Explain the term "selective internal radiation therapy" (SIRT).

Selective internal radiation therapy (SIRT) is a targeted treatment option specifically used to treat liver tumors. It involves the use of radiolabeled microspheres, which are tiny beads that contain a radioactive isotope, usually Yttrium-90. These microspheres are delivered directly into the blood vessels supplying the tumor, allowing for localized radiation treatment while sparing healthy surrounding tissue.

The key aspect of SIRT is its specificity; the microspheres are designed to preferentially lodge in the tumor's blood supply. This targeted approach maximizes the radiation dose to the tumor while minimizing the exposure to adjacent normal liver tissues, often reducing adverse effects associated with traditional external beam radiotherapy.

In the context of the other options, external radiation refers to radiation delivered from outside the body, systemic chemotherapy focuses on using chemical substances systemically, and traditional radiotherapy does not use the specific targeted approach characteristic of SIRT. Thus, the distinct method of employing radiolabeled microspheres within SIRT uniquely positions it as an effective choice for certain liver cancer treatments.