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.

Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams.

PURPOSE: The aim of the present study is to provide a comprehensive set of detector specific correction factors for beam output measurements for small beams, for a wide range of real time and passive detectors. The detector specific correction factors determined in this study may be potentially useful as a reference data set for small beam dosimetry measurements. METHODS: Dose response of passive and real time detectors was investigated for small field sizes shaped with a micromultileaf collimator ranging from 0.6 × 0.6 cm(2) to 4.2 × 4.2 cm(2) and the measurements were extended to larger fields of up to 10 × 10 cm(2). Measurements were performed at 5 cm depth, in a 6 MV photon beam. Detectors used included alanine, thermoluminescent dosimeters (TLDs), stereotactic diode, electron diode, photon diode, radiophotoluminescent dosimeters (RPLDs), radioluminescence detector based on carbon-doped aluminium oxide (Al2O3:C), organic plastic scintillators, diamond detectors, liquid filled ion chamber, and a range of small volume air filled ionization chambers (volumes ranging from 0.002 cm(3) to 0.3 cm(3)). All detector measurements were corrected for volume averaging effect and compared with dose ratios determined from alanine to derive a detector correction factors that account for beam perturbation related to nonwater equivalence of the detector materials. RESULTS: For the detectors used in this study, volume averaging corrections ranged from unity for the smallest detectors such as the diodes, 1.148 for the 0.14 cm(3) air filled ionization chamber and were as high as 1.924 for the 0.3 cm(3) ionization chamber. After applying volume averaging corrections, the detector readings were consistent among themselves and with alanine measurements for several small detectors but they differed for larger detectors, in particular for some small ionization chambers with volumes larger than 0.1 cm(3). CONCLUSIONS: The results demonstrate how important it is for the appropriate corrections to be applied to give consistent and accurate measurements for a range of detectors in small beam geometry. The results further demonstrate that depending on the choice of detectors, there is a potential for large errors when effects such as volume averaging, perturbation and differences in material properties of detectors are not taken into account. As the commissioning of small fields for clinical treatment has to rely on accurate dose measurements, the authors recommend the use of detectors that require relatively little correction, such as unshielded diodes, diamond detectors or microchambers, and solid state detectors such as alanine, TLD, Al2O3:C, or scintillators.

Positional dependence of the CT number with use of a cone-beam CT scanner for an electron density phantom in particle beam therapy.

We evaluated the CT number accuracy in determining a CT calibration method for treatment planning by use of a 256 multi-slice CT (MSCT) scanner. An electron density phantom, which extends full length in the longitudinal direction, was scanned by the 256 MSCT scanner in a single rotation. We inserted four types of samples (air, 100 % ethanol, 40 wt% aqueous K(2)HPO(4), and water) into the phantom. The regions of interest were set for all image slices, and the average CT number was calculated in the transverse and longitudinal sections. For the transverse image direction, the CT number became lower toward the center of the phantom with almost all samples. The causes of these phenomena might be attributed to the effects of scattered radiation and those of beam hardening due to the high-atomic-number material (40 wt% aqueous K(2)HPO(4)). However, the phenomena were too complicated to allow us to determine their causes only from the present studies. Meanwhile, for increasing the accuracy of the CT number calibration, a single sample should be inserted, and this step should be repeated with different samples. For the longitudinal direction, the CT number for a 40 wt % aqueous K(2)HPO(4) sample increased by 30 HU from the anode to the cathode side due to variations in the X-ray energy caused by the heel effect. The caveat is that the CT number varies in the longitudinal direction when a CT scanner with a wide beam width is used.

Development of a multi-layer ionization chamber for heavy-ion radiotherapy.

A multi-layer ionization chamber (MLIC) which consists of a stack of parallel plate-type ionization chambers in which the parallel configuration is in the depth direction was developed at the National Institute of Radiological Science (NIRS) and has been used as a field dosimeter for MU calibration since 2002. Although the MLIC can measure depth dose distributions at one time, a correction is needed to obtain an accurate dose at the center of the spread-out Bragg peak (SOBP) in a water phantom. We attributed the observed difference between the correct dose at the center of the SOBP and the measured dose to the lack of water equivalence of the MLIC. In order to overcome this problem, a new MLIC was developed. It consists of polymethyl-methacrylate (PMMA) plates and graphite electrodes instead of flame retardant type 4 (FR4) and copper electrodes. The calibration coefficients of the MLICs were obtained by 160 MeV proton beam irradiation. For carbon-ion beams, the PMMA type MLIC has the capability to measure depth dose distributions in the water phantom with less than 2% error, including the fragment tail region.

FcgammaRIIB deficiency with Fas mutation is sufficient for the development of systemic autoimmune disease.

MRL.Fas(lpr/lpr) mice, a model for systemic lupus erythematosus (SLE) and arthritis in humans, have a Fas mutation that results in spontaneous development of systemic autoimmune diseases and a short life span. Half of them die by 5-6 months of age due to massive progression of systemic autoimmune diseases, such as lupus nephritis. However, C57BL/6 (B6).Fas(lpr/lpr) strain does not develop such disorders within the normal life span, indicating that suppressor gene(s) in B6 mice may control the onset and exacerbation of disease. Here, we show that the gene for a unique inhibitory Fc receptor for IgG (Fc gamma RIIB) is a critical SLE suppressor. Fc gamma RIIB-deficient B6.Fas(lpr/lpr) (B6.IIB(-/-)Fas(lpr/lpr)) mice developed systemic autoimmune diseases, including anti-DNA and anti-type II collagen autoantibodies and cryoglobulin production, immune complex glomerulonephritis and arthritis. They were short-lived, due to enhanced autoantibody production by B cells culminating in fatal lupus nephritis. Thus, Fc gamma RIIB deletion with Fas mutation is sufficient for the development of systemic autoimmunity in B6 mice. The inhibitory signaling cascade via Fc gamma RIIB may be critical for suppressing SLE in humans.