SPECT/CT carves a niche in emerging radiotherapy

They are nearly microscopic, yet ganged together they can pack a big punch. They are microbeads embedded with radioisotopes and infused into body parts containing cancer tumors. The idea is to surround and destroy the tumor. The problem is making sure the microbeads stay put. SPECT/CT can provide the information to make it work.

This novel form of radiotherapy, called radiomicrosphere therapy, offers new hope for patients with inoperable hepatic metastases or unresectable primary liver tumors (hepatocellular carcinoma or cholangiocarcinoma). The tiny beads packed with yttrium-emitting radioisotope might also buy some additional time for patients waiting for liver transplants by preventing tumor.

These benefits come only if the cancer-killing microbeads can be kept around the tumor. Unlike monoclonal antibody therapy, these beads are not naturally drawn to nor do they meld with the cancer. Instead they are infused in a nearby region and then carried via the circulatory system to the tumor. Normal flow would take them to the target and beyond, but the therapy calls for interventional radiologists to block their exit.

They do it by embolizing vasculature downstream from the tumor, usually with microcoils. SPECT/CT ensures this embolization has done the job before microbeads carrying the cancer-killing isotopes are infused.

In essence, a dry run is performed with microbeads carrying technetium instead of the yttrium isotope. SPECT CT tracks them, documenting flow to and, hopefully, not away from the tumor. Of particular concern when treating patients with liver cancer is to spot extrahepatic shunts that would take the microbeads to the lung and gastrointestinal tract.

In a study reported last week at the molecular imaging meeting of the SNM, SPECT/CT detected such possible extrahepatic shunting in 23 of 99 patients at the Cleveland Clinic in Cleveland, Ohio. By comparison, planar nuclear imaging spotted shunting in only nine of these 23 patients.

In one patient, the imaging hybrid found that the liver cancer had grown into and was obstructing the hepatic vein through which the microbeads were to be delivered.

Seeing such obstructions allows the treatment to be modified, according to the Cleveland Clinic researchers, just as spotting extrahepatic shunting will identify patients who need further embolization before the actual radiotherapy begins.