The DosiMap, a new 2D scintillating dosimeter for IMRT quality assurance: Characterization of two Čerenkov discrimination methods

Abstract

New radiation therapy techniques such as IMRT present significant efficiency due to their highly conformal dose distributions. A consequence of the complexity of their dose distributions (high gradients, small irradiation fields, low dose distribution, ...) is the requirement for better precision quality assurance than in classical radiotherapy in order to compare the conformation of the delivered dose with the planned dose distribution and to guarantee the quality of the treatment. Currently this control is mostly performed by matrices of ionization chambers, diode detectors, dosimetric films, portal imaging, or dosimetric gels. Another approach is scintillation dosimetry, which has been developed in the last 15 years mainly through scintillating fiber devices. Despite having many advantages over other methods it is still at an experimental level for routine dosimetry because the Cerenkov radiation produced under irradiation represents an important stem effect. A new 2D water equivalent scintillating dosimeter, the DosiMap, and two different Cerenkov discrimination methods were developed with the collaboration of the Laboratoire de Physique Corpusculaire of Caen, the Comprehensive Cancer Center François Baclesse, and the ELDIM Co., in the frame of the MAESTRO European project. The DosiMap consists of a plastic scintillating sheet placed inside a transparent polystyrene phantom. The light distribution produced under irradiation is recorded by a CCD camera. Our first Cerenkov discrimination technique is subtractive. It uses a chessboard pattern placed in front of the scintillator, which provides a background signal containing only Cerenkov light. Our second discrimination technique is colorimetric. It performs a spectral analysis of the light signal, which allows the unfolding of the Cerenkov radiation and the scintillation. Tests were carried out with our DosiMap prototype and the performances of the two discrimination methods were assessed. The comparison of the dose measurements performed with the DosiMap and with dosimetric films for three different irradiation configurations showed discrepancies smaller than 3.5% for a 2 mm spatial resolution. Two innovative discrimination solutions were demonstrated to separate the scintillation from the Cerenkov radiation. It was also shown that the DosiMap, which is water equivalent, fast, and user friendly, is a very promising tool for radiotherapy quality assurance.

Projection imaging of photon beams using Čerenkov-excited fluorescence

“…In a sense, this portion of the Čerenkov light becomes a source of contamination, analogous to the stem effect in plastic scintillation dosimetry and could be reduced by using an optical filter centered on the fluorescent emission peak, or fully removed using spectral or temporal methods (Frelin et al , 2008; Liu et al , 2011; Therriault-Proulx et al , 2011). All three potential sources of error will be investigated in a future study.…”

“…In the context of profiling static megavoltage linear accelerator (LINAC) beams for routine quality assurance (QA), typically only a small subset of the full volumetric dose is directly measured by virtue of an ionization chamber measurement in water. Several methods have been proposed to move towards 2D and/or full 3D beam profiling without the use of ionization chambers, including techniques that use plastic or liquid scintillators, as well as gel-based dosimetry (Beddar et al , 1992a; Beddar et al , 1992b; Frelin et al , 2008; Collomb-Patton et al , 2009; Guillot et al , 2011; Kirov et al , 2000; Ponisch et al , 2009; Beddar et al , 2009; Archambault et al , 2012; Maryanski et al , 1994; Maryanski et al , 1996; Kelly et al , 1998; McJury et al , 2000). However, both methods have limitations in that they are not truly water equivalent.…”

“…Although the existence of Čerenkov effect (optical photons generated as an energetic electron travels faster than the local speed of light in a dielectric medium) during radiation therapy was initially documented two decades ago, historically the phenomenon was only considered a source of noise in scintillation optical dosimetry (Beddar et al , 1992c; Frelin et al , 2008; Liu et al , 2011; Therriault-Proulx et al , 2011). However, a recent study into an optical-based dose imaging modality investigated the direct use of the induced Čerenkov emission in an irradiated water volume as an indirect surrogate for the imparted dose (Glaser et al, 2012a ; Čerenkov, 1934; Frank and Tamm, 1937).…”

Quantifying the impact of lead doping on plastic scintillator response to radiation

“…Numerous studies have demonstrated that plastic scintillators are excellent detectors for use in clinical radiation dosimetry . Several factors contribute to their suitability.…”

Liquid scintillator for 2D dosimetry for high‐energy photon beams

“…However both detector designs are limited to a single 2D imaging plane perpendicular to the direction of the beam and thus no depth dose distribution can be directly measured. Recently, Frelin et al 7 presented a new 2D scintillation dosimeter for IMRT QA, which consisted of a plastic scintillating sheet placed inside a transparent polystyrene phantom, and investigated methods to correct for Čerenkov radiation. Kirov et al 8 investigated the use of liquid scintillators for brachytherapy applications and, therefore, studied the dosimetric properties of different scintillating solutions with the aim of developing water equivalent dosimeter media at the lower energies that are suitable for use with the most commonly used radioactive sources.…”