Diagnosis of Prostate Cancer via Transanal Endoscopic Ultrasound–Guided Fine-Needle Biopsy in a Patient With Bone Lesions and a Rapid Clinical Response to Enzalutamide Plus Degarelix

In patients presenting with bone lesions suggestive of metastatic prostate cancer, rapid acquisition of diagnostic tissue is essential. Although transrectal ultrasound-guided (TRUS) or transperineal biopsy is standard, these approaches may be impractical in certain clinical settings. Markedly elevated serum prostate-specific antigen (PSA) levels can strongly suggest a prostatic origin, but histological confirmation remains necessary to initiate androgen receptor signaling inhibitor (ARSI)-based therapy. A 67-year-old man presented with diffuse arthralgia, impaired ambulation, and widespread osteosclerotic bone lesions on CT. Serum PSA was markedly elevated (5,230 ng/mL). Pelvic MRI could not be performed because of severe claustrophobia, and TRUS/transperineal biopsy was not feasible due to the absence of urology services at our institution. Bone biopsy was considered but judged less feasible than an endoscopic approach. Transanal endoscopic ultrasound (EUS) revealed a hypoechoic lesion in the right prostatic lobe, and EUS-guided fine-needle biopsy (EUS-FNB) using a 19-gauge needle yielded core tissue demonstrating PSA-positive adenocarcinoma. Despite an initial performance status (PS) of 4, driven primarily by pain, enzalutamide plus degarelix was initiated. The patient experienced rapid improvement in mobility and pain, with PSA falling to 11.3 ng/mL at two months and stable bone lesions on imaging. He returned to work in three months. This case illustrates that transanal EUS-FNB can provide a minimally invasive, feasible, and timely diagnostic option for patients in whom conventional prostate biopsy routes are impractical. Early histologic confirmation enabled prompt initiation of ARSI-based therapy, resulting in rapid symptomatic and biochemical improvement. EUS-FNB may be considered in selected cases of suspected prostate cancer when alternative biopsy approaches are not feasible.