A 58 year old man, a known case of carcinoma lung undergoing systemic treatment, presented with progressive fatigue and generalized weakness. Laboratory evaluation revealed pancytopenia with haemoglobin of 7.8 g/dL, total leukocyte count of 2.1 × 10⁹/L, and platelet count of 48 × 10⁹/L. Peripheral blood smear showed normocytic normochromic anemia with leukopenia and thrombocytopenia without circulating atypical cells or blasts. In view of unexplained pancytopenia in a patient with a known malignancy, bone marrow examination was performed to evaluate for therapy related marrow suppression or marrow infiltration.

Bone marrow aspirate was markedly hemodiluted and yielded no diagnostic material. Trephine biopsy demonstrated diffuse infiltration of the marrow by malignant epithelial cells arranged predominantly in nests, cords, and occasional glandular structures replacing normal hematopoietic elements. The tumor cells were large, exhibiting moderate eosinophilic cytoplasm, pleomorphic vesicular nuclei, irregular nuclear membranes, and conspicuous nucleoli. Residual hematopoiesis was markedly suppressed, correlating with the patient’s pancytopenia.

Immunohistochemistry was performed to establish the lineage and primary site of the metastatic tumor. The neoplastic cells showed strong diffuse positivity for pan-cytokeratin (PanCK), CK7 and Napsin A. They were negative for CK20, TTF-1, P40, P63 and PAX8. Although TTF-1 negativity can occasionally pose a diagnostic challenge, loss of TTF-1 expression has been reported in a subset of treated or poorly differentiated pulmonary adenocarcinomas. Considering the patient’s known history of lung adenocarcinoma, the characteristic morphology, CK7 and Napsin A positivity, and the immunoprofile excluding other common primary sites, a diagnosis of bone marrow metastasis from pulmonary adenocarcinoma was established.

Contrast-enhanced computed tomography (CECT) of the chest revealed an irregular spiculated mass measuring approximately 4.8 × 3.9 cm in the right upper lobe with associated mediastinal lymphadenopathy. Multiple osteosclerotic lesions involving the vertebral bodies, pelvis, and ribs were identified, consistent with skeletal metastases. No radiological evidence of a renal, thyroid, or gastrointestinal primary lesion was noted.

Bone marrow metastasis from lung adenocarcinoma is an uncommon but clinically significant cause of pancytopenia and may mimic treatment-induced myelosuppression. This case highlights the indispensable role of bone marrow trephine biopsy when aspirate material is non-diagnostic due to hemodilution. Correlation with clinical history, imaging findings, and a focused immunohistochemical panel is essential for accurate diagnosis. Recognition of marrow involvement has important prognostic and therapeutic implications, as it signifies advanced-stage disease and guides subsequent management decisions.