Penile Metastasis from Prostate Cancer Presenting as Malignant Priapism Detected Using Gallium-68 Prostate-specific Membrane Antigen Positron Emission Tomography/Computed Tomography

Introduction

Gallium-68 prostate-specific membrane antigen positron emission tomography/computer tomography (Ga-68 PSMA PET/CT) is the best available imaging tool in the evaluation of prostate cancer.[1] The penis is an uncommon site for metastasis originating from a prostate cancer. A review of the literature on penile metastases returned approximately 400 published cases, with priapism being the initial presentation in 20%–50% of cases.[2] Our case describes the image findings of Ga-68 PSMA PET/CT in an unusual case of prostate cancer with penile metastasis presenting with priapism.

Case Report

A 79-year-old male presented with a complaint of painful urinary outflow obstruction and persistent erection. Biopsy of prostate gland revealed carcinoma prostate (poorly differentiated adenocarcinoma, Gleason's score 3 + 4, initial prostate-specific antigen (PSA) >100 ng/ml). He was referred for the whole body Ga-68 PSMA PET/CT [Figure 1] for staging which showed an enhancing lesion in the prostate gland (with a maximum standardized uptake value [SUVmax] of 16) and thickening of entire shaft of penis with intense PSMA uptake (SUVmax of 35) [Figure 2]. He underwent bilateral orchidectomy and radiotherapy and is on follow-up.

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Figure 1 Whole body Gallium-68 prostate-specific membrane antigen positron emission tomography/computed tomography maximum projection image shows uptake in primary prostate lesion and penile metastasis

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Figure 2 Sagittal computer tomography (a), positron emission tomography (b), positron emission tomography/computed tomography (c), maximum projection image (d), Gallium-68 prostate-specific membrane antigen positron emission tomography/computer tomography showing soft-tissue enhancing lesion in the prostate gland and corpora cavernosa thickening of penis with intense prostate-specific membrane antigen uptake suggesting metastasis

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Discussion

Secondary penile lesions are a rare phenomenon, first described in 1870 by Eberth. Reviews of published case reports reveal that organs along the genitourinary tract, such as the prostate and bladder, are the most common primary sites. Of the 394 documented cases, 129 (33%) cases were of prostate origin, and bladder cancer was a close second with 118 published cases (30%).[2] Clinical manifestations of penile metastases include penile masses or nodules, ulceration, obstructive or irritative urinary symptoms, hematuria, and malignant priapism in 20%–50% of the documented cases.[3]

Different mechanisms of persistent erection due to malignancy have been described, with a distinction between low-flow and high-flow priapism. Most incidences of malignant priapism are considered to be low-flow priapisms and are believed to be due to neoplastic invasions into the cavernous sinuses and venous system, causing a complete blockage, and a consequent unrelenting erection. The penile shaft is the most common anatomical site to be affected in case of a penile metastasis. Metastatic spread of prostate cancer to the penis occurs by several routes. Retrograde venous or direct lymphatic/vascular invasion and direct extension through the lumen of vas deferens are the most common mechanisms.[4]

PSMA is a transmembrane protein that has considerable over expression on most prostate cancer cells. The whole body Ga-68 PSMA PET/CT have a high sensitivity in detecting lesions of prostate cancer with a high tumor to background contrast, even at low-serum PSA levels.[5] Brain, liver, and penis are some of the organs which are uncommonly involved in carcinoma of the prostate. There are very few case reports till date which revealed detection of these sites on Ga-68 PSMA PET/CT. A case series' described three unusual sites of metastases (brain, penis, and liver) from prostate cancer.[67] Our case describes the first case of Ga-068 PSMA PET/CT detecting penile metastasis with priapism in the initial staging of prostate cancer.

Regardless of mechanism of spread or site of primary cancer, the prognosis of secondary penile malignancies is generally poor. It is reported that the average survival of such patients is approximately 9 months, with an overall survival of <18 months. Treatment of secondary penile lesions with malignant priapism has generally been aimed at palliation and improved quality of life, including surgical management of priapism and a possibility of penectomy for cutaneous lesions, urinary symptoms, and pain.[8]

In conclusion, metastatic prostatic cancer presents with various signs and symptoms, and one should keep in mind the possibility of the penis as the site for metastasis. Ga-68 PSMA PET/CT scan allows detection of the metastatic sites, even the rare sites making it a powerful diagnostic tool for an assessment of the extent of disease in patients of prostate cancer.