前立腺肥大症のまれな病態：前立腺画像検査で正常な巨大孤立性骨盤腫瘍としての純粋な間質性肥大症

Benign prostatic hyperplasia (BPH) is the most common benign lesion of the prostate in aging men. However, diagnosing BPH can be challenging when it presents as an exophytic pelvic mass, particularly if radiologic continuity with the prostate and prostatic glandular components are not clearly seen. Here, we report a 57-year-old male patient with a 2-year history of obstructive voiding dysfunction. Radiologic investigations, including both computed tomography and magnetic resonance imaging, revealed a large (up to 11.9 cm) solid pelvic mass located superior to the prostate with marked mass effect. No direct continuity with the prostate or multilocular/cystic structure was identified on imaging. Pathologic evaluation of the biopsy specimen showed a hypocellular lesion consisting of bland spindle cell proliferation and small blood vessels within a loose fibrous stroma. The subsequent resection specimen showed similar histologic morphology, and prostatic glands were not identified. Immunohistochemical staining demonstrated that the tumor cells were positive for CD34, desmin, androgen receptor, and progesterone receptor. Ki67 staining indicated a very low proliferative index (<1%). The constellation of histomorphological and immunohistochemical profiles was consistent with BPH with pure stromal proliferation. To the best of our knowledge, this represents the first documented instance of BPH with pure stromal hyperplasia manifesting as a distinct pelvic mass without radiologic continuity with the prostate. This case report highlights the need for clinicians and pathologists to consider BPH in the potential diagnosis of large pelvic lesions, especially in the condition that standard prostate imaging appears normal and prostatic glands are absent.