Background:
Lung carcinoma is the leading cause of cancer-related mortality, with 50% of cases presenting metastases at the time of diagnosis. Metastatic peritoneal localisation of lung carcinoma is an exceptionally rare situation, accounting for 3% of lung cancers. The rarity is heightened when dealing with small cell lung carcinoma cases, making reported instances even more exceptional.
Accurate diagnosis and proper disease staging are crucial in determining whether the disease is metastatic or non-metastatic, influencing subsequent management.

A 58-year-old patient with a history of non-metastatic small cell lung carcinoma diagnosed in October 2021 (2 years ago) underwent radiotherapy and achieved remission. In February 2023, the patient experienced abdominal pain and hematemesis with melena. Esophagogastroduodenoscopy revealed an ulcerative, polypoid gastric process. Biopsy confirmed moderately differentiated gastric adenocarcinoma. A whole-body CT scan identified new peritoneal nodules.
Laparoscopy with biopsy was performed, and specimens were sent for histopathological examination.

Results:
Histologically, the specimen showed a diffuse tumor proliferation consisting of generally small, highly mitotic cells, displaying irregular, hyperchromatic nuclei and scant, basophilic cytoplasm. Immunohistochemical analysis revealed the following:
-Positive staining of tumor cells for TTF1 and CK7 and negative staining for CK20, indicating a pulmonary origin and ruling out gastric origin.
-Tumor cells expressed chromogranin A and synaptophysin, with a high proliferative index (Ki67) estimated at 60%.
Considering all data, the diagnosis retained is a secondary localisation of undifferentiated carcinoma, with immunohistochemical profiling primarily aligning with small cell lung carcinoma.

Discussion and Conclusions:
Small cell carcinoma, a therapeutic emergency, poses a significant health problem due to its high incidence and mortality. Diagnosis relies on histology supported by immunohistochemistry. Patients often present acute symptoms due to the rapid local growth of intrathoracic tumors, especially at the pulmonary hilum, or distant extrapulmonary dissemination in 40% of cases involving lymph nodes, liver, adrenal glands, bones, or brain.
Peritoneal metastasis is exceptionally rare, with the most common histological type being pulmonary adenocarcinoma (80% of cases), and the least common scenario is the one reported in this abstract: small-cell carcinoma.
Thorough morphological and immunohistochemical studies, guided by the suspected histological type and patient history, are mandatory for a conclusive diagnosis.