[Verification of Dose Distribution in Cervical Cancer Brachytherapy Using Metal and Plastic Applicators].

PURPOSE: To validate the point-A dose and dose distribution of metal and resin applicators in comparison with those of TG-43U1. METHODS: The metal and resin applicators consisting of tandem and ovoid were modeled by the egs_brachy. The doses to point A and dose distributions considering each applicator were calculated and compared to those of TG-43U1. RESULTS: The dose to point A considering the metal applicator was 3.2% lower than that of TG-43U1, but there was no difference in the dose to point A considering the resin applicator. The dose distribution considering the metal applicator was lower than that of TG-43U1 at all calculation points, but there was no difference in the dose distribution considering the resin applicator at almost all calculation points. CONCLUSION: In this study, the dose distribution considering the metal applicator was lower than that of TG-43U1 at all calculation points, but there was no difference in the dose distribution considering the resin applicator at almost all calculation points. Therefore, TG-43U1 can accurately calculate the dose distribution when changing from the metal applicator to the resin applicator.

Verification of morphological and physical properties for the development of a lung substitute phantom using microspheres.

This paper proposes a new concept of phantom development, along with the utilization of new materials that can reproduce lung morphology and density. A lung substitute phantom using microspheres was fabricated; then, its dosimetric utility in radiotherapy was investigated, during which the density was adjusted to closely resemble the morphology of the actual human lung. Microspheres were used to reproduce alveoli, which are the main components of the lung. By changing the ratio of urethane, which is commonly used in soft tissue phantoms, to microspheres, we reproduced the density change of the lungs due to respiration. Here, we fabricated two slab-like lung substitutes to emulate commercially used phantoms. Although there is room for improvement in terms of practicality, the substitutes were easy to fabricate. Microscopic observation of the cut surface of the phantoms showed that the morphology of the phantoms mimicked the alveoli more faithfully than commercial phantoms. Furthermore, to compensate for the energy-independent mass attenuation and mass collision inhibition ability required by the tissue substitute phantom, we examined the physical properties of the phantom and confirmed that there was negligible energy dependence.

Mouthpiece polymer-gel dosimeter for in vivo oral dosimetry during head and neck radiotherapy.

In this study, we developed a mouthpiece-type gel dosimeter to prevent the oral mucositis caused by the perturbation effect of dental alloys in the radiotherapy of the head and neck regions and to enable in vivo dosimetry. Understanding the dose distribution in the oral cavity during radiotherapy helps identify the possible site for oral mucositis during treatment. Here agarose, which has a higher melting point than gelatin, was added as a coagulant to stabilize the shape of the dosimeter. The strength and dose response of the dosimeter were investigated. The strength was measured at room temperature, 20°C-40 °C, which is higher than the intraoral temperature. The dose-response curves were obtained by magnetic resonance imaging with R2 ranging from 0 to 25 Gy. The strength and dose response of the mouthpiece-type gel dosimeter were approximately 4 and 2.1 times higher than those of polyacrylamide gel and tetrakis hydroxymethyl phosphonium chloride dosimeters commonly used in the prescribed doses per fraction of treatment. The dosimeter is composed of 4 wt% MgCl2 and 1.5 wt% agarose; thus, it can retain the water equivalence. Through in vivo oral dosimetry in three dimensions for head and neck radiotherapy with dental alloys using the mouthpiece-type gel dosimeter, we obtained three-dimensional dose distributions in the dosimeter. The properties of the dosimeter show that it can be used in the clinic, depending on the prescribed dose.

[Determination and Verification of Parameters of Lévy Distribution Incident Energy Spectrum of High-energy Electron Beam].

PURPOSE: The incident electron energy spectrum was determined by an estimation formula based on the Lévy distribution in order to calculate the PDD and OAR that is consistent with the measurement. METHODS: EGSnrc was used to calculate PDD and OAR at nominal energies of 4, 6, 9, 12, and 15 MeV. The parameters for determining the incident electron energy spectrum were adjusted to be a reasonable value in the error between the measured and the calculated values. RESULTS: Location and scaling parameters were determined to be 0.5 and 0.001, respectively. The calculated PDD based on the determination formula was in agreement with the measurement within 2 mm/2% at all depths. The OAR also was in agreement with the measurement within 2 mm/2%. CONCLUSION: In this study, the incident electron energy spectrum was estimated by determining the location and scaling parameters. This method is simpler and more accurate than previously reported, and can be applied to the calculation of dose distributions in Monte Carlo simulations.

Investigation of fiducial marker recognition possibility by water equivalent length in real-time tracking radiotherapy.

Real-time tumor tracking radiotherapy (RTRT) systems typically use fiducial markers implanted near the tumor to track the target using X-ray fluoroscopy. Template pattern matching, used in tracking, is often used to automatically localize the fiducial markers. In radiotherapy of the liver, the thickness of the body that can recognize the fiducial markers must be clinically assessed. The purpose of this study was to quantify the recognition of fiducial markers according to body thickness in stereotactic body radiotherapy of the liver using clinical images obtained using SyncTraX FX4. The recognition scores of fiducial markers were examined in relation to water equivalent length (WEL), tube current, and each flat panel detector. The relationship between the contrast ratio of the fiducial marker and the background and the WEL was also investigated. The average recognition score was found to be less than 20 when the WEL was greater than 25 cm. The probability of successful tracking of image recognition was mostly smaller than 0.8 when the WEL was over 30 cm. The relationship between WEL and tube current did not significantly differ between 100 and 140 mA, but there was a significant difference (p < 0.05) for all other combinations. To ensure tracking of fiducial markers during SBRT, if the WEL representing body thickness is longer than 25 cm, the X-ray fluoroscopy arrangement should be determined based on the WEL.

Quantitative analysis of the intra-beam respiratory motion with baseline drift for respiratory-gating lung stereotactic body radiation therapy.

This study aimed to quantitatively clarify the baseline drift for each respiratory cycle in two respiratory-gating methods using the intra-beam respiratory motion data of lung cancer patients. The residual motion and dose distribution were calculated based on intra-beam respiratory motion data with the baseline drift. To quantify the baseline drift $\Delta$ during irradiation, it was defined as the inclination between the detected expiration point and the expiration point in the next cycle in the anterior-posterior (AP), cranial-caudal (CC) and left-right (LR) directions obtained using an in-house programme. The baseline drift value reached up to 0.74 mm/s in the CC direction as per the respiratory motion data of 10 patients. The homogeneity index (HI) of the phase-gating method tended to increase because the target was irradiated even when the amplitude position of the target differed from period to period. In contrast, the amplitude-gating method enabled irradiation considering the amplitude position of the target because the gating window was set considering the amplitude position of the respiratory motion. The respiratory-gating methods and respiratory phase in respiratory-gating lung stereotactic body radiation therapy (SBRT) must be determined based on the respiratory motion of the patients.

Quantification of the temperature equilibrium time of the cavity in parallel-plate-type ionization chambers by thermal analysis.

Temperature corrections are necessary to account for the varying mass of air in the cavity volume of a vented ionization chamber. The temporal response resulting from temperature changes in a cylindrical and/or Farmer-type ionization chamber, which is the standard dosimeter, has been thoroughly discussed by some researchers. The purpose of this study was to characterise and analyse the dependence of the cavity air temperature of the parallel-plate-type ionization chamber on changes in the ambient temperature. Ionization chambers NACP-02 (IBA Dosimetry, GmbH) and Advanced Markus TN34045 (PTW, Freiburg) were modelled using thermal analysis software to present the temperature equilibrium time and the entire ionization chamber temperature distribution. The temporal response of each ionization chamber was measured for comparing the calculation results of the thermal analysis. The ionization chamber cavities of NACP-02 and TN34045 reached complete equilibrium in 670 and 750 s, respectively. Heat transfer occurred faster at the centre of the front wall of TN34045 than at the outside of the centre except for the edges. Further, the non-uniformity of temperature in the cavity was in the range of 24.2-24.8°C for NACP-02 and 23.7-24.4°C for TN34045 at 200 s after the ionization chamber was installed in the water phantom. The previous proposal to wait for about 15 mins after submerging the chamber in a water phantom before the measurement is demonstrated to be appropriate for parallel-plate-type ionization chambers.

Quantifying temperature-equilibrium time using temperature analysis inside a Farmer ionization chamber.

In this study, we propose a methodology for temperature determination of the temperature and pressure correction factor, PTP, by analyzing the temperature distribution of the modeled ionization chamber taking into account the thermal effect of a water phantom on neighboring materials in the process. Additionally, we present an appropriate temperature-equilibrium time for conducting measurements. The temporal response in the cavity is acquired at 20-s intervals using a Farmer ionization chamber and an electrometer. The initial temperature of the water phantom is 20-25°C with continuous heating/cooling. The temporal response is measured until temperature equilibrium is confirmed, specifically when a temperature difference of 1-5°C is observed between the ionization chamber and the water phantom. Using an ionization-chamber model, temperature distribution is simulated between 20 and 25°C with various parameters set to receive heating and cooling from surrounding media. The results suggest that the temporal response of the ionization chamber essentially coincides with the temperature change at the tip and middle; moreover, the predicted temperature change for temporal response and the simulated temperature of water are different by ~0.16°C at the tip and ~0.79°C at the bottom. Overall, the temperature-equilibration time for absorbed dosimetry is affected by two factors: the cavity wall and the stem side of the cavity; moreover, 400 s is required to obtain complete temperature equilibrium in the water phantom. This analytical study supports the experimental value obtained in previous research. Therefore, analytical representation of the temperature distribution in the ionization chamber is possible.

Design and characteristics of an agar additive polymer gel dosimeter.

Herein, we investigate the use of agar and gelatin in a polymer gel dosimeter. The polymer gel is enclosed in a vinyl film to obtain a dosimeter of arbitrary shape and maintain the shape at room temperature. The resulting polymer gel dosimeter could preserve its shape across a wide temperature range. Excluding the surface region, the obtained dose distribution was within 3% of that determined in an ionization chamber.

Note: Utilization of polymer gel as a bolus compensator and a dosimeter in the near-surface buildup region for breast-conserving therapy.

Tangential beam radiotherapy is routinely used for radiation therapy after breast conserving surgery. A tissue-equivalent bolus placed on the irradiated area shifts the depth of the dose distribution; this bolus provides uniform dose distribution to the breast. The gel bolus made by the BANG-Pro(®) polymer gel and in an oxygen non-transmission pack was applicable as a dosimeter to measure dose distribution in near-surface buildup region. We validated the use of the gel bolus to improve in the whole-breast/chest wall, including the near-surface buildup region.

[Evaluation of relative electrometer calibration factor with user beam.].

To establish traceability of absorbed dose to water, a cobalt calibration coefficient is transferred to a reference ionization chamber by the standard dosimetry laboratory in the radiotherapy field. In Japan, the calibration is done against a set of an ionization chamber and an electrometer as a system. Nowadays, solely electrometer calibration is desirable to measure absorbed dose with more than one combination of ionization chamber and electrometer. Unfortunately, there is no domestic electrometer calibration service for nano-ampere range appropriate for ionization current measurement in the radiotherapy field. In this report, a relative electrometer calibration factor was determined by comparison between a calibrated combination and a non-calibrated electrometer under identical irradiation conditions at the user site. To estimate uncertainties of user electrometer calibration, comparison was made between calibration factors obtained by ionization current under a linac photon beam and DC current with a precision DC source instrument. It was found that the variation of electrometer readings to pulse ionization current is negligible under the steady photon beam output and dose monitor system. Therefore relative electrometer calibration under identical irradiation conditions at the user site was judged valid until a domestic nano-ampere electrometer calibration service becomes available.

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.

Two-dimensional networks based on Mn4 complex linked by dicyanamide anion: from single-molecule magnet to classical magnet behavior.

Three two-dimensional (2D) network compounds based on Mn(III)/Mn(II) tetranuclear single-molecule magnets (SMMs) connected by dicyanamide (dcn-) linkers have been synthesized: [Mn4(hmp)4(Hpdm)2(dcn)2](ClO4)2 x 2 H2O x 2 MeCN (2), [Mn4(hmp)4Br2(OMe)2(dcn)2] x 0.5 H2O x 2 THF (3), [Mn4(hmp)6(dcn)2](ClO4)2 (4), where Hhmp and H2pdm are 2-hydroxymethylpyridine and pyridine-2,6-dimethanol, respectively. The [Mn4]/dcn- system appears very versatile, but enables its chemistry to be rationalized by a fine-tune of the synthetic conditions. The double cuboidal [Mn4] unit is preserved in the whole family of compounds, despite strong modifications of its Mn(II) coordination sphere. The chemical control of the coordination number of dcn- on the Mn(II) sites has been the key to obtain the following series of compounds: a discrete cluster, [Mn4(hmp)6(NO3)2(dcn)2] x 2 MeCN (1), 2D networks (2, 3, and 4), and the previously reported 3D compound, [Mn4(hmp)4(mu3-OH)2][Mn(II)(dcn)6] x 2 MeCN x THF. Direct current magnetic measurements show that both Mn2+-Mn3+ and Mn3+-Mn3+ intra-[Mn4] magnetic interactions are ferromagnetic leading to an S(T) = 9 ground state for the [Mn4] unit. Despite the very similar 2D lattices in 2-4, the two kinds of orientation of the [Mn4] unit (i.e., angle variations between the two easy axes) lead to different magnetic properties ranging from SMM behavior for 2 and 1 to a long-range canted antiferromagnetic order for 4. Compound 3 is more complicated as the magnetic measurements strongly suggest the presence of a canted antiferromagnetic order below 2.1 K, although the magnetization slow relaxation is simultaneously observed. Heat capacity measurements confirm the long-range magnetic order in 4, while in 3, the critical behavior is frozen by the slow relaxation of the anisotropic [Mn4] units.

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.

[Polarity effects of commercial plane-parallel ionization chamber in therapeutic electron dosimetry-Results with C-134A ionization chamber.].

Plane-parallel ionization chambers that exhibit polar effects with low energy electron beams are recommended for therapeutic electron dosimetry. In this study, the polarity effects of a C-134A ionization chamber, a major commercially available plane-parallel ionization chamber in Japan, were characterized as a function of mean energy at various depths. Polarity effects were measured at representative depths along depth dose curves of nominal 4, 6, 9, 12 and 15 MeV electron beams, and were compared with previously reported results. Polarity errors for the ionization chamber studied were shown to monotonically increase with decreasing mean energy at a given depth and were maximal at about 1-2 MeV. It was also shown that polarity errors depended on the energy of the incident electron beam. The polarity error of the C-134A ionization chamber was larger than that of other previously investigated plane-parallel ionization chambers. Because the magnitude of polarity effects should be determined throughout the depth dose curve in therapeutic electron dosimetry, it is always necessary to measure ionization readings taken at both polarities.

[Sensitometry of Mammographic Screen-film System Using Bootstrap Aluminum Step-Wedge.].

Recently, a few types of step-wedges for bootstrap sensitometry with a mammographic screen-film system have been proposed. In this study, the bootstrap sensitometry with the mammographic screen-film system was studied for two types of aluminum step-wedges. Characteristic X-ray energy curves were determined using mammographic and general radiographic aluminum step-wedges devised to prevent scattered X-rays generated from one step penetrating into the region of another one, and dependence of the characteristic curves on the wedges was also discussed. No difference was found in the characteristic curves due to the difference in the step-wedges for mammography and general radiography although there was a slight difference in shape at the shoulder portion for the two types of step-wedges. Therefore, it was concluded that aluminum step-wedges for mammography and general radiography could be employed in bootstrap sensitometry with the mammographic screen-film system.

Effects of density changes in the chest on lung stereotactic radiotherapy.

To experimentally and theoretically evaluate dose distribution during lung stereotactic radiotherapy, we investigated the relative electron densities in lung and tumor tissues using X-ray computed tomography images obtained from 30 patients in three breathing states: free breathing, inspiration breath-hold, and expiration breath-hold. We also calculated dose distribution using Monte Carlo simulation for lung tissue with two relative electron densities. The effect of changes in relative electron density on dose distribution in lung tissue was evaluated using calculated differential and integral dose volume histograms. The relative electron density of lung tissue was 0.22 in free breathing, 0.23 in shallow expiration, and 0.17 in shallow inspiration, and there was a tendency for relative electron density to decrease with age. The relative electron density of tumor tissue was approximately 0.9, with little variation due to differences in breathing state. As the relative electron density of lung tissue decreases, the low-dose region expands and leads to changes in the marginal dose.

[Measurement of focal spot in X-ray tube using imaging plate].

Comparisons of focal-spot size measurements with a direct-exposure X-ray film, screen-film (S/F) system, and imaging plate (IP) were carried out for slit and star resolution pattern camera techniques. The focal-spot measurements of X-ray units with the nominal size of 0.1-1 mm were performed, and measurement using the computed radiography (CR) system was investigated. Compared with the direct-exposure X-ray film method, the measurements of focal-spot size by the S/F system were slightly small for all focal spots. The measurements obtained with IP were slightly large for large and small focal spots, although the measurement of micro focal-spot size was overestimated, extending the tolerance level owing to the sampling. With the star pattern camera technique, differences between the three kinds of detectors were minimal. Therefore, it was thought that measurement of focal-spot size using the CR system would be practical for both large and small focal spots if this method were used for quality control in medical facilities.

[Absorbed dose measurement of photon beam with Farmer-type ionization chambers in Japanese dosimetry protocols].

The Japan Society of Medical Physics (JSMP) has published a new dosimetry protocol "JSMP-01" (standard dosimetry of absorbed dose in external beam radiotherapy) which conforms to the recommendations of the International Atomic Energy Agency (IAEA TRS-398) and the American Association of Physicists in Medicine (AAPM TG-51) protocols for the calibration of radiotherapy beams. Since the new protocol offers the physical data for the Famer-type ionization chambers of the various wall materials, the user can measure the absorbed dose at reference point (D(r)) using most of the commercially available Famer-type ionization chambers. In this paper, the six Famer-type ionization chambers of the various wall materials are examined for photon beam by two ways. To verify the JSMP-01 protocol as the first way, D(r) was cross-measured based on the JSMP-01 protocol using a Farmer-type ionization chamber of the acrylic wall material which is called "JARP-chamber" and the Farmer-type ionization chambers of the various wall materials, and compared. To compare the basic data in previous and new protocols as the second way, D(r) was measured based on the previous protocol (JSMP-86) and the JSMP-01 protocol using the Farmer-type ionization chambers of the various wall materials. Dose calculation was made using common exposure calibration factor for (60)Co gamma-rays (Nc) for each of the Farmer-type ionization chambers. Measurement was made with each ionization chamber for 6 and 10 MV photon beams in two facilities. D(r) were found to agree to that of JARP-chamber within about +/- 1% despite significant differences of ratio of calibration factor (k(D,X)) and beam quality conversion factor (k(Q)) for photon beams. The ratios JSMP-01/JSMP-86 of the reference dose were found to lie on between 0.999 and 1.004 for 6 MV and on between 0.999 and 1.005 for 10 MV depending upon the Farmer-type ionization chambers used. The largest discrepancies between the previous and new protocols arise from the use of different data of k(D,x) x k(Q) and C(lambda) for the absorbed dose conversion factors of each ionization chamber.