Intraluminal brachytherapy with a transcatheter Iridium-192 (Ir-192) wire, as a method to deliver a boost to patients with tumors of the extrahepatic bile ducts, has been well described. A major limitation of current imaging techniques is the inability to accurately define the proximal, distal, and most importantly the lateral extent of the tumor. We have found endoluminal ultrasound, which to this date has been used primarily to measure arterial wall layers, can successfully determine tumor volume in the bile ducts. The small diameter of these ultrasound probes allows easy insertion into a biliary duct drainage tube. Given the uncertainties in the determination of tumor size in the bile ducts, ultrasound is an ideal method by which to obtain the measurements. We present a case of recurrent islet cell carcinoma treated with external beam radiation therapy and an Ir-192 intraluminal brachytherapy boost in which the ultrasound probe was used to determine the tumor volume and response to therapy.