Occult epidermal growth factor receptor-mutant lung adenocarcinoma complicated by prostatic metastasis: a case report.

Herein, we report a case of occult epidermal growth factor receptor (EGFR)-mutant lung adenocarcinoma complicated by prostatic metastasis. A 75-year-old male with >30 years of smoking history presented with lower back pain as the initial symptom. Respiratory symptoms, including cough and sputum production, were absent. PET-computed tomography revealed the presence of bone and prostatic metastases, without any lung abnormalities. Biopsies of the space-occupying bone and metastatic lesions suggested that the metastases originated from primary lung adenocarcinoma. Genetic testing indicated EGFR 21L858R(+). The patient had an abnormal serum carcinoembryonic antigen level but a normal prostate-specific antigen level. Following a multidisciplinary discussion, a diagnosis of occult primary lung adenocarcinoma complicated by bone and prostatic metastases (TxN0M1b, Stage IVB) was considered. Following targeted therapy with oral osimertinib, the patient achieved a partial response, with alleviation of pain symptoms alleviated and normalization of carcinoembryonic antigen levels. In the absence of tissue biopsy, such cases can often be misdiagnosed as prostate cancer complicated by multiple bone metastases. Hence, the present case highlights the importance of comprehensive diagnostic testing, including tissue biopsy, to accurately identify the underlying cause of metastatic disease.