Spectral method for the correction of the Cerenkov light effect in plastic scintillation detectors: a comparison study of calibration procedures and validation in Cerenkov light-dominated situations.

PURPOSE The purposes of this work were: (1) To determine if a spectral method can accurately correct the Cerenkov light effect in plastic scintillation detectors (PSDs) for situations where the Cerenkov light is dominant over the scintillation light and (2) to develop a procedural guideline for accurately determining the calibration factors of PSDs. METHODS The authors demonstrate, by using the equations of the spectral method, that the condition for accurately correcting the effect of Cerenkov light is that the ratio of the two calibration factors must be equal to the ratio of the Cerenkov light measured within the two different spectral regions used for analysis. Based on this proof, the authors propose two new procedures to determine the calibration factors of PSDs, which were designed to respect this condition. A PSD that consists of a cylindrical polystyrene scintillating fiber (1.6 mm3) coupled to a plastic optical fiber was calibrated by using these new procedures and the two reference procedures described in the literature. To validate the extracted calibration factors, relative dose profiles and output factors for a 6 MV photon beam from a medical linac were measured with the PSD and an ionization chamber. Emphasis was placed on situations where the Cerenkov light is dominant over the scintillation light and on situations dissimilar to the calibration conditions. RESULTS The authors found that the accuracy of the spectral method depends on the procedure used to determine the calibration factors of the PSD and on the attenuation properties of the optical fiber used. The results from the relative dose profile measurements showed that the spectral method can correct the Cerenkov light effect with an accuracy level of 1%. The results obtained also indicate that PSDs measure output factors that are lower than those measured with ionization chambers for square field sizes larger than 25 x 25 cm2, in general agreement with previously published Monte Carlo results. CONCLUSIONS The authors conclude that the spectral method can be used to accurately correct the Cerenkov light effect in PSDs. The authors confirmed the importance of maximizing the difference of Cerenkov light production between calibration measurements. The authors also found that the attenuation of the optical fiber, which is assumed to be constant in the original formulation of the spectral method, may cause a variation of the calibration factors in some experimental setups.

[1]  R A Sutton,et al.  Water equivalence of plastic organic scintillators in megavoltage radiotherapy bremsstrahlung beams. , 2000, Physics in medicine and biology.

[2]  Natalka Suchowerska,et al.  Plastic scintillation dosimetry: optimization of light collection efficiency. , 2003, Physics in medicine and biology.

[3]  G. Ding.  Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator. , 2002, Physics in medicine and biology.

[4]  Natalka Suchowerska,et al.  Plastic scintillation dosimetry for radiation therapy: minimizing capture of Cerenkov radiation noise. , 2004, Physics in medicine and biology.

[5]  G. Knoll.  Radiation Detection And Measurement, 3rd Ed , 2009 .

[6]  P. Andreo,et al.  Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water , 2001 .

[7]  Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector. , 2010, Medical physics.

[8]  Natalka Suchowerska,et al.  A prototype scintillation dosimeter customized for small and dynamic megavoltage radiation fields , 2010, Physics in medicine and biology.

[9]  Radhe Mohan,et al.  Monte Carlo calculations of the absorbed dose and energy dependence of plastic scintillators. , 2005, Medical physics.

[10]  R. Fox,et al.  Classical Electrodynamics, 3rd ed. , 1999 .

[11]  Thomas R. Mackie,et al.  Cerenkov light generated in optical fibres and other light pipes irradiated by electron beams , 1992 .

[12]  G. Ding.  Using Monte Carlo simulations to commission photon beam output factors--a feasibility study. , 2003, Physics in medicine and biology.

[13]  G. Knoll.  Radiation detection and measurement , 1979 .

[14]  N Suchowerska,et al.  Cerenkov-free scintillation dosimetry in external beam radiotherapy with an air core light guide , 2008, Physics in medicine and biology.

[15]  A Batalla,et al.  The DOSIMAP, a high spatial resolution tissue equivalent 2D dosimeter for LINAC QA and IMRT verification. , 2009, Medical physics.

[16]  A Isambert,et al.  Spectral discrimination of Čerenkov radiation in scintillating dosimeters. , 2005, Medical physics.

[17]  F. Nüsslin,et al.  A virtual photon energy fluence model for Monte Carlo dose calculation. , 2003, Medical physics.

[18]  Louis Archambault,et al.  Water-equivalent dosimeter array for small-field external beam radiotherapy. , 2007, Medical physics.

[19]  F. H. Attix,et al.  Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: II. Properties and measurements. , 1992, Physics in medicine and biology.

[20]  N Suchowerska,et al.  A plastic scintillation dosimeter for high dose rate brachytherapy , 2006, Physics in medicine and biology.

[21]  J. Seuntjens,et al.  Characterization of cylindrical ionization chambers for patient specific IMRT QA , 2009, Journal of applied clinical medical physics.

[22]  L. Beaulieu,et al.  Clinical prototype of a plastic water-equivalent scintillating fiber dosimeter array for QA applicationsa). , 2008, Medical physics.

[23]  R. Mohan,et al.  Exploration of the potential of liquid scintillators for real-time 3D dosimetry of intensity modulated proton beams. , 2009, Medical physics.

[24]  J. M. Fontbonne,et al.  Scintillating fiber dosimeter for radiation therapy accelerator , 2001 .

[25]  Louis Archambault,et al.  Measurement accuracy and cerenkov removal for high performance, high spatial resolution scintillation dosimetry. , 2005, Medical physics.

[26]  G. Ban,et al.  Comparative Study of Plastic Scintillators for Dosimetric Applications , 2008, IEEE Transactions on Nuclear Science.

[27]  J. Seuntjens,et al.  A new formalism for reference dosimetry of small and nonstandard fields. , 2008, Medical physics.

[28]  D. Rogers,et al.  AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. , 1999, Medical physics.

[29]  F. H. Attix,et al.  Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical consideration. , 1992, Physics in medicine and biology.