Comments on 'Cellular response to modulated radiation fields'.

The authors of a recent paper (Claridge Mackonis et al 2007 Phys. Med. Biol. 52 5469-82) measured cell survival in spatially modulated radiation fields. They claim to have identified two new types of radiation-induced bystander effect. We conclude that their claims are not supported by their data.

An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: II. Clinical electron beams.

The energy dependence of alanine/EPR dosimetry for 8, 12, 18 and 22 MeV clinical electron beams was investigated by experiment and by Monte Carlo simulations. Alanine pellets in a waterproof holder were irradiated in a water phantom using an Elekta Precise linear accelerator. The dose rates at the reference point were determined following the TG-51 protocol using an NACP-02 parallel-plate chamber calibrated in a (60)Co beam. The EPR spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. Experimentally, we found no significant change in alanine/EPR response to absorbed dose-to-water over the energy range 8-22 MeV at an uncertainty level of 0.6%. However, the response for high-energy electrons is about 1.3 (+/-1.1)% lower than for (60)Co. The EGSnrc Monte Carlo system was used to calculate the ratio of absorbed dose-to-alanine to absorbed dose-to-water and it was shown that there is 1.3 (+/-0.2)% reduction in this ratio from the (60)Co beam to the electron beams, which confirms the experimental results. Alanine/EPR response per unit absorbed dose-to-alanine was also investigated and it is the same for high-energy electrons and (60)Co gamma-rays.

Commissioning of an NRC-type sealed water calorimeter at METAS using 60Co gamma-rays.

As part of a collaborative project between the National Research Council of Canada (NRC) and the Swiss Federal Office of Metrology and Accreditation (METAS), a sealed water calorimeter was built at NRC and transferred to METAS. The calorimeter is operated at 4 degrees C and uses two thermistor probes in a sealed glass vessel containing high-purity water to measure the radiation-induced temperature rise. The various correction factors have been evaluated and the estimated standard uncertainty on the absorbed dose to water is 0.41%. An extensive set of measurements using 60Co gamma-rays was carried out at NRC and two ionization chambers were calibrated against the absorbed dose determined calorimetrically. The chambers were also calibrated against the NRC standard for absorbed dose. After transferring the calorimeter to METAS, a similar set of measurements was carried out using their 60Co beam and the same two ionization chambers were calibrated against the absorbed dose to water established at METAS. The discrepancy between the three sets of calibration coefficients was smaller than the estimated standard uncertainty of 0.47% on the ratio of any pair of calibration coefficients.

An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: I. Clinical x-ray beams.

The energy dependence of alanine/EPR dosimetry, in terms of absorbed dose-to-water for clinical 6, 10, 25 MV x-rays and 60Co gamma-rays was investigated by measurements and Monte Carlo (MC) calculations. The dose rates were traceable to the NRC primary standard for absorbed dose, a sealed water calorimetry. The electron paramagnetic resonance (EPR) spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. The DOSRZnrc Monte Carlo code of the EGSnrc system was used to simulate the experimental conditions with BEAM code calculated input spectra of x-rays and gamma-rays. Within the experimental uncertainty of 0.5%, the alanine EPR response to absorbed dose-to-water for x-rays was not dependent on beam quality from 6 MV to 25 MV, but on average, it was about 0.6% lower than its response to 60Co gamma-rays. Combining experimental data with Monte Carlo calculations, it is found that the alanine/EPR response per unit absorbed dose-to-alanine is the same for clinical x-rays and 60Co gamma-rays within the uncertainty of 0.6%. Monte Carlo simulations showed that neither the presence of PMMA holder nor varying the dosimeter thickness between 1 mm and 5 mm has significant effect of the energy dependence of alanine/EPR dosimetry within the calculation uncertainty of 0.3%.

Fricke dosimetry: the difference between G(Fe3+) for 60Co gamma-rays and high-energy x-rays.

A calibration of the Fricke dosimeter is a measurement of epsilon G(Fe3+). Although G(Fe3+) is expected to be approximately energy independent for all low-LET radiation, existing data are not adequate to rule out the possibility of changes of a few per cent with beam quality. When a high-precision Fricke dosimeter, which has been calibrated for one particular low-LET beam quality, is used to measure the absorbed dose for another low-LET beam quality, the accuracy of the absorbed dose measurement is limited by the uncertainty in the value of G(Fe3+). The ratio of G(Fe3+) for high-energy x-rays (20 and 30 MV) to G(Fe3+) for 60Co gamma-rays, G(Fe3+)MV(Co), was measured to be 1.007(+/-0.003) (confidence level of 68%) using two different types of water calorimeter, a stirred-water calorimeter (20 MV) and a sealed-water calorimeter (20, 30 MV). This value is consistent with our calculations based on the LET dependence of G(primary products) and, as well, with published measurements and theoretical treatments of G(Fe3+).

Properties of Nd3+-doped and undoped tetragonal PbWO4, NaY(WO4)2, CaWO4, and undoped monoclinic ZnWO4 and CdWO4 as laser-active and stimulated raman scattering-active crystals.

Spectroscopic, laser, and chi((3)) nonlinear optical properties of tetragonal PbWO(4), NaY(WO(4))(2), CaWO(4), and monoclinic CdWO(4) and ZnWO(4) were investigated. Particular attention was paid to Nd(3+)-doped and undoped PbWO(4) and NaY(WO(4))(2) crystals. Their absorption and luminescence intensity characteristics, including the peak cross sections of induced transitions, were determined. Pulsed and continuous-wave lasing in the two 4F(3/2)-->4I(11/2) and 4F(3/2)-->4I(13/2) channels was excited. For these five tungstates, highly efficient (greater than 50%) multiple Stokes generation and anti-Stokes picosecond generation were achieved. All the observed scattered laser components were identified. These results were analyzed and compared with spectroscopic data from spontaneous Raman scattering. A new crystalline Raman laser based on PbWO(4) was developed for the chi((3)) conversion frequency of 1-microm pump radiation to the first Stokes emission with efficiency up to 40%. We classify all the tungstates as promising media for lasers and neodymium-doped crystals for self-stimulated Raman scattering lasers.

GafChromic MD-55: investigated as a precision dosimeter.

GafChromic MD-55 is a fairly new, thin film dosimeter that develops a blue color (lambda max = 676 nm) when irradiated with ionizing radiation. The increase in absorbance is roughly proportional to the absorbed dose. In this study, GafChromic MD-55 was irradiated with 60Co gamma rays. A double irradiation method was used in which a dosimeter is given an unknown dose followed by a known, calibration dose. With this method, GafChromic MD-55 was used to measure doses in the vicinity of 6 Gy with an uncertainty of less than 1%. It was found that the measured optical density of GafChromic MD-55, as presently fabricated, is affected by the polarization of the analyzing light, an important consideration when using GafChromic MD-55 as a precision dosimeter. GafChromic MD-55 was found to consist of seven layers. The response to polarized light was measured for the whole dosimeter and for the three Mylar films which form part of the dosimeter.

Water calorimetry for radiation dosimetry.

Calorimetry has a long history as a technique for establishing the absorbed dose, and graphite calorimetry has often been used to establish absorbed dose standards for use in radiation therapy. However, a conversion process is necessary to convert from dose to graphite to dose to water, which is the quantity of clinical interest. In order to more directly measure the dose to water, considerable effort has been devoted in the last fifteen years to the development of water calorimetry. This article reviews these developments and summarizes the present status of water calorimetry. Absorbed dose standards based on water calorimetry and with a relative standard uncertainty of 0.5-1% now seem achievable.

The survival of aerobic and anoxic human glioma and melanoma cells after irradiation at ultrahigh and clinical dose rates.

This in vitro study was undertaken to determine if ultrahigh dose rates could improve the radiation response of human tumors. Two cell lines, human glioma (U-87 MG), which is radioresistant, and human melanoma (HT-144), which is radiosensitive, were irradiated at ultrahigh and high dose rates under aerobic and anoxic conditions to determine if their oxygen enhancement ratios are modified by dose rate. In fact, the survival curves, and hence the oxygen enhancement ratios, were found to be independent of the dose rate. The oxygen enhancement ratio for glioma cells irradiated in plateau phase was 2.8 (+/- 0.3). The oxygen enhancement ratio was 2.7 (+/- 0.4) for melanoma cells in plateau phase and 2.8 (+/- 0.3) in exponential phase. These results indicate that there is no advantage in treating these tumors using ultrahigh dose rates instead of conventional dose rates.

Two-stage cell shrinkage and the OER for radiation-induced apoptosis of rat thymocytes.

Suspensions of rat thymocytes were given 0.09-100 Gy using 60Co gamma-rays. The radiation-induced changes in the thymocytes were examined from minutes to hours post-irradiation using electron microscopy, agarose gel electrophoresis, staining and Coulter Counter sizing. Sizing by Coulter Counter showed, for the first time, that thymocytes which undergo apoptosis shrink in two distinct stages, first by a sudden decrease from an original volume of 99 microns 3 to a volume of 76 microns 3, followed by a gradual decrease to 57 microns 3 over the space of a few hours. The oxygen enhancement ratio for apoptosis was measured to be about 3.5, similar to the value for reproductive death for many mammalian cells.

A direct comparison of water calorimetry and Fricke dosimetry.

Considerable effort has been devoted to measuring the absorbed dose to water using water calorimetry. Most of these efforts have been hampered by a lack of adequate knowledge of the heat defect of water. We argue that there is now sufficient information to establish with considerable confidence the heat defect of high-purity water containing various dissolved gases. For the present work we used water saturated with a 50/50 mixture of H2 and O2 gases, for which the heat defect is calculated to be -2.1%. As a test of this assignment, we have compared the absorbed dose to water as measured using water calorimetry with that obtained from Fricke dosimetry. The water calorimeter consisted of a small sealed vessel containing 100 ml of stirred water saturated with a 50/50 mixture of H2 and O2 gases. It was irradiated with 20 MV x-rays at a dose rate of about 0.4 Gy s-1. The same vessel was then filled with Fricke dosemeter solution, and irradiated under identical conditions. Our Fricke dosimetry is based on the Svensson and Brahme value of epsilon G (3.515 x 10(-3) 1 cm-1 J-1) and agrees to within 0.2% with the dose to water for 60Co gamma-rays obtained via graphite calorimetry. We find that for 20 MV x-rays, the dose to water determined by water calorimetry is 1.006 +/- 0.004 times the dose determined by Fricke dosimetry. Within 0.6(+/- 0.4)%, this result supports the calculated heat defect of -2.1% for water saturated with a 50/50 mixture of H2 and O2 gases.

The effect of various dissolved gases on the heat defect of water.

Recent measurements of the absorbed dose to air-saturated water, made using water calorimetry and assuming a zero heat defect for irradiated water, gave results 2%-5% higher than those determined by more conventional means. According to the current radiation chemical model for air-saturated water, the dose measured by water calorimetry assuming a zero heat defect should actually be 2% too low because of the endothermicity of the radiolysis processes in water. In order to examine possible sources for this discrepancy, we have constructed a small calorimeter (holding 100 ml of water) with which to measure the temperature rise in irradiated water saturated with various gases. The gases used were air, oxygen, argon, nitrogen, and hydrogen/oxygen mixtures. Irradiations were carried out with 20-MV x rays at a dose rate of 0.41 Gy/s. Our results are consistent with model calculations, except for some differences for accumulated doses of less than 100 Gy. The discrepancies we find at low doses and the discrepancies observed by others using water calorimeters may arise from impurities in the water.