Laser heating of thermoluminescent plates: application to intraoperative irradiation measurements

Laser heating of thermoluminescent dosimeter plate (TLDP) makes possible radiation dose mapping. The application of this technique to intraoperative radiotherapy is examined, the first results show the interest of this approach. The intraoperative radiotherapy seems to be an interesting but complex technique: it allows, during a surgical operation, to irradiate either the tumor, or the tumoral bed. To protect the neighbouring tissues, a cone is used to localize the field to the target volume. The whole dose is delivered in a single electron irradiation in complex configuration that makes more critical the dose control and measurement. Only in-situ dose mapping makes possible the control that will ensure the treatment efficiency. Many parameters must be considered, the irradiated volume has to be as uniform as possible (it is necessary to avoid over and under dosage), the irradiation of the surrounding healthy tissues must be minimized and controlled. For many years, we have been developing for various applications a new dose mapping system based on laser heating TLDP. The plates are made of a 50 μm thick thermoluminescent film. The film itself is a silk screen printed layer of thermoluminescent CaSO4: Dy powder mixed with a binder, on a flexible kapton support. The local thermoluminescence is measured when a CO2 laser (λ=10.6μm) beam is scanned on the TLDP. The dose distribution map is obtained by plotting the thermoluminescence intensity as a function of the laser beam position. We have developed sealed soft protection cases for TLDP that can be sterilized. This sterilization was performed in warm humid atmosphere at 55 deg. C during 5 hours. 48 hours later we verified that the response of TLDP had not changed. We also measure the response of our TLDP using an electron beam (6 and 13.5 MeV) for doses ranging from 1 to 20 Gy. As expected, the thermoluminescence response is linear, i.e. the luminescence intensity is proportional to dose, and does not depend on the dose rate. Deep doses are the same, when measured in a polystirene phantom with a TLDP, or in a water container with an ionization chamber. In irradiated polystirene phantom we measure the dose distribution at 1 mm depth and at the maximum depth for 6 MeV and 13.5 MeV electrons. For irradiation we use an electron beam positioner (plexiglass wall+reducer brass ring, inner diameter 7 cm). The large dynamic range of the TLDP allows to visualize the maximum dose in the center of the field as well as the lateral dose related to scattering. For simple irradiation configurations the results are in good agreement with calculations and ionisation chamber measurements. Our long experience in TLDP laser heating was helpful for solving efficiently the specific problems encountered in intraoperative dosimetry. Our present results illustrate the interest of this method in the field of radiotherapy, and more specifically for intraoperative irradiations. We are currently developing a new expertness in this field. This technique is a good alternative to radiological films, since it provides some advantages such as digital storage, large dynamic dose range and easy processing