1. A method for planning and implementing a selective internal radiation therapy (SIRT), comprising the steps of: providing a processor with a large volume dataset of a patient who is to undergo SIRT, said large volume dataset encompassing the entire liver of the patient, said liver having a tumor therein fed with blood by feeder vessels emanating from a main branch; in the processor, automatically segmenting the liver from surrounding tissue in said large volume dataset by executing a computerize segmentation algorithm, and automatically calculating the volume of the liver from the segmented liver; injecting contrast agent into said main branch for said feeder vessels and acquiring a contrast-enhanced tomographic volume dataset from the patient comprising a contrast-enhanced tumor and contrast-enhanced feeder vessels that supply blood to said contrast-enhanced tumor; in said processor, automatically segmenting said contrast-enhanced tumor and said contrast-enhanced feeder vessels from surrounding tissue in said contrast-enhanced volume dataset by executing a computerize segmentation algorithm; in said processor, automatically calculating the volume of the tumor from the segmented contrast-enhanced tumor; determining a lung shunt percentage for said patient; determining the body surface area (BSA) of the patient; in said processor, automatically calculating an amount of therapeutic agent for use in said SIRT for treating said tumor, from the BSA, the lung shunt percentage, the liver volume, and the tumor volume; obtaining a live fluoroscopic image of the liver of the patient during SIRT and overlaying the segmented contrast-enhanced liver and the segmented contrast-enhanced feeder vessels on said live fluoroscopic image, to obtain an overlay image; using said overlay image, positioning a catheter with respect to at least one of said feeder vessels; and via said catheter, administering the calculated amount of said therapeutic agent during SIRT.