Effect of Magnetic Field Strength on Plastic Scintillation Detector Response.

PURPOSE: To characterize the response of plastic scintillation detectors (PSDs) to high-energy photon radiation as a function of magnetic field strength. MATERIALS AND METHODS: PSDs were placed inside a plastic phantom held at the center point between 2 magnets and irradiated using a 6-MV photon beam from a linear accelerator. The magnetic field was varied from 0 T to 1.5 T by 0.3-T increments. The light emission and stem-effect-corrected response as a function of magnetic field strength were obtained for both a commercial PSD (Exradin W1, Standard Imaging) and an in-house hyperspectral PSD. Spectral signatures were obtained for the in-house PSD, and light emission from a bare fiber was also measured. RESULTS: Light emission increased as magnetic field strength increased for all detectors tested. The tested PSDs exhibited an increase in light intensity of 10% to 20%, mostly owing to the increase in Cerenkov light produced within and transmitted along the optical fiber. When corrected for stem effects, the increase in PSD response went down to 2.4% for both detectors. This most likely represents the change in the inherent dose deposition within the phantom. CONCLUSION: PSDs with a suitable stem-effect removal approach were less dependent on magnetic field strength and had better water equivalence than did ion chambers tested in previous studies. PSDs therefore show great promise for use in both quality assurance and in-vivo dosimetry applications in a magnetic field environment.

WE-G-BRB-05: On the Importance of Fluorescence Within the Stem Effect of Scintillation Detectors.

PURPOSE: To quantify the nature and composition of the light produced in optical fibers under different irradiation conditions and evaluate its impact on dosimetry. METHODS: Irradiation of a bare PMMA optical fiber (Mitsubishi ESKA Premier) was performed using a superficial therapy unit, an Ir-192 HDR brachytherapy source, a Co-60 external-beam unit as well as photon and electron beams from a linear accelerator. Spectra of the radiation-induced visible light in the fiber were acquired and signals were compared as a function of depth and irradiation type. Irradiation of a 75 kVp beam from the superficial therapy unit was used to isolate the fluorescence spectrum. Isolation of the Cerenkov spectrum component was obtained from irradiation of a 15 MeV electron beam at a 45 degree angle. Relative composition in fluorescence and Cerenkov of the stem effect light has been determined for all irradiations. RESULTS: The total stem effect spectra can be represented by a linear superposition of the fluorescence and Cerenkov spectra. The fluorescence contribution was shown to strongly differ between the superficial therapy unit (99%±1%), the Ir-192 HDR source (25%±3%) and higher energy irradiations (3%±2%). Variations within each energy regime (kV, HDR brachytherapy and MV) were small at 3% or lower. These were observed for irradiations at angle or when the fiber was near the surface. This study suggests it is better to calibrate the stem effect of a scintillation detector using the same irradiation modality. CONCLUSIONS: Stem effect light was shown to be composed of fluorescence and Cerenkov light in different proportions depending on the geometry of the experimental setup, nature of the irradiation, and irradiation energy. Calibrating detectors separately for fluorescence and Cerenkov may lead to better performance of the stem effect removal technique.

WE-G-BRB-04: BEST IN PHYSICS (THERAPY) - A Novel Multi-Point Plastic Scintillation Detector for in Vivo Dosimetry and Quality Assurance in Radiation Therapy.

PURPOSE: To develop a novel multi-point plastic scintillation detector (mPSD) capable of accurately measuring dose at multiple positions simultaneously with the use of a single optical guide. METHODS: We built a new generation of plastic scintillation detectors composed of multiple scintillating elements along a same optical transmission line. Three different scintillating fibers were optically coupled to a single collecting optical fiber. A primary challenge for this new type of detector is that the output signal is a superposition of multiple scintillation spectra and contaminating elements. Acquisition with a spectrometry setup allows for the implementation of a new hyperspectral approach that accounts for each light-emitting component separately, and allows spectral unmixing. The mPSD and an ion chamber were irradiated in a water phantom with a 6 MV photon beam. Profiles and depth-dose curves were measured and compared between detectors. This detector and the corresponding calibration approach were also applied to Ir- 192 HDR brachytherapy. RESULTS: Doses measured with the mPSD were in good agreement with the ion chamber measurements for external beam irradiations. Average relative differences of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)% were observed for each scintillating element. The mPSD measurements tended to be at least as accurate as published measurements from single-point PSDs. For the Ir-192 HDR brachytherapy application, the average difference between the treatment planning system and the measurements were (4.6±1.0)% per dwell-position and (2.1±1.0)% per catheter. The accuracy of each scintillating element was shown to depend on light attenuation and on the similarity of its scintillation spectrum in comparison to the other light emitters. CONCLUSIONS: The feasibility and accuracy of mPSDs using a single transmission line was demonstrated. In addition to well-documented advantages of single-point PSDs, the multi-point capability of this single-fiber detector makes mPSDs a very promising new technique for quality assurance and on-line in vivo dosimetry.

Sci-Thur PM: YIS - 10: A new optically encoded single-fiber plastic scintillation detector for multi-point radiation dosimetry.

PURPOSE: To develop a new multi-point plastic scintillation detector (mPSD) that allows for simultaneous dose measurements at multiple points and uses a single optical guide. MATERIALS AND METHODS: Two different prototypes were built. A two-point mPSD was built and light discrimination was based on the use of multiple color filters at the outputs of a network of optical fiber splitters. Light intensity was measured by an EMCCD camera. For the three-point mPSD, the light discrimination setup was replaced by a low-noise spectrometer. Depth-dose and profiles measurements were obtained on a 6 MV photon beam with the mPSDs inside a water phantom. An ion chamber was also used for comparison purpose. Finally, the three-point mPSD was tested under an Ir-192 high-dose-rate (HDR) brachytherapy dose delivery and compared to the treatment planning system. RESULTS: A good agreement was found between the measured and expected dose for both mPSDs. The average relative differences to the ion chamber measurement for the two-point mPSD were of (2.4 ± 1.6)% and (1.3 ± 0.8)%. For the three-point mPSD, these differences were of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)%. The latter mPSD was shown very versatile, being able to measure dose from HDR brachytherapy with an average accuracy of (2.3±1.0)% per catheter. CONCLUSIONS: The practical feasibility of mPSDs using a single optical guide has been demonstrated under irradiation from a 6 MV photon beam and an Ir-192 HDR brachytherapy source. Their application for pre-treatment quality assurance and in vivo dosimetry will be various.

Sci-Sat AM: Brachy - 07: Plastic scintillation detector validation for kV dosimetry.

PURPOSE: To characterize the plastic scintillation detectors (PSDs) response in the diagnostic energy range. A fast and adaptable method for real-time dosimetry in superficial x-ray therapy and interventional radiology is proposed. METHOD: A PSD (1 mm diameter and 10 mm long) is coupled to a 5 m long optical fiber. Scintillation photons are guided to a polychromatic photodiode which provides an electrical current proportional to the input light signal. If the incident energy spectrum is known, the dose measured in the PSD's polystyrene sensitive volume can be converted to score dose in any other media such as air, water or soft tissues using the large cavity theory (LCT). A software simulating x-ray tube spectra and filtration has been benchmarked and is used for analysis. The method is confirmed by Monte Carlo simulations. RESULTS: PSDs cannot be assumed energy independent with low-energy photons as a factor 2 has been observed in the energy response between 80 kVp and 150 kVp. When the dose is converted to the desired medium, the PSD's energy dependence is compensated and a 2.1% standard deviation was observed upon the studied energy ranges, which is inside the measurement and calculation uncertainties. Percent depth dose (PDD) measurements are in good agreement with Monte Carlo simulations and results can be improved if the proposed method is applied to compensate beam hardening. CONCLUSION: PSDs present great potential for real-time dose measurements with radiologic photon energy.