an apparatus and method for tailoring the radiation intensity about a radiation source , enabling affected zones of a cancerous tissue mass to receive full radiation dosage while limiting the exposure of unaffected regions where radiation exposure is not desired . the assembly includes a radiation attenuator partially covering the outer periphery of a delivery portion of a high dosage radiation catheter , thereby altering the radiation profile that exits the assembly . a method for treating a cancer affected tissue mass includes inserting a plurality of said catheters into a tissue mass about the center of mass and orienting said catheter so that said the maximum radiation intensity is facing substantially toward the center of mass . the radiation source is allowed to dwell in the catheter for a prescribed period of time to deliver the desired dosage of radiation . the radiation source is then removed from the catheter .