Application of novel thermoluminescence foils (2D) for QA of proton eye radiotherapy beams

Full text: In modern radiation therapy such as intensity modulation radiation therapy, image-guided brachytherapy or proton therapy through improved dose conformation, one is able to better match the target volume and to spare the surrounding healthy tissue. The high dose gradients and complicated patterns of dynamically varying dose distribution require two dimensional (2D) dosimetry, with spatial resolution better than 1 mm, preferably with real-time readout. In clinical dosimetry, thermoluminescence detectors are routinely applied as point detectors in the form of chips, rods or microcubes for in-phantom measurements of dose distribution. Among the many TLD materials available, LiF:Mg,Ti (TLD-100 or MTS-N) is most frequently applied in clinical dosimetry. 2D or 3D dosimetry can be performed in anthropomorphic phantoms using several TLDs suitably placed in the phantom and exposed simultaneously or sequentially. The spatial resolution of dose measurements is limited to the physical dimensions of the TL detectors and ranges between some 6 mm (rods) to approximately 1 mm (micro-cubes). Alternatively, sheets of X ray photographic emulsion or Gafchromic films have been used for 2D passive dosimetry in medical physics. The disadvantages of these film techniques are not entirely flat energy response of X ray films but rather low sensitivity of the Gafchromic films, as well as fading. Many 2D dosimetry systems, typically with surface deposited TLD powder heated with a scanning laser beam, have been tested in the last two decades but such systems were never commercialized. Application of sensitive coupled charge device (CCD) cameras used for readout of 2D TLD detectors opened new possibilities for 2D dosimetry. The currently available CCD cameras have sufficient sensitivity to record the weak TL signal and visualize its emission source - the irradiated and heated TL detector - as a two dimensional digital image array. A prototype planar TL reader with a sensitive CCD was developed at the Institute of Nuclear Physics in Krakow (Polish acronym IFJ) in cooperation with the Mikrolab company. We present the dosimetric properties of our recently developed two dimensional (2D) thermoluminescence (TL) dosimetry system, consisting of 2D TLD foil and planar TLD reader with 78 mm heater and 12-bit CCD PCO camera with a resolution of 640x480 pixels. The image manipulation may be performed with specially dedicated reading and analysing software, which allows for quantitative analysis of the 2D pictures. The TLD foil, of thickness 0.3 mm and different diameter (up to 70 mm), was developed as a mixture of highly sensitive LiF:Mg,Cu,P powder and ETFE polymer. The new 2D TLD system has been applied for studies, which may contribute to the improvement of quality assurance for proton radiotherapy of eye melanoma. A special eye phantom has been developed, in which several TLD foils or Gafchromic foils can be installed to verify 2D distribution of dose. The measurements have been performed at 60Co beam and 60 MeV proton beams at AIC-144 cyclotron in Krakow and at INFN, Katania. The used detectors have a spatial resolution better than 0.5 mm and a measurable dose range typical for radiotherapy. The beam profiles for different configurations of the beam preparation system have been tested. The results obtained from these measurements were compared with these measured with ionization chambers and profiles calculated using 'Eclipse Ocular Proton Planning' showing good consistency, with discrepancies up to 0.5mm. We believe that our 2D thermoluminescence technique will become a promising tool for quality assurance tests in radiotherapy, particularly proton radiotherapy, and in the assessment of dose delivered to patients undergoing radiotherapy. The potential advantages of the 2D TLD technique, as compared to silver-halide X ray films and radiochromic films, are the flat energy response of lithium fluoride and the re-usability of the 2D TLD sheets. Radiochromic film also provides a flat energy response and high spatial resolution but due to its m uch lower sensitivity, doses above approximately 3 Gy are required to provide accurate dosimetry. Moreover, radiochromic films are not re-usable and their readout is preceded by several hours of waiting for 'dye coloration'. Several problems must be solved before the 2D TLD technique can be introduced as a routine method in radiotherapy dosimetry. Some limitations are inherently connected with the properties of the thermoluminescence technique such as fading, sublinear (or supralinear) dose response at doses exceeding about 1 Gy, or variable energy/LET response. Based on experience gained from applying TLDs in the dosimetry of radiotherapy beams, e.g. in the mailed dosimetry audits of radiotherapy units where TLD point detectors in the form of chips, pellets or encapsulated LiF powder are used, to reach an overall uncertainty better than 2-3% is quite difficult. Such low uncertainty can be obtained only in an experienced laboratory after reproducible preparation and annealing of TLD material and suitably correcting for energy response, dose response, fading, instability of TLD reader, etc. In 2D TLD dosimetry many additional problems are encountered, such as a distribution of sensitivity of the individual pixels in the CCD camera, optical distortion of the camera lenses and the non-homogeneously sensitive TLD sheet. We have demonstrated that it is possible to provide individual sensitivity corrections for individual fragments of the sheet, but such a procedure should be coupled with advanced software able to identify fiducial markers on the detector and able to suitably re-evaluate the TL signal read from individual pixels or from groups of pixels. (author)