Thermoradiotherapy of intraocular tumors in an animal model: concurrent vs. sequential brachytherapy and ferromagnetic hyperthermia

To compare concurrent vs. sequential ferromagnetic thermoradiotherapy in vivo.

Greene melanomas were implanted subretinally in rabbits and observed until they were 3-5 mm in diameter. Episcleral plaques were assembled with 125I seeds for radiation therapy, or with ferromagnetic (FM) thermoseeds and nonradioactive I seeds for hyperthermia. Rabbits were implanted by centering a plaque over the intraocular melanoma. After a given dose of radiation had been delivered, the plaque was removed and a nonradioactive plaque containing FM thermoseeds was inserted into the same extrascleral space. One hour later, hyperthermia (46-47 degrees C at the plaque-scleral interface) was initiated and continued for a period of 1 h by placing the rabbits in a magnetic induction coil powered to 1200 W. Tumor size was determined at 1- to 2-week intervals by indirect ophthalmoscopy and by ultrasound.

Dose-response analysis of 27 treated eye melanomas showed 50% local tumor control at 43 Gy for 125I alone and 29.4 Gy for 125I followed by FM hyperthermia. The thermal enhancement ratio was 1.4.

Comparison with a previously published thermal enhancement ratio of 4.4 (for concurrent 125I and FM hyperthermia) leads us to conclude that thermal enhancement of 125I brachytherapy is more efficient in this tumor model system when hyperthermia is delivered during, rather than after, the irradiation process.