Dosimetric characterization of a ⁹⁰ Sr ⁹⁰ Y Pterygium applicator with radiochromic film and Monte Carlo simulations

Background: β-emitting radionuclides, such as 90Sr90Y, are widely used in clinical settings for the treatment of both benign and malignant lesions, particularly as surface applicators. Despite their clinical relevance, the three-dimensional dose distributions delivered by these applicators remain inadequately characterized using Monte Carlo simulations, the current gold standard for dose calculation. This study aims to address these limitations by characterizing the three-dimensional dose distribution of a commonly used 90Sr90Y Pterygium applicator. The goals include generating accurate percent-depth-dose (PDD) curves and validating a custom irradiation setup using radiochromic film and Monte Carlo simulations to enable accessible, reproducible, and highly precise radiobiology experiments. Methods: A Monte Carlo dose calculation software based on Geant4 10.02.p02 was developed, and the Amersham SIA 20 Pterygium applicator, a stacked film setup with 30 EBT-XD GafChromic ® films, and a film-cell irradiation setup (a film layer, cell monolayer, and growth media) were modeled. The dose rate was averaged over the 8.2 mm diameter active area on the surface in water in both the stacked film setup and the film-cell setup. The spectrum of the source was also calculated. An experimental PDD was generated by irradiating stacked films and was compared to the Monte Carlo simulations. Results: The measured and computed PDDs agreed within 2% within 2.6 mm of depth. The dose rates were 28.30, 26.48, 21.23, and 22.76 cGy/s on the surface in water, in the film active layer, in the cell monolayer, and in the growth media, respectively, compared to the manufacturer's nominal value of 27.1 cGy/s. Conclusions: A Monte Carlo-validated PDD curve of the source was generated in a stacked film setup using EBT-XD GafChromic ® film. A custom film-cell irradiation setup was characterized for future radiobiology experiments.