PET-CT Tumor Detection in Patients with Paraneoplastic Syndromes

By Adília Hormigo, MD, PhD, Attending Neurologist, Memorial Sloan-Kettering Cancer Center, and Assistant Professor of Neurology, Weill Medical College of Cornell University. Dr. Hormigo reports no financial relationships relevant to this field of study.

Synopsis: Colocalized PET-CT is the most sensitive and accurate method for identifying an underlying neoplasm in paraneoplastic neurological syndromes.

Source: McKeon A, et al. Positron emission tomography - computed tomography in paraneoplastic neurologic disorders. Systematic analysis and review. ArchNeurol 2010;67: (doi:10.1001/archneurol.2009.336).

Paraneoplastic neurologic syndromes refer to symptoms or signs resulting from direct damage to the nervous system remote from the primary site of a neoplasm or any of its metastases. Although these syndromes are rare, they can be the initial presentation of an unknown cancer. Both body computed tomography (CT) and positron emission tomography (PET) scanning have been used for tumor detection.

McKeon and colleagues assess the rate of cancer detection in patients with paraneoplastic neurologic syndromes by using PET-CT. They retrospectively analyzed the data in 56 patients with presumed paraneoplastic neurologic syndromes, whose tumor had not been detected by the conventional work-up including a CT. In 22 patients (39%) PET-CT was suggestive of a cancer and the presence of a neuronal or cytoplasmic paraneoplastic autoantibody in the serum was significantly associated with PET-CT abnormality (P < 0.001). The correlation of anatomic localization of CT with metabolic abnormalities in PET led the authors to recommend this test when a paraneoplastic neurologic disorder was suspected.

Commentary

The tumor that causes a paraneoplastic neurologic disorder may be too small to be easily detected and often the neurologic disorder appears before the cancer is identified. The work-up for the neurologic syndrome depends on its localization and may include MRI, electrodiagnostic studies, CSF evaluation, and paraneoplastic antibody measurement in the serum. If no tumor is found, the patient should be monitored carefully and studies should be repeated at frequent intervals to look for the appearance of a cancer. Failure to find a cancer does not mean that the disorder is not paraneoplastic.

Previous studies suggested that PET scanning is probably the best screening method and the performance of both PET and CT increases sensitivity. While fluorodeoxyglucose (FDG) PET detects radiolabeled glucose and is a measurement of metabolism, a CT scan gives anatomic information. When you combine the two tests, PET-CT, in a scanner, you obtain the localization of the abnormal metabolic activity. The combination of the two techniques in one scanner increases the yield of tumor detection when compared to the two tests performed separately.