Dosimetric characterization of COMS eye plaques loaded with IsoSeed I25.S16 seeds using Monte Carlo simulations for ophthalmic brachytherapy

Uveal melanoma is the most common intraocular tumor, with enucleation being the standard treatment of choice. However, alternative therapies exist, including ophthalmic brachytherapy, which provides an effective option by offering equivalent tumor control while preserving the eye and, in some cases, maintaining visual function. In ophthalmic brachytherapy, dedicated plaques containing radioactive sources are used to deliver highly localized irradiation, utilizing low-energy x- and γ-rays or electron emissions. These plaques come in different sizes to accommodate the patient’s tumor dimensions and are surgically placed directly over the tumor site in an operating room setting. In clinical practice, the dosimetric characterization of these plaques follows the AAPM Task Group 43 formalism, which assumes a homogeneous medium for dose calculations, neglecting factors such as the presence of high-Z materials, like the metal alloys used in the plaques. This approximation can significantly affect the actual dose distribution, which is used to estimate the treatment time during which the loaded plaque remains applied to the patient to deliver the prescribed dose. This thesis, carried out at the Istituto Oncologico Veneto, aimed to perform a dosimetric characterization of COMS plaques loaded with IsoSeed I25.S16 sources through Monte Carlo simulations, assessing the discrepancies between AAPM-standardized dose calculations and those that account for source heterogeneity. Simulations were complemented and validated through experimental measurements using tissue-equivalent phantoms and radiochromic films.