Small field output correction factors of the microSilicon detector and a deeper understanding of their origin by quantifying perturbation factors

Purpose

The aim of this study is the experimental and Monte Carlo‐based determination of small field correction factors for the unshielded silicon detector microSilicon for a standard linear accelerator as well as the Cyberknife System. In addition, a detailed Monte Carlo analysis has been performed by modifying the detector models stepwise to study the influences of the detector’s components.

Methods

Small field output correction factors have been determined for the new unshielded silicon diode detector, microSilicon (type 60023, PTW Freiburg, Germany) as well as for the predecessors Diode E (type 60017, PTW Freiburg, Germany) and Diode SRS (type 60018, PTW Freiburg, Germany) for a Varian TrueBeam linear accelerator at 6 MV and a Cyberknife system. For the experimental determination, an Exradin W1 scintillation detector (Standard Imaging, Middleton, USA) has been used as reference. The Monte Carlo simulations have been performed with EGSnrc and phase space files from IAEA as well as detector models according to manufacturer blueprints. To investigate the influence of the detector’s components, the detector models have been modified stepwise.

Results

The correction factors for the smallest field size investigated at the TrueBeam linear accelerator (equivalent dosimetric square field side length S_clin = 6.3 mm) are 0.983 and 0.939 for the microSilicon and Diode E, respectively. At the Cyberknife system, the correction factors of the microSilicon are 0.967 at the smallest 5‐mm collimator compared to 0.928 for the Diode SRS. Monte Carlo simulations show comparable results from the measurements and literature.

Conclusion

The microSilicon (type 60023) detector requires less correction than its predecessors, Diode E (type 60017) and Diode SRS (type 60018). The detector housing has been demonstrated to cause the largest perturbation, mainly due to the enhanced density of the epoxy encapsulation surrounding the silicon chip. This density has been rendered more water equivalent in case of the microSilicon detector to minimize the associated perturbation. The sensitive volume itself has been shown not to cause observable field size‐dependent perturbation except for the volume‐averaging effect, where the slightly larger diameter of the sensitive volume of the microSilicon (1.5 mm) is still small at the smallest field size investigated with corrections <2%. The new microSilicon fulfils the 5% correction limit recommended by the TRS 483 for output factor measurements at all conditions investigated in this work.

Separation of structurally related primary aliphatic amines using hydrophilic interaction chromatography with fluorescence detection after postcolumn derivatization with o-phthaldialdehyde/mercaptoethanol

The retention behavior of primary aliphatic amines (homologous series of aliphatic alkyl amines and cycloalkyl amines) and positional isomers of alkylamines in the hydrophilic interaction chromatography mode was studied. The study was carried out on a TSKgel Amide-80 column followed by postcolumn derivatization with fluorescence detection to describe the retention mechanism of tested compounds. The effect of chromatographic conditions including column temperature, acetonitrile content in the mobile phase, mobile phase pH (ranging from 3.5 to 6.8), and salt concentration in the mobile phase was investigated. The final mobile phase consisted of acetonitrile and solution of 20 mM potassium formate pH 3.5 in ratio 80:20 v/v. The analyses were carried out at mobile phase flow rate of 1.0 mL/min and the column temperature of 20°C. The developed method was fully validated in terms of linearity, sensitivity (limit of detection and limit of quantification), accuracy, and precision according to International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use guidelines. The proposed new methods were proved to be highly sensitive, simple, and rapid, and were successfully applied to the determinations of isopropylamine, cyclohexylamine, and cyclopropylamine in relevant active pharmaceutical ingredients.

The mean photon energy ĒF at the point of measurement determines the detector-specific radiation quality correction factor kQ,M in (192)Ir brachytherapy dosimetry

The application of various radiation detectors for brachytherapy dosimetry has motivated this study of the energy dependence of radiation quality correction factor kQ,M, the quotient of the detector responses under calibration conditions at a (60)Co unit and under the given non-reference conditions at the point of measurement, M, occurring in photon brachytherapy. The investigated detectors comprise TLD, radiochromic film, ESR, Si diode, plastic scintillator and diamond crystal detectors as well as ionization chambers of various sizes, whose measured response-energy relationships, taken from the literature, served as input data. Brachytherapy photon fields were Monte-Carlo simulated for an ideal isotropic (192)Ir point source, a model spherical (192)Ir source with steel encapsulation and a commercial HDR GammaMed Plus source. The radial source distance was varied within cylindrical water phantoms with outer radii ranging from 10 to 30cm and heights from 20 to 60cm. By application of this semiempirical method - originally developed for teletherapy dosimetry - it has been shown that factor kQ,M is closely correlated with a single variable, the fluence-weighted mean photon energy ĒF at the point of measurement. The radial profiles of ĒF obtained with either the commercial (192)Ir source or the two simplified source variants show little variation. The observed correlations between parameters kQ,M and ĒF are represented by fitting formulae for all investigated detectors, and further variation of the detector type is foreseen. The herewith established close correlation of radiation quality correction factor kQ,M with local mean photon energy ĒF can be regarded as a simple regularity, facilitating the practical application of correction factor kQ,M for in-phantom dosimetry around (192)Ir brachytherapy sources. ĒF values can be assessed by Monte Carlo simulation or measurement. A technique describing the local measurement of ĒF will be published separately.