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.