Validation of absolute dose calculation of a single 125I-seed for ophthalmic brachytherapy within distance range 1-15...

Intraocular tumors can be treated with brachytherapy. Dosimetry of ophthalmic applicators is challenging due to short distance ranges and steep dose gradients. Measurements are sensitive to the structure of the dosimeter, and correction methods specific to 125I-source are needed for validation measurements of dose distribution.

We measured dose profiles of I25.S16 seed using diode detector in a water phantom at 2 and 5 mm above surface of acrylic (PMMA) and gold backing. Volume averaging effect was corrected by the ratio of dose calculated with AAPM TG-43 formalism at the center point and the dose average within the detector element area. The angular sensitivity was measured at a constant radius around the seed. The effect of gold backing was evaluated from measurements with and without gold slab behind the seed. Measurements were normalized to calculated value at the distance of 10 mm from the seed using the air-kerma rate measured with a calibrated well chamber.

1D/3D gamma comparison pass rates 97% with criteria 0.3 mm/3% and 60% with 0.5 mm/5% were reached for a seed attached to PMMA and gold, respectively, in the range ± 10 mm.

TG-43 calculated single seed distributions in PMMA were equivalent to measurements corrected for angular sensitivity and detector area. Gold-to-water ratio corrected calculations were not similarly comparable, and simple one-dimensional correction method is not accurate enough laterally. Diode dosimetry with these correction methods can be used for quality assurance and validation of clinical dose calculation of ophthalmic applicators.