The aim of this paper is to generalize and extend the mathematical formalism used with plastic scintillation detectors (PSDs). By doing so, we show the feasibility of multi-point PSD. The new formalism is based on the sole hypothesis that a PSD optical signal is a linear superposition of spectra. Two calibration scenarios were developed. Both involve solving a linear equation of the form Y = XB, but the process and input data depend on the information available on the detector system. Simulations were carried out to validate both scenarios and demonstrate the advantages of the new formalism. In this paper, we prove the following results. (1) Multi-point PSDs are feasible. Simulations have shown that six different spectra could be resolved accurately even in the presence of up to 10% Gaussian noise. (2) The new formalism leads to more precise PSD measurements. (3) By using the condition number of the measurement matrix, the ideal sets of calibration measurements can be identified. (4) By using principal component analysis it was possible to identify the best set of wavelength filters. We have shown through numerical simulations that multi-point detectors are feasible. This has potential for applications such as in vivo dose verification. Furthermore, our new formalism can be used to improve the robustness and ease of use of PSDs.

The goal of this study was to evaluate the nature of the stem effect light produced within an optical fiber, to quantify its composition, and to evaluate the efficiency of the chromatic technique to remove the stem effect. Spectrometry studies were performed during irradiations of a bare PMMA optical fiber with kilovoltage x-rays from a superficial therapy unit, an Ir-192 high-dose-rate brachytherapy source, a Co-60 external-therapy unit, and megavoltage electrons and x-rays from a linear accelerator. Stem effect spectra can be accurately modeled by a linear combination of the Cerenkov light and fluorescence emitted spectra. Fluorescence light contributes more for lower-energy modalities. Cerenkov light contributes more as the energy increases above the threshold for its production. The chromatic stem effect removal technique is accurate in most of the situations. However, noticeable differences were obtained between very specific high-energy irradiation conditions. It would be advantageous to implement an additional channel in the chromatic stem effect removal chain or implement a spectral approach to independently remove the Cerenkov and the fluorescence components from the signal of interest. This would increase the accuracy and versatility of the actual chromatic stem effect removal technique.

The purpose is to evaluate the impact of interseed attenuation and prostate composition for prostate treatment plans with 125I permanent seed implants using the Monte Carlo (MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG-43 based calculation, superposition MC, full MC with water prostate, and full MC with realistic prostate tissue. The prostate tissue description is from the ICRP report 23 (W. S. Snyer, M. J. Cook, E. S. Nasset, L. R. Karkhausen, G. P. Howells, and I. H. Tipton, "Report of the task group on reference man," Technical Report 23, International Commission on Radiological Protection, 1974). According to the comparisons, the seed density has an influence on interseed attenuation. A plan with a typical low seed density (42 0.6 mCi seeds in a 26 cm3 prostate) suffers a 1.2% drop in the CTV D90 value due to interseed attenuation. A drop of 3.0% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate composition is similar for all seed densities and prostate sizes. The difference between MC simulations in water and MC simulations in prostate tissue is between 4.4% and 4.8% for the D90 parameter. Overall, the effect on D90 is ranging from 5.8% to 12.8% when comparing clinically approved TG-43 and MC simulations in prostate tissue. The impact varies from one patient to the other and depends on the prostate size and the number of seeds. This effect can reach a significant level when reporting correlations between clinical effect and deposited dose.