Energy dependence of a scintillating fiber detector for preclinical dosimetry with an image guided micro-irradiator.

The dosimetry of preclinical micro-irradiators is challenging due to their millimetric beams and medium x-ray energy range. Plastic scintillator dosimeters (PSD) are good candidates for such a purpose as they provide a high spatial resolution although they show an energy dependence below 100 keV. The purpose of this study was to assess the energy dependence of a dedicated PSD (called DosiRat) for micro-irradiators dosimetry. The response of the PSD relative to air kerma was measured for different beam qualities (40-225 kV) with the X-RAD 225Cx irradiator. The corresponding energy spectra, determined by Monte Carlo simulations, allowed for correcting the differences in absorbed dose between the DosiRat material (polystyrene) and the air and therefore allowed to compare DosiRat intrinsic energy response to the Birks scintillation quenching model. The energy response of DosiRat was then assessed under preclinical conditions through percentage depth dose curves (PDD) and relative output factor (ROF) measurements in water for beam diameters ranging from 1 to 25 mm. DosiRat energy response showed a coefficient of variation of 23% from 40 to 225 kV, mainly explained by the mass energy-absorption coefficient variation between polystyrene and air. A remaining variation was shown to be caused by the quenching of the scintillation and was correctly reproduced by the Birks model (with kB = 10.27 mg MeV-1 cm-2). PDD and ROF measurements highlighted an energy response variation with depth and collimation up to 10%. A dose accuracy better than 1% was finally achieved with appropriate calibration and correction factors (CF), for beam collimations larger than the detector ([Formula: see text]2 mm diameter). DosiRat energy dependence was fully characterized in preclinical energy range and shown to be negligible with convenient calibration and corrections factors. It provided accurate dosimetry for medium energy (225 kV) and millimetric beams (down to 2.5 mm).

The DosiMap, a new 2D scintillating dosimeter for IMRT quality assurance: characterization of two Cerenkov discrimination methods.

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.

Implementation and evaluation of respiratory gating in small animal radiotherapy.

Major advance was done in preclinical radiotherapy thanks to the development of image guided micro-irradiator. Nevertheless, some applications still can benefit of improvements, such as the irradiation of mobile tumors. This preclinical radiotherapy case presents increased difficulties compared to clinical practice because of the waveform of small animals breathing cycle, its frequency and amplitude. To answer this issue, we developed a specific beam shutter and implemented respiratory gating on the X-RAD 225Cx preclinical irradiator. In the first step of this study, the shutter was accurately characterized. Opening and closing speed of 1.28 and 0.33 mm ms-1 were respectively measured, and a transmission of 0.7% of the beam was measured with the shutter fully closed. Beam-on times were also determined for various gating parameters and highlighted a difference of 57 ms between the beam delivery duration and the gate width. This discrepancy was compensated during the respiratory monitoring adjustment. In a second step, a respiratory protocol was evaluated with two vertical beams of 2.5 and 5 mm diameters, for motion amplitudes ranging from 0.5 to 4 mm. This evaluation demonstrated the effectiveness of our set up to perform motion compensation for amplitude as small as 0.5 mm despite a dose gradient of 1.47 cGy mm-1 observed with the 5 mm irradiation field, due to the shutter opening and closing durations. We also investigated the efficiency of a scintillating fiber dosimeter, adapted to small beams and providing real-time dose rate measurements. This detector showed very good performances to detect motion in small irradiation fields and would be very suitable to monitor the number of delivered gates until the planned delivered dose is achieved. This study presented a new respiratory gating set up and showed that very efficient motion compensation could be achieved in small animal radiotherapy.

Spectral discrimination of Cerenkov radiation in scintillating dosimeters.

Radiation therapy accelerators require highly accurate dose deposition and the output must be monitored frequently and regularly. Ionization chambers are the primary tool for this control, but their size, their high voltage needed, and the correction needed for electrons make them unsuitable for use during patient treatment. We have developed a small (1-mm-diam and 1-mm-long active part), flexible, and water-equivalent dosimeter. It is suitable for photon and electron beams without corrections, and performs on line dose measurements. This detector is based on only one scintillating fiber and a CCD camera. A new signal processing is used to remove the effect of Cerenkov radiation background, which only requires a preliminary calibration. Central-axis depth-dose distribution comparisons have been achieved with standard ionization chambers, over a range from 8 to 25 MV photons and from 6 to 21 MeV electrons in order to validate this calibration. Results show a very good agreement, with less than 1% difference between the two detectors.