A 70-year-old man was referred to our institution with chronic obstructive urinary symptoms and constipation for 2 years before admission. His medical history was unremarkable. Outside laboratory tests revealed an elevated serum prostate-specific antigen level (21.7 ng/mL [normal range, 0.00–4.00 ng/mL]), and urinalysis results were negative. He was admitted for evaluation of possible prostate cancer. The patient reported no specific symptoms of infection and denied fever, dysuria, hematuria, and abdominal pain. He had no family history of prostate cancer. On physical examination, he was afebrile, and the digital rectal examination was not painful. These findings were evidence of an enlarged prostate and a hard multilobulated mass, which was palpable bilaterally. The urologic team performed MRI of the prostate (Figs 1–4)Multiparametric MRI was performed with a 3-T imager without an endorectal coil and in compliance with Prostate Imaging Reporting and Data System guidelines. Axial, sagittal, and coronal T2-weighted MRI scans of the prostate (Fig 1a–1c) showed a large predominant exophytic and multicystic pelvic mass posterosuperior to the prostate and without a plan for clear separation from this gland, which led us to believe the mass originated from the prostate. Parasagittal T2-weighted MRI scans (Fig 1d, 1e) showed the relationship between the mass and the seminal vesicles. While the left seminal vesicle appeared normal (Fig 1e), the right seminal vesicle appeared atrophic and slightly infiltrated (Fig 1d). The cystic lesion had multiple septa and mixed hypo- and hyperintense areas, manifesting some fluid-fluid levels (Fig 2a) and absence of marked diffusion restriction on scans obtained with diffusion-weighted and apparent diffusion coefficient (ADC) sequences (b value = 1000 sec/ mm2) (Fig 2b, 2c). The fluid content represented slight hyperintensity on T1-weighted images and high signal intensity on T2-weighted images (Fig 2a, 2d), suggesting a proteinaceous fluid that was compatible with mucin. The fluid-fluid levels suggested deposition of heavier substances and, as it presents high signal intensity on T1-weighted images (Fig 2d) and low signal intensity on T2-weighted images (Fig 2a), more probably represents hemorrhagic fluids. Axial T1-weighted contrast-enhanced MRI scan (Fig 2e) shows enhancement of internal septa. There was a solid portion of the lesion centered on the left peripheral zone of the middle third of the prostate that was not enhanced on the postcontrast T1-weighted image (Fig 3e), had a diffusion restriction (Fig 3b, 3c), and had low signal intensity on a T2-weighted image (Fig 3a), corresponding to a low concentration of the mucine site of the tumor. Cystic and enlarged lymph nodes in the external and common right iliac chains were seen, with an enhancement pattern and a signal intensity similar to those of the prostate cystic mass on axial T2-weighted (Fig 4a) and contrast-enhanced T1-weighted MRI scans (Fig 4b), that most likely represents mucinous content, suggesting secondary involvement. These findings were compatible with diagnosis of mucinous prostate adenocarcinoma. The patient also underwent chest CT and bone scintigraphy for oncologic staging, with no images that were suspicious for secondary involvement. What is your diagnosis?