Technical note: characterization of spectral changes with measurement geometry and magnetic field strength in light guides used for scintillation dosimetry.

The purpose of this work was to characterize the stem-effect signal and the Cerenkov light ratio (CLR) in various light guides as functions of measurement geometry and magnetic field strength. Two PMMA-, two silica-, and one polystyrene-based light guides were considered in this work. Spectra measurements were performed as functions of depth, fiber-beam angle, and magnetic field strength using an optical spectrometer. All measurements were performed using a clinical linear accelerator at a nominal photon beam energy of 6 MV. Depths ranging from 1 cm to 10 cm, fiber-beam angles ranging from 90 degrees to 30 degrees, and magnetic field strengths ranging from 0 T to ± 1.40 T were investigated. The CLR was calculated from each spectrum by taking the ratio of the integral signal between 400 nm and 500 nm to the integral signal between 500 nm and 600 nm. A maximum increase of 80.5% in the stem-effect signal was observed in the magnetic field. Variations in spectral shape and, consequently, the CLR were observed for all of the fibers as functions of magnetic field strength and measurement geometry, particularly for wavelengths less than 400 nm. The plastic fibers exhibited decreases in the CLR as a function of magnetic field strength at all depths investigated, whereas the silica fibers exhibited increases in the CLR with decreasing magnetic field strength. A maximum variation of 11.1% in the CLR was observed for the polystyrene fiber due to the magnetic field. The sensitivity of the CLR to the magnetic field decreased as the fiber-beam angle decreased. The measured spectral response, shape, and CLR were found to be sensitive to the applied magnetic field strength and polarity where the variations in response were unique to each fiber.

Spectral characterization of plastic scintillation detector response as a function of magnetic field strength.

The purpose of this work was to characterize intensity and spectral response changes in a plastic scintillation detector (PSD) as a function of magnetic field strength. Spectra measurements as a function of magnetic field strength were performed using an optical spectrometer. The response of both a PSD and PMMA fiber were investigated to isolate the changes in response from the scintillator and the noise signal as a function of magnetic field strength. All irradiations were performed in water at a photon beam energy of 6 MV. Magnetic field strengths of (0, ±0.35, ±0.70, ±1.05, and ±1.40) T were investigated. Four noise subtraction techniques were investigated to evaluate the impact on the resulting noise-subtracted scintillator response with magnetic field strength. The noise subtraction methods included direct spectral subtraction, the spectral method, and variants thereof. The PMMA fiber exhibited changes in response of up to 50% with magnetic field strength due to the directional light emission from [Formula: see text]erenkov radiation. The PSD showed increases in response of up to 10% when not corrected for the noise signal, which agrees with previous investigations of scintillator response in magnetic fields. Decreases in the [Formula: see text]erenkov light ratio with negative field strength were observed with a maximum change at -1.40 T of 3.2% compared to 0 T. The change in the noise-subtracted PSD response as a function of magnetic field strength varied with the noise subtraction technique used. Even after noise subtraction, the PSD exhibited changes in response of up to 5.5% over the four noise subtraction methods investigated.

Technical Note: An investigation of polarity effects for wide-angle free-air chambers.

PURPOSE: Wide-angle free-air chambers (WAFACs) are used as primary standard measurement devices for establishing the air-kerma strength of low-energy, low-dose rate brachytherapy seeds. The National Research Council of Canada (NRC) is commissioning a primary standard wide-angle free-air chamber (NRC WAFAC) to serve the calibration needs of Canadian clients. The University of Wisconsin has developed a similar variable-aperture free-air chamber (UW VAFAC) to be used as a research tool. As part of the NRC commissioning, measurements were carried out for both polarities of the applied bias voltage and the resulting effects were observed to be very large. Similar effects were identified with the UW VAFAC. The authors describe the measurements carried out to determine the underlying causes of the polarity effect and the approach used to eliminate it. METHODS: The NRC WAFAC is based on the WAFAC design developed at the National Institute of Standards and Technology in the USA. Charge measurements for (125)I and (241)Am sources were carried out for both negative and positive polarities on the NRC WAFAC and UW VAFAC. Two aperture sizes were also investigated with the UW VAFAC. In addition, measurements on the NRC WAFAC were carried out with a small bias between the collecting electrode and the shield foil at the downstream end of the chamber. To mitigate all of the polarity effects, the downstream surface of the collecting electrode was covered with a thin layer of graphite on both the NRC and UW chambers. RESULTS: Both chamber designs showed a difference of more than 30 % between the charge collected with positive and negative bias voltages for the smallest electrode separation. It was shown for the NRC WAFAC that charge could be collected in the small gap downstream of the collecting volume by applying a voltage between the shield foil and the collecting electrode, even though an insulating foil (Mylar or polyimide film) separated the conducting surface from the small gap region. The unwanted additional current was shown to be proportional to the size of the aperture for the UW VAFAC. The extra ionization produced in the small gap region was eliminated for both chambers by covering the insulating side of the collecting electrode with a grounded conducting layer. CONCLUSIONS: The small gap region downstream of the collecting electrode in the NRC WAFAC and UW VAFAC can serve as an unwanted source of ion current. It is concluded that a residual electric field in the small gap region may lead to ion transport and to charge being trapped on the surface of the foil. The foil then acts as a capacitor with an equal charge, but of opposite sign, being attracted to the conducting surface. Covering the back of the collecting electrode surface with a grounded conducting layer eliminated the polarity effect.

Initial characterization of a gel patch dosimeter for in vivo dosimetry.

In vivo dosimetry is a greatly underutilized tool for patient safety in clinical external beam radiotherapy treatments, despite being recommended by several national and international organizations (AAPM, ICRU, IAEA, NACP). The reasons for this underutilization mostly relate to the feasibility and cost of in vivo dosimetry methods. Due to the increase in the number of beam angles and dose per fraction in modern treatments, there is a compelling need for a novel dosimeter that is robust and affordable while able to operate properly in these complex conditions. This work presents a gel patch dosimeter as a novel method of in vivo dosimetry. DEFGEL, a 6% T normoxic polyacrylamide gel, was injected into 1 cm thick acrylic molds to create 1 cm thick small cylindrical patch dosimeters. To evaluate the change in optical density due to radiation induced polymerization, dosimeters were scanned before and after irradiation using an in-house developed laser densitometer. The dose-responses of three separate batches of gel were evaluated and compared to check for linearity and repeatability. The response development time was evaluated to ensure that the patch dosimeter could be high throughput. Additionally, the potential of this system to be used as an in vivo dosimeter was tested with a clinically relevant end-to-end in vivo phantom test. All irradiations were performed with a Varian Clinac 21EX at the University of Wisconsin Medical Radiation Research Center (UWMRRC). The dose-response of all three batches of gel was found to be linear within the range of 2-20 Gy. At doses below 0.5 Gy the statistical uncertainties were prohibitively large to make quantitative assessments of the results. The three batches demonstrated good repeatability in the range of 2 Gy to up to 10 Gy, with only slight variations in response at higher doses. For low doses the dosimeter fully developed within an hour while at higher doses they fully developed within four hours. During the in vivo phantom test the predicted patch absorbed dose was 4.23 Gy while the readout dose was evaluated to be 4.37 Gy, which corresponds to a 3.2% discrepancy. The dosimeter and densitometer pairing shows promise as an in vivo dosimetry system, especially for hypofractionated or MRI-guided radiotherapy treatments where higher doses are prescribed.

Determination of the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters for 125I and 103Pd brachytherapy sources relative to 60Co.

PURPOSE: To determine the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters (TLD-100) for (125)I and (103)Pd brachytherapy sources relative to (60)Co. METHODS: LiF:Mg,Ti TLDs were irradiated with low-energy brachytherapy sources and with a (60)Co teletherapy source. The brachytherapy sources measured were the Best 2301 (125)I seed, the OncoSeed 6711 (125)I seed, and the Best 2335 (103)Pd seed. The TLD light output per measured air-kerma strength was determined for the brachytherapy source irradiations, and the TLD light output per air kerma was determined for the (60)Co irradiations. Monte Carlo (MC) simulations were used to calculate the dose-to-TLD rate per air-kerma strength for the brachytherapy source irradiations and the dose to TLD per air kerma for the (60)Co irradiations. The measured and MC-calculated results for all irradiations were used to determine the TLD intrinsic energy dependence for (125)I and (103)Pd relative to (60)Co. RESULTS: The relative TLD intrinsic energy dependences (relative to (60)Co) and associated uncertainties (k = 1) were determined to be 0.883 ± 1.3%, 0.870 ± 1.4%, and 0.871 ± 1.5% for the Best 2301 seed, OncoSeed 6711 seed, and Best 2335 seed, respectively. CONCLUSIONS: The intrinsic energy dependence of TLD-100 is dependent on photon energy, exhibiting changes of 13%-15% for (125)I and (103)Pd sources relative to (60)Co. TLD measurements of absolute dose around (125)I and (103)Pd brachytherapy sources should explicitly account for the relative TLD intrinsic energy dependence in order to improve dosimetric accuracy.

Air-kerma strength determination of a 169Yb high dose rate brachytherapy source.

The increased demand for high dose rate (HDR) brachytherapy as an alternative to external beam radiotherapy has led to the introduction of a HDR brachytherapy isotope 169Yb. This source offers a dose rate similar to 192Ir HDR sources, at about one fourth the effective photon energy. This work presents the calibration of this source in terms of air-kerma strength, based on an adaptation of the current, National Institute of Standards and Technology traceable, in air measurement technique currently used for 192Ir HDR sources. Several additional measurement correction factors were required, including corrections for air scatter, air attenuation, and ion recombination. A new method 169Yb is introduced for determining the ion chamber calibration coefficient Nk(169Yb). An uncertainty analysis was also performed, indicating an overall measurement expanded uncertainty in the air-kerma strength (k=2) of 2.2%.

Parallel evoluton in lichen-forming fungi.

The asexual lichen-forming fungus Parmelia hypotropa has two common chemical races that differ sharply in biogeography and niche characteristics and appear to have been derived by morphologic parallelism from chemically identical races of the closely related Parmelia perforata, which is sexual. Among the lichen fungi many chemically variable asexual morphs, which conventional taxonomies treat as species, are probably polyphyletic like Parmelia hypotropa.

Habitat selection by chemically differentiated races of lichens.

The maritime European lichens of the aggregate species Ramalina siliquosa represent six chemical races. Where the races are sympatric they populate different habitats. Such intensive local ecological sorting of morphologically similar individuals accumulating different, highly specialized metabolic end products appears to be unknown in other plants.