Brachytherapy involves the delivery of radiation therapy locally via specially designed catheters placed in direct apposition to the treated tissue. The most common radioisotope used in this application is which of the following?

a. 125I

Brachytherapy is a technique in which the source of the radiation is adjacent to or within the targeted tissue. In this method, isotopes such as 198Au and 125I are placed in special catheters, the positioning of which is based on precise geometric considerations. The placement of the radiation source is critical because, in principle, a high dose of energy is delivered to the immediate vicinity of the target and then decreases rapidly with distance. The advantage of this technique is the ability to deliver high, concentrated doses to the target while limiting damage to nearby normal tissue. Clinical applicability is somewhat limited, but there is considerable experience with brachytherapy in the treatment of some oral tumors, gynecologic malignancies, and soft tissue tumors of the extremities.