Paraneoplastic Hypereosinophilia Revealing Disseminated Colorectal Signet Ring Cell Carcinoma

Background: Hypereosinophilia, defined as a peripheral blood eosinophil count greater than 1.5 × 109/L, can arise from allergic, infectious, autoimmune, or malignant conditions. In solid tumors, it is rare and most often linked to mucin-secreting carcinomas, while on extremely rare occasions, it accompanies signet ring cell carcinoma, a highly aggressive form of adenocarcinoma. Case Presentation: A 64-year-old woman presented with dyspnea and hypereosinophilia (2.9 × 109/L). She was admitted with suspected eosinophilic pneumonia, but extensive testing was inconclusive. After bone marrow biopsy, her condition deteriorated; histology revealed metastatic signet ring cell carcinoma. PET/CT showed skeletal metastases without apparent local recurrence, although colonoscopy could not be performed to definitively rule it out. Retrospective review uncovered a 2 mm rectal polyp with signet ring cell carcinoma (SRCC) removed two years earlier. Peripheral eosinophilia progressively increased from 0.16 × 109/L ten months earlier to a peak of 4.29 × 109/L one month prior to admission. She died four weeks after discharge. Conclusions: To the best of our knowledge, this case represents one of the smallest reported primary colorectal SRCC lesions (2 mm) presenting with disseminated disease and paraneoplastic hypereosinophilia as the first diagnostic clue. Monitoring peripheral blood eosinophil counts may provide additional insight into disease activity and prognosis in solid tumors.