Optimisation of eye plaque dosimetry using Monte Carlo method

Field: Medicine (Health Sciences)

Cancer of the eye is a rare and challenging disease. Uveal melanoma is the most prevalent of ocular malignancies in adults, with Australia holding one of the highest rates of incidence worldwide. While rare, the consequences are serious and more often than not, the sufferer will incur vision loss as a result of treatment, or, in time, will face losing the eye. Brachytherapy using radioactive eye plaques is the preferred method of treatment for ocular melanoma, yielding the best results for the patient in terms of vision and survival. Currently, methods of plaque brachytherapy dosimetry are limited by the size and spatial resolution of detectors such as thermoluminescent dosimeters (TLDs), where steep dose gradients create significant challenges. It is not only the detector size but the complexity of source geometry that hinder accurate dosimetry further still. These limitations lead to incomplete dose distributions and inevitably, inaccurate treatment planning for the quality assurance of eye plaques before clinical use.\nDose planning software for I-125 plaque brachytherapy using the 15mm ROPES plaque has been developed based on interpolation of data from the published dosimetric parameters of the Task Group No. 43 (TG43) AAPM revised protocol (Rivard et al.,2004). This application includes the ability to export dose maps in the format of a 256 x256 pixel array analogous to the output of the Medipix2 silicon pixelated counting device recently used in physical measurements at the Centre for Medical Radiation Physics (CMRP). Using the Geant4 photon transport toolkit, Monte Carlo dosimetry was performed for a single I-125 model 6711 seed to optimise the dose rate distribution data calculated the software. Dose point data was obtained every millimetre up to 25mm in the radial direction and every five degrees in polar angle. Similarly, Monte Carlo method was used to compare dose distributions for the Ru-106 CCD plaque with those generated by the Plaque Simulator (BEBIG GmbH, Berlin, Germany). The cylindrical symmetry of the plaque allowed data to be obtained in toroids of radii in millimetre increments up to 25mm, and depth from the plaque surface also in millimetre increments up to 25mm.\nThe results of the I-125 simulation were used to calculate the TG43 dosimetric parameters and were in good agreement with the published data. Radial dose functions over the scoring range fell within 3% with a maximum deviation from TG43 data of only 2.8% (occurring 1mm away from the source on the plaque central axis). Anisotropy functions were obtained within 5% uncertainty for all polar angles. The Ru-106 plaque results differed considerably from the dose data generated by the Plaque Simulator withthe most significant deviation occurring at very small distances from the plaque inner surface (within 1cm).\nMonte Carlo method is a useful technique for dosimetry of plaque brachytherapy sources used in the treatment of ocular melanoma. The Geant4 toolkit is capable of accurately scoring dose at defined small radial distances from the source previously unaccounted for. The dose point array obtained for the model 6711 seed can be input into the dose planning software for dose optimisation and combined with the results of recent physical measurements using the Medipix2, can achieve the quality assurance of eye plaque brachytherapy treatment.