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PET Sensitivity and Lymph Nodes

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Last Modified: December 18, 2005

Dear OncoLink "Ask The Experts,"

I am from Australia and I was hoping you may be able to point me in the direction of any relevant research articles (or give your advice). My husband had a PET scan before his surgery for squamous cell carcinoma of his lung. The PET scan did not identify any lymph node involvement, but when he had the surgery, 4 lymph nodes were involved. What does this mean? PET scan was not sensitive enough? The cancer had just arrived in the lymph nodes? Or any other reason?

Peeyush Bhargava, MD, Chief Fellow in the Department of Nuclear Medicine at the University of Pennsylvania, responds:

In patients with lung cancer , PET may miss nodal involvement, especially if the lymph nodes are less than a centimeter in size, or if the primary tumor itself does not show significant FDG ( Fluorodeoxyglucose) activity (such as carcinoids and alveolar cell lung cancers). FDG activity refers to how the tumor takes up the injection that is given before the PET scan; this injection is what allows the cancer to be seen on the scan.

That is the reason we now do a study called PET-CT (PET scanner and CT scanner combined in one machine) instead of a PET alone, in order to improve the sensitivity and specificity of PET. The CT scan allows the radiologist to better visualize the organs and nodes that may not take up FDG well, thus the two studies can complement each other.