PET/CT uncovers cancer underlying neurologic disorders

Cancer patients sometimes develop neurologic complications directly caused by their cancers. Sometimes, however, these disorders are due not to the growth of a cancer tumor but to cancer-fighting antibodies that mistakenly attack cells in the nervous system. PET/CT may help physicians tell the difference.

Andrew McKeon, assistant professor of neurology, and colleagues at the Mayo Clinic in Rochester, MN, will report in next month’s Archives of Neurology that whole-body scans with PET/CT appear to be more accurate than some common tests in detecting cancer in patients with related neurologic complications.

Typically small and restricted to single sites, cancers underlying paraneoplastic neurologic disorders are seldom found on routine evaluations. Consequently, cancers underlying paraneoplastic neurologic disorders often are not uncovered until autopsy.

Reflecting this shortcoming, routine evaluations of 56 patients suspected by Mayo physicians of having these disorders produced no evidence of tumors. PET/CT scans, however, revealed abnormalities suggestive of cancer in 22 of these 56. Ten of the patients flagged with PET/CT had cancer diagnoses associated with their neurologic disorders confirmed by biopsy or another method.

Of the proven cancers, two were in the thyroid, one in the tonsil, three in the lungs, one in the colon, and three in the lymph nodes with unknown primary cancer sites. Nine of the 10 were found at an early stage. Their detection facilitated early treatment, according to the Mayo team. Cancer remission was reported in seven patients. Five showed improvement in neurologic symptoms after a median of 11 months of follow-up.

Patients with symptoms of paraneoplastic neurologic disorders may routinely be evaluated by physical examination; CT of the chest, abdomen, and pelvis; mammography; and testicular ultrasonography or prostate-specific antigen. Before their PET/CTs, patients participating in the Mayo study underwent a median of three such tests. The most common was CT of the chest, abdomen, and pelvis.

The Mayo team was not surprised by CT’s failure, as four of the 10 cancers detected using PET/CT were outside the chest, abdomen, and pelvis. (They resided in the thyroid, cervical lymph node, or tonsil.) The other six, according to the Mayo team, were too small to be detected by CT.

When paraneoplastic neurologic disorder is suspected, therefore, PET/CT should be done first, rather than after CT, to reduce radiation exposure, the researchers advise.

While the Mayo study indicated the ability of PET/CT to uncover cancers that might otherwise have been missed, scans with this modality are not sufficient to exclude cancer. Some types, including cancers of the bladder and kidney, are difficult to detect with PET/CT, the investigators noted.