A 58-Year-Old Man with a Painful Gluteal Mass as the First Presentation of Metastatic Adenocarcinoma of the Lung.

BACKGROUND Lung cancer is the second most common cancer, with the highest mortality rate. It frequently metastasizes to the nervous system, bone, adrenal gland, and liver. Rarely, it metastasizes to soft tissues, including cutaneous, subcutaneous, and skeletal muscles, with an overall prevalence rate of 2.3%. In most cases, soft-tissue metastases develop after an initial diagnosis of the primary internal malignancy and late in the disease course. In exceedingly rare cases, they may coincide with or occur before primary cancer has been detected. In our case, the initial manifestation of primary lung adenocarcinoma was a gluteal mass. CASE REPORT We present the case of a 58-year-old man with no other medical comorbidities other than a 40-pack-year smoking history, who initially presented with a solitary painful right-buttock mass. Imaging revealed a solid right gluteal soft-tissue mass along with lumbar, lung, hepatic, bilateral renal, and adrenal lesions concerning for an underlying metastatic pathology. A gluteal mass biopsy showed poorly differentiated adenocarcinoma with immunohistochemistry (TTF-1+CK7+CD20-) favoring primary lung cancer. CONCLUSIONS Although it is an unusual and uncommon presenting entity of lung cancer, our case report accentuates how a simple solitary cutaneous palpable mass can be an alarming sign of a serious underlying occult malignancy. Moreover, our case report also highlights the diagnostic and prognostic value of immunohistochemistry characteristics of the tumor and how it can guide the clinician to identify the primary site, which, in this case, was adenocarcinoma of the lung.