Multi-channel fibre optic dosimeter based on Optically Stimulated Luminescence for dose verification during radiotherapy treatments

Within the WP 3.2, CEA LIST (Saclay) investigates OSL dosimetry for dose verification in radiotherapy. It collaborates with the Institut Gustave Roussy (Villejuif) on clinical validations and Laboratoire National Henri Becquerel (LNHB, Saclay) on pre-clinical validation tests.

IGR is the first European cancer treatment center. It provides a clinical expertise, accelerator equipments as well as usual dosimetry measurements.

LNHB is a certified laboratory for the measurement of dose. It provides the equipment and methodology to calibrate the sensors with respect to absolute dose.

The quality control in radiotherapy needs an efficient, reliable, easy to use and cost-effective multi-sensor dosimeter system in order to check the conformity between the prescribed doses and the absorbed doses delivered to the patient, especially in the context of IMRT for which high dose gradients are observed.

The measurement and data calculation/storage must be performed in a restricted time (less than some minutes) in order to be compatible with a treatment protocol. The sensors should be radiation transparent in order not to attenuate or scatter the radiation. Finally, they should be calibrated for all the energies used.

Since OSL dosimeters are truly integrating dosimeters, they may be read after treatment. They offer significant advantages over active dosimeters such as avoiding Cerenkov and scintillation perturbations associated to a high accuracy in dose measurement. However, these sensors may be also used as scintillators to measure the dose rate. For most treatments, the absorbed dose may be measured after irradiation (delayed measurement) but some treatments (such as Total Body Irradiation - TBI) necessitates a real-time estimation of the cumulated dose in order to stop the irradiation when it reaches the required level.

The OSL technique conveys the additional advantage over TL of measurement in operation (no need to collect the sensor) and over MOSFET, longer lifetime and lower maintenance cost.

 Optically Stimulated Luminescence (OSL) dosimetry is similar to the well-known thermoluminescence dosimetry (TL) except that the luminescence is stimulated by light instead of heat, opening the way to a remote and operational optical interrogation.

OSL dosimetry with alumina conveys a lot of advantages (high sensitivity, very good linearity, no fading at room temperature, low Z (~10)). The fiber sensors are compatible with in-vivo applications (small (mm size) and sterilizable). They are immune to electromagnetic perturbations and exhibit low energy and angular dependences.

The OSL dosimeter system proposed by the CEA LIST operates in a continuous-wave mode (CW-OSL) which is simple and convenient for performing delayed OSL measurements. Moreover, the system incorporates an optical fibre switch (16 channels) (fig. 1-2) in a cost-effective approach.

Fig. 1: View of the OSL dosimeter developed by the CEA LIST

Fig. 2: Principle of the CEA LIST OSL dosimeter system

 The instrumentation, located in the irradiation room (protected by screens), is handled from the control room by a laptop through a dedicated software written in LabView^.

The OSL signals are read sequentially after treatment and integrated to provide the doses. The light stimulation also fully bleaches the sensor crystals so that they can be remotely read in operation (reusable for the next patient).

During Total Body Irradiation (TBI) treatments, the optical switch is periodically rotated to scan the sensors emissions and a routine calculates the cumulated dose for each sensor (period ~ 2 seconds for 12 sensors) by integrating the radioluminescence (RL) signals emitted by the sensors (in the absence of laser stimulation). Then, the absorbed dose (i.e. at the end of the treatment) is measured by OSL which also bleaches the crystal for a next usage. To compensate for the - albeit relatively weak - stem effect perturbation (fibre Cerenkov and scintillation), a second identical optical fibre (without detector) is placed in parallel with and aside the first one. The luminescence from the reference fibre is then subtracted from the RL signal measured with the sensor.

Each fibre sensor consist of an optical cable with a SMA connector at one end and a molded cylindrical head at the other (fig. 3), made of polymers for radiation transparency. The a-Al₂O₃ crystal is 1 mm long and has a diameter of 1 mm. It is protected by a sleeve rigidly fixed inside the cable and affixed near the end of a silica fibre (Æ = 600 µm, NA = 0.37). It is remotely stimulated via the optical fibre using a doubled Nd-YAG laser (532 nm, 150 mW). The OSL is collected and guided back along the same fibre to a photomultiplier tube through adequate optical filters.

 Fig. 3: OSL fibre sensors for external radiotherapy (Ø=6 mm)

Tests are performed using an X-ray generator (80 keV, 0.15 Gy/min) and a Saturn 43 linear accelerator (6 MV, 12 MV, 20 MV). The Fig. 4 shows a continuous irradiation by the X-ray generator during 2 minutes. The RL (observed during the irradiation exposure) provides an estimation of the dose rate. Then, the laser shutter is opened to stimulate the OSL and the OSL signal (with a decaying shape) is recorded and integrated (dose resolution ~ 1 mGy). The crystal is bleached within 24 seconds (> 99.9 % of trap depletion) for 15 mW at the fibre end. An additional time of about 6 s is needed to record the background noise that is subtracted to provide the dose measurement. OSL readings may be programmed as the irradiation advances, following the planned treatment.

Fig. 4: Typical RL/OSL signals obtained with the fibre sensors depicted on fig. 3

 Pre-clinical validation tests are planned on a Saturn 43 linear accelerator to check the energy response (for several energies ranging from 1 MeV to 20 MeV), angular response and dose reproducibility. Clinical validations will then be carried out at IGR facilities to validate the instrumentation in real medical conditions on body simulators (phantoms).

An industrial transfer is planned for 2009 towards a french SME specialized in medical dosimetry. However, other licenses are expected.

[1] O. Roy, S. Magne, J.C. Gaucher, L. Albert, L. Dusseau, J.C. Bessière and P. Ferdinand, All Optical Fiber Sensor based on Optically Stimulated Luminescence for Radiation Detection, Optical Fiber Sensor Conference (OFS12), 1997, Williamsburg (Virginia), USA

[2] S. Magne, P. Ferdinand, Fiber Optic remote g dosimeters based on Optically Stimulated Luminescence: State-of-the art at CEA, IRPA 11, Madrid, 23-27 May 2004

[3] G. Ranchoux, S. Magne, J.P. Bouvet and P. Ferdinand, Fiber Remote Optoelectronic gamma dosimetry based on Optically Stimulated Luminescence of Al₂O₃:C, Rad. Prot. Dos. 100, 1-4 (2002) 255-260

[4] S. Magne, L. Auger, A. Isambert, A. Bridier, P. Ferdinand, J. Barthe, Développement d’un dosimètre multivoies à fibres optiques pour la radiothérapie, fondé sur la mesure de la scintillation et de l’OSL de l’alumine, 45^èmes journées Scientifiques de la SFPM (Société Française de Physique Médicale), 7-9 juin 2006, Lyon (ENS)

[5] S. Magne, L. Auger, A. Isambert, A. Bridier, P. Ferdinand, J. Barthe, Multi-channel fibre optic dosimeter based on OSL for dose verification during radiotherapy treatments, 18^th Optical Fiber Sensor Conference, Cancun, Mexico, October 23-27, 2006.

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