MRI of penile metastatization from malignant melanoma

Clinical History

An 83-year-old man complained of progressive penile swelling over the last month. His medical history recorded previous exeresis of a parasternal T3N1 cutaneous malignant melanoma with axillary nodal dissection one year earlier.
Clinically, multiple hard-consistency, mildly painful nodules were palpable along the penile shaft. He could urinate without significant difficulty.

Imaging Findings

Unenhanced MRI acquisition was performed using a phased-array coil positioned over the penis fixed in a neutral position. MRI images (Fig. 1) showed diffuse replacement of corpus spongiosum and corpora cavernosa along the entire penis shaft and root by the presence of several moderate-sized confluent masses with T1-hyperintense signal compared to the normal, low intensity of the involved anatomical structures, and solid, intermediate-to-high T2-signal intensity. The lesions, consistent with neoplastic deposits with melanin content, caused multifocal bulging of the low-intensity-signal albuginea corresponding to the clinical findings.

Discussion

Background: metastatic involvement of the penis is a distinctly uncommon occurrence in oncology practice, usually associated with advanced disease. The vast majority (75%) of penile metastases result from metachronous dissemination from genitourinary tumors such as urothelial bladder and prostatic cancers, whereas the remaining cases are related to colorectal or pulmonary primaries and leukemia, in descending order or frequency [1, 2]. To our best knowledge, only three cases of melanoma metastases to the penis have been described previously in the literature [3, 4].
Clinical perspective: penile metastasization usually presents as painless, solitary of multifocal palpable masses on the penile shaft, causing dysuria and sometimes priapism. Treatment is palliative and radiotherapy, cystostomy or partial penectomy may be required to relieve symptoms.
Imaging perspective: with its direct multiplanar imaging capabilities, excellent soft tissue contrast and spatial resolution, MRI performed with surface phased-array coils is increasingly adopted to image the soft tissue and the vascular anatomy of the penis, and currently considered the most accurate and reliable non-invasive modality to stage penile tumors. Typically, both primary carcinomas and penile metastases appear as solitary or multiple, ill-defined and infiltrating lesions, with a low signal intensity on both T1- and T2-weighted MR images compared with the adjacent corpora cavernosa and corpus spongiosum [5, 6].
On MRI, melanin content decreases T1 relaxation time and results in T1 hyperintensity when sufficient quantities are present. As shown by this case, the characteristic MRI signal intensity features (particularly with T1-hyperintensity consistent with melanin content) very uncommonly observed in other malignancies and specific to the peculiar histology of the primary neoplasm, may allow to confirm melanoma metastasization, thereby obviating the need for invasive biopsy confirmation [7].
Outcome: the uncommon penile metastatic involvement is usually diagnosed in patients with widespread neoplastic dissemination, and is associated with a poor prognosis (median survival 5-6 months) [1, 2, 7]. In our patient, restaging with total-body computed tomography (CT) detected several pulmonary, subcutaneous, nodal, liver, spleen and adrenal metastases. In consideration of the patient’s good performance status, renal, hepatic and cardiopulmonary function, systemic chemotherapy was started, including paclitaxel as the main drug. Six weeks later, he suddenly died because of intestinal perforation.
Take home message: penile metastasization from extragenital primary tumours is very rare, and exquisitely depicted by MRI. Signal intensity indicating presence of melanin may be observed in the exceptional occurrence of melanoma metastases.

Differential Diagnosis List

Final Diagnosis

Penile metastases from cutaneous malignant melanoma.

URL:	https://www.eurorad.org/case/10204
DOI:	10.1594/EURORAD/CASE.10204
ISSN:	1563-4086