairs, the reference dosimetry has not been performed under uniform conditions and the beam quality correction factor kQ for an ordinary 6 MV linear accelerator has been temporally substituted for the kQ of the CyberKnife in many sites. Therefore, the reference conditions and kQ as a function of the beam quality index in a new way are required. The dose flatness and the error of dosimeter reading caused by radiation fields and detector size were analyzed to determine the reference conditions. Owing to the absence of beam flattening filter, the dose flatness of the CyberKnife beam was inferior to that of an ordinary 6 MV linear accelerator. And if the absorbed dose is measured with an ionization chamber which has cavity length of 2.4, 1.0 and 0.7 cm in reference dosimetry, the dose at the beam axis for a field of 6.0 cm collimator was underestimated 1.5%, 0.4%, and 0.2% on a calculation. Therefore, the maximum field shaped with a 6.0 cm collimator and ionization chamber which has a cavity length of 1.0 cm or shorter were recommended as the conditions of reference dosimetry. Furthermore, to determine the kQ for the CyberKnife, the realistic energy spectrum of photons and electrons in water was simulated with the BEAMnrc. The absence of beam flattening filter also caused softer photon energy spectrum than that of an ordinary 6 MV linear accelerator. Consequently, the kQ for ionization chambers of a suitable size were determined and tabulated as a function of measurable beam quality indexes in the CyberKnife beam.

Investigations of different kilovoltage x‐ray energy for three‐dimensional converging stereotactic radiotherapy system: Monte Carlo simulations with CT data

We are investigating three-dimensional converging stereotactic radiotherapy (3DCSRT) with suitable medium-energy x rays as treatment for small lung tumors with better dose homogeneity at the target. A computed tomography (CT) system dedicated for non-coplanar converging radiotherapy was simulated with BEAMnrc (EGS4) Monte-Carlo code for x-ray energy of 147.5, 200, 300, and 500 kilovoltage (kVp). The system was validated by comparing calculated and measured percentage of depth dose in a water phantom for the energy of 120 and 147.5 kVp. A thorax phantom and CT data from lung tumors (<20 cm3) were used to compare dose homogeneities of kVp energies with MV energies of 4, 6, and 10 MV. Three non-coplanar arcs (0 degrees and +/-25 degrees ) around the center of the target were employed. The Monte Carlo dose data format was converted to the XiO RTP format to compare dose homogeneity, differential, and integral dose volume histograms of kVp and MV energies. In terms of dose homogeneity and DVHs, dose distributions at the target of all kVp energies with the thorax phantom were better than MV energies, with mean dose absorption at the ribs (human data) of 100%, 85%, 50%, 30% for 147.5, 200, 300, and 500 kVp, respectively. Considering dose distributions and reduction of the enhanced dose absorption at the ribs, a minimum of 500 kVp is suitable for the lung kVp 3DCSRT system.

Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac

PURPOSEnMicrodosimetry has been developed for the evaluation of radiation quality, and single-event dose-mean lineal energy y(D) is well-used to represent the radiation quality. In this study, the changes of the relative biological effectiveness (RBE) values under the therapeutic conditions using a 6 MV linac were investigated with a microdosimetric method.nnnMETHODSnThe y(D) values under the various irradiation conditions for x-rays from a 6 MV linac were measured with a tissue-equivalent proportional counter (TEPC) at an extremely low dose rate of a few tens of microGy/min by decreasing the gun grid voltage of the linac. According to the microdosimetric kinetic model (MK model), the RBE(MK) values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the y(D) values are obtained from TEPC measurements. The Monte Carlo code GEANT4 was also used to calculate the photon energy distributions and to investigate the changes of the y(D) values under the various conditions.nnnRESULTSnThe changes of the y(D) values were less than approximately 10% when the field size and the depth in a phantom varied. However, in the measurements perpendicular to a central beam axis, large changes were observed between the y(D) values inside the field and those outside the field. The maximum increase of approximately 50% in the y(D) value outside the field was obtained compared with those inside the field. The GEANT4 calculations showed that there existed a large relative number of low energy photons outside of the field as compared with inside of the field. The percentages of the photon fluences below 200 keV outside the field were approximately 40% against approximately 8% inside the field. By using the MK model, the field size and the depth dependence of the RBEMK values were less than approximately 2% inside the field. However, the RBEMK values outside the field were 6.6% higher than those inside the field.nnnCONCLUSIONSnThe increase of the RBE(MK) values by 6.6% outside the field was observed. This increase is caused by the change of the photon energy distributions, especially the increase of the relative number of low energy photons outside the field.

Production design and evaluation of a novel breast phantom with various breast glandular fractions.

The purpose of this study was to evaluate the production design of a novel breast phantom, which has adjustable breast glandular fractions and potential application in the mammography quality assurance/quality control system. The breast phantom was based on a urethane resin that was used to adjust the breast glandular fraction by varying the amount of plasticizer added. The resin was cured at constant temperature and humidity. Theoretical phantom properties, such as elemental composition, specific density, effective atomic number, electron density, and linear attenuation coefficients, at various energies were compared to those of breast tissue tabulated in the ICRU 44. These properties were also compared to polymethyl methacrylate resin and BR12. The novel breast phantom was made to represent breast glandular content calculated from breast tissue of the ICRU 44. We hypothesized that the breast phantom theoretical properties are approximately equal to those of the BR12, which is known for being an excellent substitute breast phantom. It was found that the phantom can be used to improve both mammography performance and dosimetry.

Contribution of Cerenkov radiation in high-energy x-ray and electron beam film dosimetry using water-substitute phantoms

The contribution of Cerenkov radiation in high-energy film dosimetry was investigated using commercially available water-substitute phantoms. Doses were evaluated using six phantoms: RMI-451, Mix-DP, WE-211, WE-Black, PMMA and PMMA-Black. The contribution of Cerenkov radiation was determined from the shielded and unshielded evaluation doses when a bare film was inserted into the phantom in a dark room and irradiated. For both x-ray and electron beams, Cerenkov radiation produced a phantom-dependent increase in the unshielded dose when compared with the shielded dose. We also found that the darker the phantom, the smaller the contribution of Cerenkov radiation. These results suggest that for film dosimetry using bare film, the accuracy of dose evaluation may be improved by using phantoms with high opacity.

Commissioning of 6 MV medical linac for dynamic MLC-based IMRT on Monte Carlo code GEANT4

Monte Carlo simulation is the most accurate tool for calculating dose distributions. In particular, the Electron Gamma shower computer code has been widely used for multi-purpose research in radiotherapy, but Monte Carlo GEANT4 (GEometry ANd Tracking) is rare for radiotherapy with photon beams and needs to be verified further under various irradiation conditions, particularly multi-leaf collimator-based intensity-modulated radiation therapy (MLC-based IMRT). In this study, GEANT4 was used for modeling of a 6 MV linac for dynamic MLC-based IMRT. To verify the modeling of our linac, we compared the calculated data with the measured depth-dose for a 10xa0×xa010xa0cm2 field and the measured dose profile for a 35xa0×xa035xa0cm2 field. Moreover, 120 MLCs were modeled on the GEANT4. Five tests of MLC modeling were performed: (I) MLC transmission, (II) MLC transmission profile including intra- and inter-leaf leakage, (III) tongue-and-groove leakage, (IV) a simple field with different field sizes by use of MLC and (V) a dynamic MLC-based IMRT field. For all tests, the calculations were compared with measurements of an ionization chamber and radiographic film. The calculations agreed with the measurements: MLC transmissions by calculations and measurements were 1.76xa0±xa00.01 and 1.87xa0±xa00.01xa0%, respectively. In gamma evaluation method (3xa0%/3xa0mm), the pass rates of the (IV) and (V) tests were 98.5 and 97.0xa0%, respectively. Furthermore, tongue-and-groove leakage could be calculated by GEANT4, and it agreed with the film measurements. The procedure of commissioning of dynamic MLC-based IMRT for GEANT4 is proposed in this study.

Interface software for DOSXYZnrc Monte Carlo dose evaluation on a commercial radiation treatment planning system

PurposeAs the conventional graphical user interface (GUI) associated with DOSXYZnrc or BEAMnrc is unable to define specific structures such as gross tumor volume (GTV) on computed tomography (CT) data, the quantitative analysis of doses in the form of dose-volume histograms (DVHs) is difficult. The purpose of this study was to develop an interface that enables us to analyze the results of DOSXYZnrc output with a commercial radiation treatment planning (RTP) system and to investigate the validity of the system.Materials and methodsInterface software to visualize three-dimensional radiotherapy Monte Carlo (MC) dose data from DOSXYZnrc on the XiO RTP system was developed. To evaluate the interface, MC doses for a variety of photon energies were calculated using the CT data of a thorax phantom and a uniform phantom as well as data from patients with lung tumors.ResultsThe dose files were analyzed on the XiO RTP system in the form of isodose distributions and DVHs. In all cases, the XiO RTP system perfectly displayed the MC doses for quantitative evaluation in the form of differential and integral DVHs.ConclusionThree-dimensional display of DOSXYZnrc doses on a dedicated RTP system could provide all the existing facilities of the system for quantitative dose analysis.

Field-size correction factors of a radiophotoluminescent glass dosimeter for small-field and intensity-modulated radiation therapy beams

PURPOSEnWe evaluated the energy responses of a radiophotoluminescent glass dosimeter (RPLD) to variations in small-field and intensity-modulated radiation therapy (IMRT) conditions using experimental measurements and Monte Carlo simulation.nnnMETHODSnSeveral sizes of the jaw and multileaf collimator fields and various plan-class IMRT-beam measurements were performed using the RPLD and an ionization chamber. The field-size correction factor for the RPLD was determined for 6- and 10-MV x rays. This correction factor, together with the perturbation factor, was also calculated using Monte Carlo simulation with the EGSnrc/egs_chamber user code. In addition, to evaluate the response of the RPLD to clinical-class-specific reference fields, the field-size correction factor for the clinical IMRT plan was measured.nnnRESULTSnThe calculated field-size correction factor ranged from 1.007 to 0.981 (for 6-MV x rays) and from 1.012 to 0.990 (for 10-MV x rays) as the jaw-field size ranged from 1xa0×xa01xa0cm2 to 20xa0×xa020xa0cm2 . The atomic composition perturbation factor for these jaw fields decreased by 3.2% and 1.9% for the 6- and 10-MV fields, respectively. The density perturbation factor was unity for field sizes ranging from 3xa0×xa03xa0cm2 to 20xa0×xa020xa0cm2 , whereas that for field sizes ranging from 3xa0×xa03xa0cm2 to 1xa0×xa01xa0cm2 decreased by 3.2% (for 6-MV x rays) and 4.3% (for 10-MV x rays). The volume-averaging factor rapidly increased for field sizes below 1.6xa0×xa01.6xa0cm2 . The results for the MLC fields were similar to those for the jaw fields. For plan-class IMRT beams, the field-size correction and perturbation factors were almost unity. The difference between the doses measured using the RPLD and ionization chamber was within 1.2% for the clinical IMRT plan at the planning-target volume (PTV) region.nnnCONCLUSIONSnFor small fields of size 1.6xa0×xa01.6xa0cm2 or less, it was clarified that the volume averaging and density perturbation were the dominant effects responsible for the variation in the RPLD response. Moreover, perturbation correction is required when measuring a field size 1.0xa0×xa01.0xa0cm2 or less. Under the IMRT conditions, the difference in the responses of the RPLD between the reference conditions and the PTV region calculated by Monte Carlo simulation did not exceed 0.8%. These results indicate that it is feasible to measure IMRT dosage using an RPLD at the PTV region.

Bremsstrahlung and photoneutron production in a steel shield for 15–22-MeV clinical electron beams

The physical data regarding bremsstrahlung and neutrons produced in a steel shield by high-energy electron beams from a medical linear accelerator were investigated. These data are required to allow the accurate prediction of shielding performance for high-energy electron beams and in the design of radiotherapy facilities. A Monte Carlo code was used to develop Monte Carlo beam models for clinical electron beams and to directly simulate bremsstrahlung and secondary neutron production in a steel shield. The effective dose and dose equivalent of bremsstrahlung X rays and secondary neutrons outside a vault were determined using a realistic radiation source. The accuracy of Monte Carlo simulations was validated experimentally by comparing the measured and calculated physical quantities. In validating the Monte Carlo simulation, the measured and calculated values showed reasonable agreement, indicating that bremsstrahlung and photoneutron production and transport were simulated accurately. The bremsstrahlung X-ray dose was the main component of the total dose outside a vault. The secondary neutron dose was 1-20 % of the bremsstrahlung X-ray dose, but the neutron dose was also at a non-negligible level. The calculated neutron dose outside the vault differed from the McGinleys reported data. These results indicate that McGinleys method overestimates the neutron dose beyond the steel shield. The physical data used here will be useful in the accurate estimation of bremsstrahlung X-ray and neutron doses for high-energy electron beams.