Beam Characterization of 10-MV Photon Beam from Medical Linear Accelerator without Flattening Filter.

AIM: This work investigated the dosimetric properties of a 10-MV photon beam emitted from a medical linear accelerator (linac) with no flattening filter (FF). The aim of this study is to analyze the radiation fluence and energy emitted from the flattening filter free (FFF) linac using Monte Carlo (MC) simulations. MATERIALS AND METHODS: The FFF linac was created by removing the FF from a linac in clinical use. Measurements of the depth dose (DD) and the off-axis profile were performed using a three-dimensional water phantom with an ionization chamber. A MC simulation for a 10-MV photon beam from this FFF linac was performed using the BEAMnrc code. RESULTS: The off-axis profiles for the FFF linac exhibited a chevron-like distribution, and the dose outside the irradiation field was found to be lower for the FFF linac than for a linac with an FF (FF linac). The DD curves for the FFF linac included many contaminant electrons in the build-up region. CONCLUSION: Therefore, for clinical use, a metal filter is additionally required to reduce the effects of the electron contamination. The mean energy of the FFF linac was found to be lower than that of the FF linac owing to the absence of beam hardening caused by the FF.

Monte Carlo simulation and measurement of radiation leakage from applicators used in external electron radiotherapy.

External electron radiotherapy is performed using a cone or applicator to collimate the beam. However, because of a trade-off between collimation and scattering/bremsstrahlung X-ray production, applicators generate a small amount of secondary radiation (leakage). We investigate the peripheral dose outside the radiation field of a Varian-type applicator. The dose and fluence outside the radiation field were analyzed in a detailed Monte Carlo simulation. The differences between the calculation results and data measured in a water phantom in an ionization chamber were less than ±1% in regions more than 3 mm below the surface of the phantom and at the depth of dose maximum. The calculated fluence was analyzed inside and outside the radiation field on a plane just above the water phantom surface. Changing the electron energy affected the off-axis fluence distribution outside the radiation field; however, the size of the applicator had little effect on this distribution. For each energy, the distributions outside the radiation field were similar to the dose distribution at shallow depths in the water phantom. The effect of secondary electrons generation by photon transmission through the alloy making up the lowest scraper was largest in the region from the field edge to directly below the cutout and at higher beam energies. The results of the Monte Carlo simulation confirm that the peripheral dose outside the field is significantly affected by radiation scattered or transmitted from the applicator, and the effect increases with the electron energy.

Dosimetric properties of radiophotoluminescent glass detector in low-energy photon beams.

PURPOSE: A radiophotoluminescent glass rod dosimeter (RGD) has recently become commercially available. It is being increasingly used for dosimetry in radiotherapy to measure the absorbed dose including scattered low-energy photons on the body surface of a patient and for postal dosimetry audit. In this article, the dosimetric properties of the RGD, including energy dependence of the dose response, reproducibly, variation in data obtained by the RGD for each energy, and angular dependence in low-energy photons, are discussed. METHODS: An RGD (GD-301, Asahi Techno Glass Corporation, Shizuoka, Japan) was irradiated with monochromatic low-energy photon beams generated by synchrotron radiation at Photon Factory, High Energy Accelerator Research Organization (KEK). The size of GD-301 was 1.5 mm in diameter and 8.5 mm in length and the active dose readout volume being 1 mm diameter and 0.6 mm depth located 0.7 mm from the end of the detector. The energy dependence of the dose response and reproducibility and variation were investigated for RGDs irradiated with a plastic holder and those irradiated without the plastic holder. Response of the RGD was obtained by not only conventional single field irradiation but also bilateral irradiation. Angular dependence of the RGD was measured in the range of 0°-90° for 13, 17, 40, and 80 keV photon beams by conventional single field irradiation. RESULTS: The dose responses had a peak at around 40 keV. For the energy range of less than 25 keV, all dose response curves steeply decreased in comparison with the ratio of mass energy absorption coefficient of the RGD to that of air. As for the reproducibility and variation in data obtained by the RGD, the coefficient of variance increased with decrease in photon energy. Furthermore, the variation for bilateral irradiation was less than that for single field irradiation. Regarding angular dependence of the RGD, for energies of 13 and 17 keV, the response decreased with increase in the irradiation angle, and the minimum values were 93.5% and 86%, respectively. CONCLUSIONS: Our results showed the dosimetric properties of the RGD, including the energy dependence of the dose response, reproducibly, variation, and angular dependence in low-energy photons and suggest that the accuracy of the absorbed dose in low-energy photons is affected by the readout method and the distribution of radiophotoluminescence centers in the RGD.

Dose distribution near thin titanium plate for skull fixation irradiated by a 4-MV photon beam.

To investigate the effects of scattered radiation when a thin titanium plate (thickness, 0.05 cm) used for skull fixation in cerebral nerve surgery is irradiated by a 4-MV photon beam. We investigated the dose distribution of radiation inside a phantom that simulates a human head fitted with a thin titanium plate used for post-surgery skull fixation and compared the distribution data measured using detectors, obtained by Monte Carlo (MC) simulations, and calculated using a radiation treatment planning system (TPS). Simulations were shown to accurately represent measured values. The effects of scattered radiation produced by high-Z materials such as titanium are not sufficiently considered currently in TPS dose calculations. Our comparisons show that the dose distribution is affected by scattered radiation around a thin high-Z material. The depth dose is measured and calculated along the central beam axis inside a water phantom with thin titanium plates at various depths. The maximum relative differences between simulation and TPS results on the entrance and exit sides of the plate were 23.1% and - 12.7%, respectively. However, the depth doses do not change in regions deeper than the plate in water. Although titanium is a high-Z material, if the titanium plate used for skull fixation in cerebral nerve surgery is thin, there is a slight change in the dose distribution in regions away from the plate. In addition, we investigated the effects of variation of photon energies, sizes of radiation field and thickness of the plate. When the target to be irradiated is far from the thin titanium plate, the dose differs little from what it would be in the absence of a plate, though the dose escalation existed in front of the metal plate.

High dose-rate intracavitary brachytherapy for cervical carcinomas with lower vaginal infiltration.

PURPOSE: This report presents the clinical applications of an automated treatment-planning program of high-dose-rate intracavitary brachytherapy (HDR-ICBT) for advanced uterine cervical cancer infiltrating the parametrium and the lower vagina. METHODS AND MATERIALS: We adopted HDR-ICBT under optimized dose distribution for 22 cervical cancer patients with tumor infiltration of the lower half of the vagina. All patients had squamous cell carcinoma with International Federation of Gynecology and Obstetrics clinical stages IIB-IVA. After whole pelvic external beam irradiation with a median dose of 30.6 Gy, a conventional ICBT was applied as "pear-shaped" isodose curve. Then 3-4 more sessions per week of this new method of ICBT were performed. With a simple determination of the treatment volume, the cervix-parametrium, and the lower vagina were covered automatically and simultaneously by this program, that was designated as "utero-vaginal brachytherapy". The mean follow-up period was 87.4 months (range, 51.8-147.9 months). RESULTS: Isodose curve for this program was "galaxy-shaped". Five-year local-progression-free survival and overall survival rates were 90.7% and 81.8%, respectively. Among those patients with late complications higher than Grade 2 Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer morbidity score, only one (4.5%) developed severe proctitis. CONCLUSIONS: Because of the favorable treatment outcomes, this treatment-planning program with a simplified target-volume based dosimetry was proposed for cervical cancer with lower vaginal infiltration.

[Field Survey Results on Output for X-ray Therapeutic Accelerators in Radiotherapy Institutions of Saitama and Tochigi Prefectures.].

Field survey on output for X-ray therapeutic accelerators took place three times in Saitama Prefecture. The result of the field survey in 1997 showed the different rate from the designated dose at peak depth of 35 beams in 18 institutions. As different rate within +/-5% stood 91.4% in all beams, so different rate within +/-3% stood 85.7% in the same beams. The average different rate from the designated dose at peak depth was 11.06%. The standard deviation of the same condition was 3.72.The result of the field survey in 2005 showed the different rate from the designated dose at correction depth of 36 beams in 18 institutions. As different rate within +/-5% stood 100% in all beams, so different rate within +/-3% stood 91.6% in the same beams. The average different rate from the designated dose at correction depth was +0.80%. The standard deviation of the same condition was 1.46.We understood that the different rate from the designated dose at radiotherapeutic institutions decreased and even the value of the standard deviation was decreasing, by receiving 3 times of field surveys that was held in Saitama Pref. Also we understood that the beam numbers of different rate within +/-5% and the beam numbers of different rate within +/-3% were going up. We recognized that the good result of accurate dose is obtained more, by doing a continual field survey. The field survey was carried out in 2006 in Tochigi Prefecture and was the insufficient result in 10% of institutions.

Visual comparisons between Cherenkov radiation from water and fluorescence from a scintillator.

A system for observing blue light of Cherenkov radiation was constructed using a Co-60 gamma-ray irradiation unit. However, there was some doubt that the observed light was not Cherenkov light, but scintillation. Therefore, the radiation from water was compared with that from a scintillator. The difference between both luminosities was examined using photographs taken in a dark irradiation room with mirrors and a camera. The radiation from the scintillator was much stronger than that from water. The differences between luminosities of the light radiated in the beam direction, at right angles to the beam and in the reverse beam direction were examined for both radiations. The luminosity from water showed very definite anisotropy, while that from the scintillator was almost isotropic. Furthermore, the light radiated in the beam direction from water was the strongest, and the strengths of the light radiated in the three directions from the scintillator were almost equivalent to each other. It was confirmed that the radiation from water irradiated by Co-60 gamma-rays was indeed Cherenkov light. The anisotropy of the radiated Cherenkov light and the isotropy of the scintillation were clearly observed in the photographs.

A method for measuring the dose distribution of the radiotherapy domain using the computed radiography system.

Knowing the dose distribution in a tissue is as important as being able to measure exposure or absorbed dose in radiotherapy. Therefore, we have developed a measurement method for the dose distribution (CR dosimetry) in the phantom based on the imaging plate (IP) of the computed radiography (CR). The IP was applied for the dose measurement as a dosimeter instead of the film used for film dosimetry. The data from the irradiated IP were processed by a personal computer with 10 bits and were depicted as absorbed dose distributions in the phantom. The image of the dose distribution was obtained from the CR system using the DICOM form. The CR dosimetry is an application of CR system currently employed in medical examinations to dosimetry in radiotherapy. A dose distribution can be easily shown by the Dose Distribution Depiction System we developed this time. Moreover, the measurement method is simpler and a result is obtained more quickly compared with film dosimetry.

[Observation of Cherenkov Radiation from Aquarium Irradiated with Co-60 Gamma-rays]

Cherenkov radiation in water at a nuclear power plant is caused by a nuclear fuel rod and is well known generally. If students can observe Cherenkov radiation at school easily, they can be impressed by the fascinating radiation. Moreover the observation may bring about interest in radiological physics profoundly. A few years ago, management of the Co-60 gamma-ray irradiation apparatus was transferred to Nagoya university school of health sciences from the related hospital. Therefore we have examined the system to observe the Cherenkov radiation in water from secondary electrons generated by Co-60 gamma-rays. At first, the Cherenkov radiation in the aquarium was led to the corridor outside the irradiation room using a mirror, and observed directly while avoiding exposure. Secondly photographs of the Cherenkov radiation from various angles were taken under conditions which consisted of several irradiation fields and pass lengths of gamma-rays in water, and were compared with each other. Our method for observing the Cherenkov radiation may be useful enough for students to raise their dedication in radiological physics study.

Evaluation of Linearity for the Radiophotoluminescence Glass Dosimeter Based on Monochromatic X-rays.

Although low energy X-rays have been utilized for mammography, their safety in medical use is a matter of concern. Characteristics of the radiophotoluminescence glass dosimeter, GD-403, consisting of a glass element and filters, were investigated with respect to monochromatic X-rays obtained from a synchrotron radiation for personal monitoring of low energy photons. We focused on low energy X-rays ranging from 8 to 20 keV to study the linearity of the GD-403 response between photon fluence and dose equivalent. The GD-403 was placed on a tough water phantom and irradiated using an 11-15 mm x 0.1-7 mm beam for modulation of the photon fluence. The tough water phantom could be moved through a distance of 110-150 mm with a stepping motor. For the dose equivalent at 1cm depth (H1), 3mm (H3) and 70 &mgr;m (H70), the GD-403 showed sufficient linearities against the photon fluences in the energy regions of 8 to 20 keV, 13 to 20 keV and 13 to 20 keV, respectively. However, H3 and H70 did not provide sufficient linearities in the energy region of 8 to 12 keV. Moreover, we compared the result in this experiment with the value calculated from the absorbed dose of air using the mass absorption coefficient for the X-ray energy ranging from 10 to 20 keV.