Penile metastasis from prostatic neuroendocrine carcinoma

A 72-year-old male patient with benign prostatic hyperplasia was brought to the emergency department with frank haematuria. Laboratory workup revealed severe anaemia, negative inflammatory biomarkers and a prostate-specific antigen (PSA) of 0.78 ng/mL. A cystoscopy was performed, and a large tumour invading the bladder outlet was found and incompletely resected.

Magnetic resonance imaging (MRI) showed a large and locally aggressive pelvic tumour arising from the prostate, with invasion of the bladder and both ureterovesical junctions. The lesion had intermediate signal intensity on T2-weighted images and restricted diffusion, with no areas suggestive of necrosis or haemorrhage (Figures 1a, 1b, 1c and 1d). Suspicious right internal iliac lymph nodes and multiple osteolytic lesions were seen, with the largest bone lesion involving the right inferior pubic ramus.

Additionally, several lesions with intermediate T2 signal, restricted diffusion and early contrast enhancement were seen involving the corpus spongiosum and both corpora cavernosa of the penis (Figures 2a, 2b, 2c, 2d and 2e). In the setting of disseminated prostatic disease, a presumptive diagnosis of penile metastasis was established.

Staging computed tomography (CT) revealed further metastatic deposits in the lungs and liver (Figures 3a, 3b and 3c), and bone scintigraphy confirmed metastatic bone involvement of the pelvic girdle.

Background

Prostatic neuroendocrine carcinoma is a rare and aggressive histologic subtype of prostate cancer, which normally occurs as a mechanism of treatment resistance during therapy for conventional prostatic adenocarcinoma [1]. Nevertheless, it can also develop de novo, which was likely the case with our patient as he had not undergone any prior treatment. These tumours have a high metastatic potential with a poor prognosis and usually express neuroendocrine markers instead of PSA [1], findings which are also present in our case.

Despite its rich vascularisation, penile metastases are rare, and most cases originate from primary malignancies of the pelvic organs in individuals between 60 and 80 years old [2]. Metastatic deposits to the penis generally occur in the setting of disseminated disease, and as such, the prognosis is also poor [3]. Patients often present with palpable nodules, predominantly affecting the corpora cavernosa in the penile shaft [2–4]. Malignant priapism is also a possible manifestation of the disease [2].

Imaging Perspective

Similarly to other prostatic malignancies, multiparametric MRI is helpful in the initial evaluation and local staging of neuroendocrine tumours, and the Prostate Imaging Reporting & Data System (PI-RADS®) is commonly employed [1,5]. MRI also has a role in guiding biopsies through the fusion of MRI and transrectal ultrasound images (MRI-TRUS fusion biopsy) [1]. Unlike usual prostatic adenocarcinoma, prostatic neuroendocrine tumours, and in particular prostatic carcinoid, have been reported to exhibit distinct imaging characteristics, such as larger lesion size and mild hyperintensity on T2-weighted imaging [1,6].

Owing to its rarity, the imaging findings of penile metastasis are not extensively described in the literature. Metastatic lesions involving the corpora cavernosa and corpus spongiosum are frequently multiple and small, in distinction to primary penile cancers, which are more commonly solitary, ill-defined and infiltrative [7]. Penile metastases have low signal intensity on both T1- and T2-weighted sequences compared to the normal corporal tissue, demonstrating restricted diffusion and early enhancement on post-contrast imaging [7,8].

Outcome

Treatment in the setting of penile metastasis is usually palliative, aiming to relieve pain and improve patients’ quality of life. Local, systemic or a combination of these therapies can be used [2,9].

Teaching Point

Radiologists should keep in mind that penile metastatic disease is a possibility in the setting of primary prostatic malignancy.

Written informed patient consent for publication has been obtained.