Dosimetry of a sonolucent material for an ultrasound-compatible gynecologic high-dose-rate brachytherapy cylinder using Monte Carlo simulation and radiochromic film.

PURPOSE: he purpose of this study was to study the dosimetric characterization of sonolucent material "TPX" to be used toward gynecologic high-dose-rate brachytherapy treatments using ultrasound-compatible cylinders in non-model-based dose calculation workflows. METHODS: Monte Carlo simulations were performed using EGSnrc application egs_brachy in cylinders of polymethylpentene (TPX) plastic, water, and PMMA. Simulations were performed of five 192Ir sources placed longitudinally in ∼3.7 cm diameter, 5.0 cm length cylinders (matching physical cylinders used in film measurements). TPX and PMMA dose distributions and percentage depth dose curves were compared relative to water. Film measurements were performed to validate egs_brachy simulations. TPX and PMMA cylinders were placed in a water tank using 3D-printed supports to position film radially and touching the surface of the cylinders. The same five 192Ir dwell positions were delivered as simulated in egs_brachy. RESULTS: The egs_brachy and film percentage depth doses agreed within film uncertainties. The egs_brachy relative dose difference between TPX and water was (0.74 ± 0.09)% and between PMMA and water was (-0.79 ± 0.09)% over the dose scoring phantom. Dose differences for TPX and PMMA relative to water were less than ± 1% within 5 cm of the cylinder surface. CONCLUSIONS: In a solid sonolucent sheath of TPX, the dosimetric differences are comparable with PMMA and other applicator materials in clinical use. No additional uncertainty to dose calculation is introduced when treating through TPX cylinders compared with current applicator materials, and therefore, it is acceptable to perform gynecologic brachytherapy treatments with a sonolucent sheath inserted during radiation delivery.

Towards a second-generation PET/MR insert with enhanced timing and count rate performance.

Previously we have developed a first-generation PET insert prototype for small animal PET/MR imaging, which used resistor-based charge division multiplexing circuits and SensL B-series silicon photomultipliers (SiPMs). In this work we present results from a second-generation readout board with improved timing and count rate performance. Three detector boards were tested: the first-generation readout board with SensL SPMArray4B (SiPM-B), the second-generation readout board with SensL ArrayC-30035-16P-PCB (SiPM-C) using the 'fast' outputs for timing, and the second generation board using Hamamatsu S11361-3050AE-04 MPPC arrays. Timing data were obtained with detector modules in coincidence with a single-pixel SensL MicroFJ-SMA-30035 reference detector and acquired using standard NIM electronics, while count rate data were acquired using the OpenPET data acquisition electronics system. The full-width at half-maximum (FWHM) coincidence time resolution (CTR) for the SiPM-B, SiPM-C and MPPC designs were 2600 ± 200 ps, 550 ± 50 ps, and 570 ± 30 ps, respectively. OpenPET waveform capture determined the mean signal durations, measured as time above 10% of the maximum amplitude, were 1850 ± 150 ns, 600 ± 25 ns, and 350 ± 25 ns, respectively, where the short signal of the MPPC resulted in reduced pileup effects at higher count rates. Decaying source measurements showed a non-paralyzable dead time of 1.30-1.41 µs for all three detectors tested, which was limited by the signal capture and processing time of the OpenPET system.

Comparison of acrylic polymer adhesive tapes and silicone optical grease in light sharing detectors for positron emission tomography.

Optical coupling is an important factor in detector design as it improves optical photon transmission by mitigating internal reflections at light-sharing boundaries. In this work we compare optical coupling materials, namely double-sided acrylic polymer tapes and silicone optical grease (SiG), in the context of positron emission tomography. Four double-sided tapes from 3 M of varying thicknesses (0.229 mm-1.016 mm) and adhesive materials ('100MP', 'A100', and 'GPA') were characterized with spectrophotometer measurements as well as photopeak amplitude and energy resolution measurements using lutetium-yttrium oxy-orthosilicate (LYSO) coupled to photomultiplier tubes (PMT) or silicon photomultipliers (SiPMs). Transmission spectra from the spectrophotometer showed over 80% transmission for all tapes at 420 nm and above, with 89.6% and 88.8% transmission for the 0.508 mm and 1.016 mm thick GPA tapes, respectively, at 420 nm. Measurements with single-pixel LYSO-PMT and 4 × 4 array (one-to-one coupled) LYSO-SiPM setups determined that SiG had the greatest photopeak amplitude, with tapes showing 2.1%-14.8% reduction in photopeak amplitude with respect to SiG. Energy resolution changed by less than 4% on a relative basis between tapes and SiG with PMT measurements, however for the SiPM array measurements the energy resolution improved from 15.6% ± 2.7% full-width at half-maximum to 11.4% ± 1.2% for SiG and 1 mm GPA respectively. Data acquired with dual-layer offset LYSO arrays (light sharing detector designs) demonstrated that a detector coupled with 1 mm thick GPA tape produced equivalent detector flood histograms to those from a design coupled with SiG and a 1 mm thick glass lightguide. No significant degradation in photopeak amplitude and energy resolution was observed over five months of measurements, indicating the tapes maintain their coupling integrity over several months. Though minimal photopeak amplitude degradation compared to SiG occurs, double-sided tapes are convenient alternatives for optical coupling materials since they diffuse light intrinsically, acting as a light guide, offer mechanical support and durability, are easily applied and removed from scintillators/photodetectors, and are relatively inexpensive and readily available.