[Show abstract][Hide abstract] ABSTRACT: Despite recent progress in carbon therapy, accurate values for physical data such as the w value in air or stopping power ratios for ionization chamber dosimetry have not been obtained. The absorbed dose to graphite obtained with the graphite calorimeter was compared with that obtained using the ionization chambers following the IAEA protocol in order to evaluate the w values in air for mono-energetic carbon beams of 135, 290, 400 and 430 MeV/n. Two cylindrical chambers (PTW type 30001 and PTW type 30011, Farmer) and two plane-parallel chambers (PTW type 23343, Markus and PTW type 34001, Roos) calibrated by the absorbed dose to graphite and exposure to the (60)Co photon beam were used. The comparisons to our calorimeter measurements revealed that, using the ionization chambers, the absorbed dose to graphite comes out low by 2-6% in this experimental energy range and with these chamber types and calibration methods. In the therapeutic energy range, the w values in air for carbon beams indicated a slight energy dependence; we, however, assumed these values to be constant for practical use because of the large uncertainty and unknown perturbation factors of the ionization chambers. The w values in air of the carbon beams were evaluated to be 35.72 J C(-1) +/- 1.5% in the energy range used in this study. This value is 3.5% larger than that recommended by the IAEA TRS 398 for heavy-ion beams. Using this evaluated result, the absorbed dose to water in the carbon beams would be increased by the same amount.
No preview · Article · Feb 2009 · Physics in Medicine and Biology
[Show abstract][Hide abstract] ABSTRACT: The postal dose audit using radio-photoluminescence glass dosimeters was begun in November 2007 in order to improve the quality of radiotherapy in Japan. However, the irradiation conditions are now limited to the reference conditions which are 10×10 cm(2) field and 10 cm depth. The application of the glass dosimeters to non-reference conditions is strongly desired. This study dealt with the field-size dependence of the glass dosimeter outputs in the 6 MV photon beams of a medical linear accelerator (Varian Clinac21EX). We irradiated glass dosimeters with square field sizes of 5, 7, 10, 13, 16, 20, 23 and 25 cm side lengths at the 10 cm depth of the water equivalent phantom (SSD=90 cm). The outputs were compared with ionization chamber outputs. The ratio of the glass dosimeter outputs to the absorbed dose to water obtained with the ionization chamber increased approximately 1.5% between 5×5 cm(2) and 25×25 cm(2). We have to consider this field-size dependence when we apply the glass dosimeters to non-reference conditions.
No preview · Article · Jan 2009 · Igaku butsuri: Nihon Igaku Butsuri Gakkai kikanshi = Japanese journal of medical physics: an official journal of Japan Society of Medical Physics
[Show abstract][Hide abstract] ABSTRACT: 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.
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).
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%.
Based on this study, postal dose audits throughout Japan will be carried out using a glass dosimeter.
No preview · Article · Mar 2008 · Radiotherapy and Oncology