NEW YORK — Use of positron emission tomography/computed tomography to assess lymph node involvement in Merkel cell carcinoma gives the best sensitivity and equal specificity, compared with more traditional imaging modalities.
"CT or MRI have been the imaging modalities of choice when staging patients with Merkel cell carcinoma," Dr. Michael B. Colgan said at the annual meeting of the American College of Mohs Surgery "We also know these techniques have shortcomings."
However, "We know from our colleagues in oncology … that PET/CT has become their go-to for staging and oftentimes restaging of disease."
Dr. Colgan, of the dermatology department at the Mayo Clinic in Rochester, Minn., looked at patients from three centers diagnosed with primary Merkel cell carcinoma between 1986 and 2008.
All patients had, as part of their cancer staging, a documented imaging study of their regional lymph node basin. Overall, 75 patients underwent a CT scan for this purpose; 34 had a PET/CT scan; and 10 patients had an MRI.
According to Dr. Colgan, the MRI results, when compared to the preferred method of histopathologic confirmation, were "dismal." The sensitivity in these cases was 0%; the specificity was 86%, the positive predictive value 0%, and the negative predictive value 67% in "detecting nodal basin involvement."
The CT scan results were slightly better - 54% sensitivity, said Dr. Colgan. "But if my radiologist tells me the CT is negative, 3 out of 10 times it's probably not the case," he said, for a negative predictive value of 70%. The specificity was 95%, and the positive predictive value was 90%.
The PET/CT results, on the other hand, offered a sensitivity of 77% - significantly higher than the other two modalities - a specificity of 95%, a positive predictive value of 91%, and a negative predictive value of 87% in detecting regional lymph node involvement.
"PET/CT can affect the stage and ultimately the clinical plan for these patients," he said.
He added that in three cases of PET/CT false negatives, there was either single node involvement or a small micrometastasis that went undetected.
"The big question when we're dealing with Merkel cell carcinoma is, if PET/CT still misses micromets [microscopic metastases], how are these scans best utilized?" asked Dr. Colgan. "That's a debate we need to have."
He pointed to some limitation of the study, including its retrospective design and the possibility of sampling bias, given that "all these patients went to get scanned."
A prospective study, of the "same patient, same node, at the same point in time" is needed.
Nevertheless, he said, especially compared with MRI, "If you were going to look at these two imaging modalities side by side, my choice is the PET/CT."
In an interview following the meeting, Dr. Colgan was asked whether PET/CT scans ought to be the standard for detecting lymph node involvement in other cancers, too.
"I would hesitate to extrapolate these results to other tumors, as each tumor has a unique metabolic signature and route of metastasis," he said.
"It is a remarkable imaging technique, but the studies need to be done."
Dr. Colgan stated that he had no conflicts of interest to disclose in relation to this study.