Hypereosinophilia is defined as an eosinophil count greater than 1.500 cells/mm. It is usually associated with organ dysfunction due to visceral eosinophilic infiltration or tissue damage mediated by eosinophilopoietic cytokines, like IL-5. Determining its etiology represents a diagnostic challenge, as numerous conditions are associated with its occurrence, especially parasitic diseases, allergic disorders, medications and neoplasms. Paraneoplastic eosinophilia usually develops in the context of tumors of the lower gastrointestinal tract, lung, Hodgkin's lymphoma, peripheral T-cell lymphomas (PTCL), and its pathophysiology is associated with interleukin-5 (IL-5) secretion, an eosinophil proliferation-inducing cytokine.

To describe a clinical case of paraneoplastic hypereosinophiliadue to lung cancer, a rare laboratory manifestation, in a case with an advanced stage of the disease and catastrophic evolution.

A 68-year-old woman, with a history of obesity and SAH, presents to the health service of the HC/FMUSP due to adynamia, dyspnea on exertion and perception of multiple violaceous cutaneous nodules on the scalp, thorax, abdomen and extremities, 45 days long. On physical examination, she had cutaneous-mucosal pallor, poor general condition, hypoxemia, and disseminated cutaneous nodules of up to 5 cm. Blood count revealed: Hb 8.9 g/d, Ht 26%, MCV 89 fl, 75.980 leukocytes/mm3 (31.920 neutrophils with left shift to promyelocytes/ 40.270 eosinophils/ 0 basophils/ 2.280 lymphocytes/ 1.520 monocytes) and 107.000 platelets/mm3; LDH = 592 U/L (NSV up to 214 U/L); IgE = 496 IU/mL (NSV up to 100 IU/mL). Myelogram showed hypercellular bone marrow with massive infiltration by elements of non-hematopoietic origin, as well as prominent medullary eosinophilia. Computed tomography (CT) showed an irregular lung mass, 5 cm in diameter, in the right upper lobe, suggestive of a primary neoplastic site, as well as secondary implants in the liver parenchyma, adrenal glands and subcutaneous cellular tissue/skin. Biopsy of subcutaneous nodules showed undifferentiated malignant neoplasm of epithelioid pattern, with immunohistochemistry: AE1/AE3 positive, 34BE12 positive, MELAN-1 negative, HMB45 negative, S100 negative, p63 negative, characterizing squamous cell carcinoma. During hospitalization, the patient rapidly evolved with tumor lisis syndrome, acute renal failure and fulminant liver failure, and died after a short period of hospitalization.

We report an atypical case of pulmonary squamous cell carcinoma, marked by paraneoplastic eosinophilia, bone marrow infiltration, cutaneous metastasis, spontaneous tumor lysissyndrome and acute liver failure. Paraneoplastic hyperosinophilia is a marker associated withpoor prognosis in solid tumors, usually reflecting an elevated tumor mass, with multi-metastatic disease, as presented in our case. Although it is a rare condition, paraneoplastic eosinophilia mustfall within the differential scope of causes of sustained elevation of eosinophils. Rapid recognition and prompt treatment of the underlying neoplasm may provide a more favorable evolution.