Technical note: The impact of storage humidity on the response of reference-class ionization chambers.

PURPOSE: A number of Farmer-type ionization chambers were tested against storage humidity to confirm whether they satisfied the criteria for the long-term stability of reference-class ionization chambers. METHODS: The ionization chambers were stored for several months in an environment with relative humidity adjustable from 20% to 80%. The ionization chambers were removed from the storage environment at variable intervals ranging from 1 to 70 days and irradiated in a Co-60 radiation beam. The responses for each ionization chamber were evaluated from the measured currents corrected for the atmospheric air density, and were compared with those predicted by the Co-60 half-life. RESULTS: Certain ionization chambers gave a constant relative response regardless of the storage humidity, while the relative responses of two types of ionization chambers changed as a function of the storage humidity. The difference between the relative responses for the low (20-30%) and high (70-80%) storage relative humidity was ∼ $\sim$ 0.7%. The response was larger for the high relative humidity storage. Immediately after the storage humidity changed, the relative response started to change by the day, and it took approximately 2 weeks to 2 months for the relative response to converge. For one type of the ionization chamber, the plastic outer wall and the outer electrode were replaced with those made of solid graphite, and it was confirmed that the remodeled ionization chamber did not exhibit the response change. CONCLUSIONS: The present results and previous reports by other authors indicate that the magnitude of the change depends on the magnitude of the water absorption of the plastic used for the outer wall and/or the electrode of the ionization chamber. Thus, it is important in the selection of the reference-class ionization chamber to note the material and structure of the outer wall and electrode of the ionization chamber. If the ionization chamber has a hygroscopic wall and electrode and it is used as a reference ionization chamber, it is necessary to pay additional attention to the humidity difference for the storage, daily irradiation, and yearly calibration especially in regions with large seasonal humidity fluctuations.

Application of a radiophotoluminescent glass dosimeter to nonreference condition dosimetry in the postal dose audit system.

PURPOSE: The purpose of this study was to obtain a set of correction factors of the radiophotoluminescent glass dosimeter (RGD) output for field size changes and wedge insertions. METHODS: Several linear accelerators were used for irradiation of the RGDs. The field sizes were changed from 5 × 5 cm to 25 × 25 cm for 4, 6, 10, and 15 MV x-ray beams. The wedge angles were 15°, 30°, 45°, and 60°. In addition to physical wedge irradiation, nonphysical (dynamic/virtual) wedge irradiations were performed. RESULTS: The obtained data were fitted with a single line for each energy, and correction factors were determined. Compared with ionization chamber outputs, the RGD outputs gradually increased with increasing field size, because of the higher RGD response to scattered low-energy photons. The output increase was about 1% per 10 cm increase in field size, with a slight difference dependent on the beam energy. For both physical and nonphysical wedged beam irradiation, there were no systematic trends in the RGD outputs, such as monotonic increase or decrease depending on the wedge angle change if the authors consider the uncertainty, which is approximately 0.6% for each set of measured points. Therefore, no correction factor was needed for all inserted wedges. Based on this work, postal dose audits using RGDs for the nonreference condition were initiated in 2010. The postal dose audit results between 2010 and 2012 were analyzed. The mean difference between the measured and stated doses was within 0.5% for all fields with field sizes between 5 × 5 cm and 25 × 25 cm and with wedge angles from 15° to 60°. The standard deviations (SDs) of the difference distribution were within the estimated uncertainty (1SD) except for the 25 × 25 cm field size data, which were not reliable because of poor statistics (n = 16). CONCLUSIONS: A set of RGD output correction factors was determined for field size changes and wedge insertions. The results obtained from recent postal dose audits were analyzed, and the mean differences between the measured and stated doses were within 0.5% for every field size and wedge angle. The SDs of the distribution were within the estimated uncertainty, except for one condition that was not reliable because of poor statistics.

Deaths within 12 months after (125)I implantation for brachytherapy of prostate cancer: an investigation of radiation safety issues in Japan (2003-2010).

PURPOSE: The International Commission on Radiological Protection recommends removing the prostate before cremation if death occurs within 12 months after (125)I brachytherapy. However, the incidence of death within this time frame has not been robustly investigated in any country. The purpose this study was to investigate the incidence and cause of death and actions taken when death has occurred within 12 months after (125)I brachytherapy for prostate cancer in Japan. METHODS AND MATERIALS: Data were extracted from the Japan Radioisotope Association database to investigate the total number of implantation cases, number of early deaths after implantation, cause of death, and postmortem actions between September 2003 and the end of June 2010 in Japan. Early death was defined as occurring within 12 months after (125)I brachytherapy for prostate cancer. RESULTS: During the study period, 15,427 patients underwent (125)I brachytherapy and 43 (0.28%) died within 12 months after implantation. For 37 of the 43 patients (86%), the brachytherapy source was retrieved together with the prostate gland at autopsy; however, autopsy could not be performed in six (14%) of the deceased patients. The largest proportion of early deaths was because of cerebrovascular or cardiovascular disease (17/43, 40%), followed by malignant tumor (15/43, 35%), and respiratory disease or infection (7/43, 16%). CONCLUSIONS: The incidence of early deaths within 12 months after (125)I brachytherapy in Japan was 0.28%. In almost all cases, the brachytherapy sources were removed in the intact prostate before the body was cremated and stored appropriately.

[Development of the 60Co gamma-ray standard field for therapy-level dosimeter calibration in terms of absorbed dose to water (N(D,w))].

A primary standard for the absorbed dose rate to water in a 60Co gamma-ray field was established at National Metrology Institute of Japan (NMIJ) in fiscal year 2011. Then, a 60Co gamma-ray standard field for therapy-level dosimeter calibration in terms of absorbed dose to water was developed at National Institute of Radiological Sciences (NIRS) as a secondary standard dosimetry laboratory (SSDL). The results of an IAEA/WHO TLD SSDL audit demonstrated that there was good agreement between NIRS stated absorbed dose to water and IAEA measurements. The IAEA guide based on the ISO standard was used to estimate the relative expanded uncertainty of the calibration factor for a therapy-level Farmer type ionization chamber in terms of absorbed dose to water (N(D,w)) with the new field. The uncertainty of N(D,w) was estimated to be 1.1% (k = 2), which corresponds to approximately one third of the value determined in the existing air kerma field. The dissemination of traceability of the calibration factor determined in the new field is expected to diminish the uncertainty of dose delivered to patients significantly.

Feasibility study of glass dosimeter postal dosimetry audit of high-energy radiotherapy photon beams.

INTRODUCTION: The characteristics of a glass dosimeter were investigated for its potential use as a tool for postal dose audits. Reproducibility, energy dependence, field size and depth dependence were compared to those of a thermoluminescence dosimeter (TLD), which has been the major tool for postal dose audits worldwide. MATERIALS AND METHODS: A glass dosimeter, GD-302M (Asahi Techno Glass Co.) and a TLD, TLD-100 chip (Harshaw Co.) were irradiated with gamma-rays from a (60)Co unit and X-rays from a medical linear accelerator (4, 6, 10 and 20 MV). RESULTS: The dosimetric characteristics of the glass dosimeter were almost equivalent to those of the TLD, in terms of utility for dosimetry under the reference condition, which is a 10 x 10 cm(2) field and 10 cm depth. Because of its reduced fading, compared to the TLD, and easy quality control with the ID number, the glass dosimeter proved to be a suitable tool for postal dose audits. Then, we conducted postal dose surveys of over 100 facilities and got good agreement, with a standard deviation of about 1.3%. CONCLUSIONS: Based on this study, postal dose audits throughout Japan will be carried out using a glass dosimeter.