18F-FDG PET for exploration of para-neoplastic syndromes with anti-h.u. antibodies and small cell lung cancer: clinical case and literature review

Introduction. - Small cell lung cancer (S.C.L.C.) is a neuroendocrine tumour representing 20% of bronchopulmonary cancers. Its metastatic potential is high, so 2/3 of diagnoses are made at disseminated stage. Anti H.u. antibodies are part of the anti neuronal antibodies, often associated to S.C.L.C.. However, 16% of cancers have positive anti H.u. with no para neoplastic syndrome (P.N.S.). (Graus, Brain, 2001). P.N.S. associated with anti H.u. are encephalomyelitis, sensitive neuropathy, chronic pseudo intestinal obstruction, cerebellar degeneration and limbic encephalitis. Case report. - A 75-year-old patient was hospitalized for exploration of an atypical tri-facial neuralgia. Anti H.u. antibodies were found and P.N.S. of a S.C.L.C. was therefore suspected. The biopsy of a thoracic parietal adenopathy confirmed the diagnostic of S.C.L.C. metastasis. Conventional imaging and 18F-FDG (Fluorodeoxyglucose) PET/CT (Positron Emission Tomography/Computed Tomography)) did not localize the primary tumour, despite advanced dissemination stage, only showing lymphatic secondary locations at the left axillary and under the diaphragm areas. Literature review. - Younes-Mhenni (Brain, 2004, 20 patients); and Linke (Neurology, 2004, 13 patients) studied patients presenting with anti H.u. antibodies (13 and eight respectively) and other anti neuronal antibodies (Y.o., Ri, C.V.2, Tr) associated with different cancers. PET sensitivity was respectively 83.3 and 90% with a specificity of 25 and 67%. In both series, specificity of anti H.u. antibodies for S.C.L.C. was estimated at 53% and 62.5%. Size is a limiting factor for S.C.L.C. detection and Watanabe (Nihon Kokyuki Gakkai Zasshi, 2001) showed that sensitivity for detection of less than 1 cm tumour was 0% in five patients. Moreover, P.N.S. can precess S.C.L.C. detection for many years and PET/CT has to be repeated. (Gaillard, Revue neurologique, 2005).In two other studies (Schumacher, EJNM, 2001, 30 patients; and Niho, Lung Cancer, 2007, 63 patients), PET and T.D.M. have been compared, showing the superiority of PET at initial staging for lymphatic node and distant metastases detection, leading to therapeutic modifications in 8% of cases. In a in vitro study, Fischer and al. (EJNMMI, 2006) showed that the number of cancerous cells to obtain a positive signal with PET was different according to the anatomo-pathological type of tumour, with a detection limit increased by a factor of 10 for S.C.L.C. detection compared to glioblastoma. Most of time S.C.L.C. are located at lung level but many extra-pulmonary locations are known. Immunohistochemical markers can help diagnosis, for instance association between T.T.F.1 and pulmonary location, without being completely specific (Ghosh, Current problems in surgery, 2005). Conclusion. - Despite some false negative, PET is a precious examination for diagnosis and staging of documented P.N.S., in particular suspicion of S.C.L.C. with anti H.u. antibodies, leading to repeat the examinations in case of negativity. (authors)