Validating plastic scintillation detectors for photon dosimetry in the radiologic energy range

Abstract

Purpose: Photon dosimetry in the kilovolt (kV) energy range represents a major challenge for diagnostic and interventional radiology and superficial therapy. Plastic scintillation detectors (PSDs) are potentially good candidates for this task. This study proposes a simple way to obtain accurate correction factors to compensate for the response of PSDs to photon energies between 80 and 150 kVp. The performance of PSDs is also investigated to determine their potential usefulness in the diagnostic energy range. Methods: A 1-mm-diameter, 10-mm-long PSD was irradiated by a Therapax SXT 150 unit using five different beam qualities made of tube potentials ranging from 80 to 150 kVp and filtration thickness ranging from 0.8 to 0.2 mmAl + 1.0 mmCu. The light emitted by the detector was collected using an 8-m-long optical fiber and a polychromatic photodiode, which converted the scintillation photons to an electrical current. The PSD response was compared with the reference free air dose rate measured with a calibrated Farmer NE2571 ionization chamber. PSD measurements were corrected using spectra-weighted corrections, accounting for mass energy-absorption coefficient differences between the sensitive volumes of the ionization chamber and the PSD, as suggested by large cavity theory (LCT). Beam spectra were obtained from x-ray simulation software and validated experimentally using a CdTe spectrometer. Correction factors were also obtained using Monte Carlo (MC) simulations. Percent depth dose (PDD) measurements were compensated for beam hardening using the LCT correction method. These PDD measurements were compared with uncorrected PSD data, PDD measurements obtained using Gafchromic films, Monte Carlo simulations, and previous data. Results: For each beam quality used, the authors observed an increase of the energy response with effective energy when no correction was applied to the PSD response. Using the LCT correction, the PSD response was almost energy independent, with a residual 2.1% coefficient of variation (COV) over the 80-150-kVp energy range. Monte Carlo corrections reduced the COV to 1.4% over this energy range. All PDD measurements were in good agreement with one another except for the uncorrected PSD data, in which an over-response was observed with depth (13% at 10 cm with a 100 kVp beam), showing that beam hardening had a non-negligible effect on the PSD response. A correction based on LCT compensated very well for this effect, reducing the over-response to 3%. Conclusion: In the diagnostic energy range, PSDs show high-energy dependence, which can be corrected using spectra-weighted mass energy-absorption coefficients, showing no considerable sign of quenching between these energies. Correction factors obtained by Monte Carlo simulations confirm that the approximations made by LCT corrections are valid. Thus, PSDs could be useful for realtime dosimetry in radiology applications.

Characterization of a scintillating fibre detector for small animal imaging and irradiation dosimetry

“…Other authors declare an energy dependence of plastic scintillator detectors sensitivity in the radiologic energy range (25-150 kVp). 13,14,[16][17][18][19] As reported by Williamson et al, 16 Lessard et al, 18 and Peralta and Rêgo, 19 it is partly related to energy absorption differences between the reference dose medium and the scintillator medium (air and polystyrene respectively in our case). As shown in Figure 4, the polystyrene-toair mass energy-absorption coefficients ratio shows significant variations ,100 keV.…”

“…[13][14][15] However, other studies also found an energy dependence of the dosemeter response, expressed as a decrease of sensitivity with decreasing energy. [16][17][18][19] In addition, in the kilovoltage energy range, the stem effect is mainly due to the fluorescence of the optical fibre and its contribution has been reported to be small. 8 Its small influence on the final signal allows to simplify the dosemeter readout system.…”

Increased radiosensitivity of colorectal tumors with intra-tumoral injection of low dose of gold nanoparticles

“…[20][21][22][23] In the current study, male nu/nu mice bearing HCT116 human colorectal tumor were treated by IT injection of tiopronin-coated GNPs (Tio-GNPs) and then irradiated with an X-ray source, which has a similar energy spectrum compared to that of the Papillon 50 X-ray unit (Figure 1). 24,25 The ligand that coats the GNP must be carefully chosen. Long ligands can attenuate the low-energy photoelectrons emitted from the GNPs after radiation, which could then lead to a considerable decrease in any radioenhancement effect.…”

Acceptance, commissioning and clinical use of the WOmed T-200 kilovoltage X-ray therapy unit

“…It is important that data used for treatment are determined in water or in a water-equivalent medium or that the irradiated medium is stated for data given in publications. 14,15 This article reports on the acceptance and commissioning work carried out on the T-200 X-ray therapy system manufactured by Wolf-Medizintechnik (WOmed) GmbH (WOmed, St Gangloff, Germany) installed at our clinic. The intended use of this machine is for superficial X-ray therapy using beams from tube potentials between 30 and 200 kV.…”