The BaFBrI:Eu2+ storage phosphor plate (SPP) is a reusable radiation image detector, widely used in diagnostic computed radiography, x-ray crystallography and radioactive tracer studies. When exposed to ionizing radiation, the SPP stores a latent image until it is scanned with a red reading laser which causes blue photostimulated luminescent (PSL) photons to be emitted. The mechanism of formation of the latent image is still poorly understood, especially for megavoltage photon beams. In order to gain insight into this mechanism and aid applications to high-energy beam dosimetry, the authors have directly determined the SPP generation efficiency, W, the energy required to produce one quantum of emitted PSL when it is irradiated by 60Co and 6 MV photon beams. This was done in four steps: 1. The SPP, in a water-equivalent plastic (WEP) phantom, was exposed to a 60Co or 6 MV beam, which had been calibrated to give a known absorbed dose to water in a water phantom at the position of the sensitive layer of the SPP. 2. Monte Carlo simulations were used to calculate the ratio of the dose to the sensitive layer in the WEP phantom to the dose to water at the same position in a water phantom. 3. A bleaching experiment was used to determine the number of photons emitted by a plate given a known dose. 4. The generation efficiency was calculated from the number of photons and the dose. This method is much more direct than previous calculations for kilovoltage x-ray beams based on quantum noise analysis. W was found, within experimental uncertainty, to be 190 eV for 60Co and 160 eV for 6 MV, independent of dose. The values for kilovoltage x-ray beams determined previously agree, within their large uncertainty, with these values for megavoltage beams.
PurposeAccelerated partial breast irradiation (APBI) with balloon and strut adjusted volume implants (SAVI) show promising results with excellent tumor control and minimal toxicity. Knowing the factors that contribute to a high skin dose, rib dose, and D95 coverage may reduce toxicity, improve tumor control, and help properly predict patient outcomes following APBI.Methods and materialsA retrospective analysis of 594 patients treated with brachytherapy based APBI at a single institution from May 2008 to September 2014 was grouped by applicator subtype. Patients were treated to a total of 34 Gy (3.4 Gy x 10 fractions over 5 days delivered BID) targeting a planning target volume (PTV) 1.0 cm beyond the lumpectomy cavity using a high dose rate source.ResultsSAVI devices had the lowest statistically significant values of DmaxSkin (81.00 ± 29.83), highest values of D90 (101.50 ± 3.66), and D95 (96.09 ± 4.55). SAVI-mini devices had the lowest statistically significant values of DmaxRib (77.66 ± 32.92) and smallest V150 (18.01 ± 3.39). Multi-lumen balloons were able to obtain the smallest V200 (5.89 ± 2.21). Strut-based applicators were more likely to achieve a DmaxSkin and a DmaxRib less than or equal to 100 %. The effect of PTV on V150 showed a strong positive relationship (p < .001). PTV and DmaxSkin showed a weak negative relationship in multi-lumen applicators (p = .016) and SAVI-mini devices (p < .001). PTV and DmaxRib showed a weak negative relationship in multi-lumen applicators (p = .009), SAVI devices (p < .001), and SAVI-mini devices (p < .001).ConclusionPTV volume is strongly correlated with V150 in all devices and V200 in strut based devices. Larger PTV volumes result in greater V150 and V200, which could help predict potential risks for hotspots and resulting toxicities in these devices. PTV volume is also weakly negatively correlated with max skin dose and max rib dose, meaning that as the PTV volumes increase one can expect slightly smaller max skin and rib doses. Strut based applicators are significantly more effective in keeping skin and rib dose constraints under 125 and 100 % when compared to any balloon based applicator.
Purpose: To investigate the mechanism underlying the energy dependence when a CR plate is exposed to therapeutic beams. Method and Materials: Small circular disks were cut from one CR plate and placed in a water‐equivalent plastic (WEP) phantom and exposed. The photostimulated luminescence (PSL) signal was recorded until the signal dropped to the background level to obtain the bleaching curve (PSL vs. time). The area under the bleaching curve (AUC) gives a measure of stored information in the CR plate. Monte Carlo simulations were used to obtain the ratio, DCR/DWater of the energy absorbed in the active layer of the CR plate in a WEP phantom to the energy absorbed in water when the entire phantom (including the CR plate) is replaced by water. Results: For electron beams, the AUC was independent of energy for any given dose to water, and the work function, W, i.e. the energy required to produce one PSL photon was also energy independent. In contrast, for photon beams, the AUC was 15% and 30% higher for 18 MV than for 6MV and Co‐60, respectively where the ratio DCR/DWater was 0.81, 1.08 and 1.11, respectively. Taking AUC data into account, the W for 18 MV had to be lower than for 6MV and Co‐60, our data showing 37% and 45%, respectively, in order to give a higher AUC despite a lower energy absorption in the active layer. While there is no obvious reason for this energy dependence, the differences in the secondary electron spectra produced by the different photon beams are probably not responsible. Conclusion: The method presented here will help researchers to both understand the response to ionizing radiation and develop new applications such as megavoltage dosimetry for IMRT verification. The energy dependence of W on beam modalities requires caution regarding CR dosimetry.
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