Lung cancer is one of the most aggressive malignancies and remains the leading cause of cancer-related mortality worldwide. It can metastasize to almost any organ, most frequently the lymph nodes, brain, bones, liver, and adrenal glands. ¹

Metastatic spread to the breast from extramammary tumors is rare, accounting for <0.5% of breast malignancies. ² Breast metastasis from lung cancer is even less common in men, given the predominantly fibrous nature of the male breast and its relatively poor vascular supply. ^{2, 3}

Despite their rarity, distinguishing breast metastases from primary breast carcinomas is of critical clinical importance, as the two entities differ significantly in terms of treatment and prognosis. ^{1, 2}

Here, we report a case of breast metastasis from lung adenocarcinoma in a middle-aged male patient.

A 45-year-old man, a chronic smoker without significant past medical or family history, presented with spinal pain evolving over two months.

On examination, he had right-sided sciatica and multiple fixed left cervical lymph nodes, with fullness of the supraclavicular fossa.

A lumbar spine CT scan revealed multiple lytic bone lesions. A subsequent whole-body CT scan demonstrated numerous bilateral pulmonary micronodules (<4 mm), associated with bilateral hilar, left supraclavicular, and axillary lymphadenopathy. No abdominal or cerebral abnormalities were identified.

Core needle biopsy of a 4 cm palpable left axillary lymph node was performed. Histopathology (H&E) revealed metastatic poorly differentiated adenocarcinoma. Immunohistochemistry showed diffuse expression of thyroid transcription factor-1 (TTF-1), confirming a pulmonary origin. Molecular testing revealed no ALK rearrangement and no EGFR mutations (exons 18–21).

The patient received cisplatin–vinorelbine chemotherapy and one session of palliative radiotherapy, which were well tolerated. After three months, repeat CT showed partial regression of supra- and subdiaphragmatic lymphadenopathy (approximately 67%).

Ten months after diagnosis, during routine follow-up, a hard, painless nodule was detected in the upper outer quadrant of the left breast. No skin or nipple changes were noted. Core needle biopsy revealed a malignant epithelial proliferation arranged in trabeculae and solid nests of atypical cells with amphophilic cytoplasm, within a fibro-inflammatory stroma. No in situ carcinoma was identified ( Figure 1: A&B).

The differential diagnostic was either two metachronous lung and breast primaries tumors or a metastasis to the breast of the lung adenocarcinoma.

Immunohistochemistry again demonstrated diffuse positivity for TTF-1 ( Figure 1: C) and CK7. Testing for CK20 and breast markers (ER, PR, mammaglobin, GCDFP-15) could not be completed due to tissue exhaustion.

The morphological and immunohistochemical findings supported eventually the diagnosis of metastatic lung adenocarcinoma to the breast.

The patient was subsequently lost to follow-up, and further therapeutic management and survival data are unavailable.

Lung cancer is the most common cause of cancer-related deaths worldwide, with a 5-year survival rate of only ~16%, mainly due to late diagnosis and frequent extrathoracic spread. ⁴ The most common metastatic sites include the liver, adrenal glands, brain, bones, and lymph nodes. ¹

Breast metastases from lung cancer are exceedingly rare, with reported prevalence ranging from 0.4% to 1.3%. ² They are much more frequent in women, whereas cases in men remain exceptional. To date, only a handful of male cases have been reported in the literature. ^{2, 4– 9}

Breast metastases may either be the first clinical manifestation of lung cancer or, as in this case, arise later during the disease course. Ipsilateral axillary lymphadenopathy is often associated. ⁶

Clinically, metastatic breast lesions often present as well-circumscribed, firm, and painless nodules, typically in the upper outer quadrant. Unlike primary breast carcinoma, they rarely cause nipple retraction or skin changes. ^{6, 10}

The main diagnostic challenge lies in differentiating metastasis from a primary breast tumor. Radiological findings are nonspecific and may mimic either primary carcinoma or benign lesions. Histological clues suggesting metastasis include the absence of in situ carcinoma, elastosis, and calcifications, which are more typical of primary breast tumors. ¹

Immunohistochemistry is essential for accurate diagnosis. TTF-1 is expressed in up to 80% of pulmonary adenocarcinomas and is rarely seen in breast carcinomas. ¹ Additional markers such as ER, PR, mammaglobin, and GCDFP-15 support a mammary origin when positive. ¹¹ In our patient, strong TTF-1 expression was decisive for confirming pulmonary origin.

Prognosis of breast metastases from lung adenocarcinoma is generally poor, with survival rarely exceeding one year after diagnosis. ^{2, 3} Distinguishing these entities is crucial, as management strategies differ considerably from those of primary breast carcinoma.

We describe a rare case of breast metastasis from primary lung adenocarcinoma in a male patient. This entity should be considered in the differential diagnosis of breast lesions in patients with known lung cancer, even in men. Accurate histopathological and immunohistochemical assessment is essential, as treatment strategies and prognosis differ markedly from primary breast carcinoma.

Not applicable.

Written informed consent for publication of clinical details and images was obtained from the patient.