Objective: To determine the in vivo efficacy of hyperfractionated external beam radiation therapy (EBRT) in comparison with standard daily EBRT in a murine model of heritable retinoblastoma. Methods: Two hundred twenty eyes from 6-week-old simian virus-40 large T-antigen-positive mice were treated with a total dose of EBRT ranging from 10-76 Gy (1000 to 7600 rad). One hundred ten eyes underwent EBRT administered in 2.0-Gy (200-rad) fractions once per day. Forty-two eyes received hyperfractionated EBRT administered in 1.2-Gy (120-rad) fractions twice per day, while 48 eyes received EBRT twice daily in fractions of 5.0 Gy (500 rad). Twenty eyes served as untreated controls. All eyes were obtained for histopathologic examination and graded positive if any tumor was present. Results: A dose-dependent inhibition of ocular tumor was observed for EBRT in these transgenic retinoblastoma mice. The tumor control dose for 50% of eyes (TCD 50) treated with 2.0 Gy fractions of EBRT was 45 Gy (4500 rad) when treatments were administered once daily. A significant increase in tumor control was observed when treatments were administered twice per day at fractions of 1.2 Gy, resulting in a TCD 50 of 33 Gy (3300 rad) (P=.003). A further increase in tumor control was observed when twice-daily EBRT was administered in 5.0 Gy fractions resulting in a TCD 50 of 28 Gy (2800 rad). Conclusions: Hyperfractionated EBRT safely and effectively controls intraocular retinoblastoma in this transgenic animal model. Use of hyperfractionation allows for a reduction in total radiation delivered dose, while shortening the total treatment time. Clinical Relevance: This treatment approach may be applicable in the management of pediatric retinoblastoma by maintaining excellent tumor control, while reducing treatment-associated complications.
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