The use of a partial transmission shield to reduce the optic chiasm doses during radiation therapy treatment of brain tumors.

Historically, brain tumors have been treated with lateral opposed beams for 40-45 Gy followed by more conformal reduced fields. Advances in treatment planning computers have led to the implementation of conformal non-axial techniques, allowing for escalation of dose. In patients where total doses exceed 50 Gy, adjacent critical structures can be protected with a partially shielded transmission block over the optic nerves and chiasm. By eliminating the conedown portion of the treatment a more cost and time effective treatment is achieved. Partially shielded blocks can be designed by using cerrobend or multileaf collimation. They can be included in the treatment plan and verified by an irregular field calculation and/ or thermoluminescent dosimeters.