Figure 1: Sagittal T2-weighted image shows a hyperintense lobulated circumferential urethral mass, predominantly at the posterior aspect of the urethra. There is superior extension into the base of the bladder, and posterior extension into the anterior vaginal wall. A Tarlov (perineural) cyst in the thecal sac is incidentally noted.

Figure 2: Axial T2-weighted images demonstrate the bulk of the mass to be within the posterior wall of the urethra. There is invasion into the anterior vaginal wall.

Figure 3: Post-gadolinium images show enhancement in the urethral mass.

Urethral cancer.

The female urethra, a thin-walled muscular channel measuring approximately 4 cm, originates at the trigone of the bladder and terminates anterior to the opening of the vagina. The proximal one-third of the urethra is lined by transitional cell epithelium whereas the lower two-thirds is lined by stratified squamous epithelium.

Routine imaging for urethral pathology should include axial high resolution small FOV T1-weighted images before and after intravenous contrast. Orthogonal axial and sagittal high-resolution T2-weighted images are also routinely obtained. The normal female urethra on axial T2-weighted and enhanced T1-weighted images has a “target” appearance with a low signal intensity outer ring (smooth muscle layers), a high-signal intensity middle ring (vascular submucosal layer) and a low intensity central dot (mucosal layer).

Benign urethral tumors are rare and include urethral leiomyoma, papillary ademona, squamous cell papillomas, transitional cell papillomas, nephrogenic adenomas, and inflammatory and fibrous polyps. Malignant tumors of the urethra are also rare and are twice as common in females (middle-aged or older women) than males. Malignant tumors include squamous cell carcinomas (60%), transitional cell carcinomas (20%), adenocarcinomas (10%), undifferentiated carcinomas and sarcomas (8%) and melanoma (2%). Metastatic tumors to the urethra include renal cell carcinoma or melanoma, and contiguous invasion from carcinomas of the bladder, uterus, cervix, vagina, and bowels in females and bladder, prostate, and testes in males. Urethral carcinomas are commonly associated with, and thus predisposed by, previous urethral trauma, infection, and caruncle. Symptoms are usually nonspecific and include hematuria, dysuria, frequency, and pain.

Anterior (distal) urethral tumors arise from the distal third of the urethra, are usually squamous cell in origin. They are usually low-grade with early presentation and good prognosis. Posterior (proximal) or “entire” urethral tumors arise from the proximal two-thirds of the urethra, and are usually transitional cell carcinoma or adenocarcinomas. These have late presentations with advanced grade and a worse prognosis. Spread is via direct invasion of contiguous structures and distally via the lymphatics. Anterior (distal) tumors usually involve the inguinal nodes with subsequent spread to the pelvic nodal groups. Posterior (proximal) urethral lesions drain to iliac, obturator, and para-aortic lymph nodes.

T1-weighted images help demonstrate local extension into the periurethral fat. Urethral tumors are low in signal intensity on unenhanced T1-weighted images and cannot be differentiated from the periurethral muscular layer. However following gadolinium-DTPA urethral tumors do demonstrate enhancement. Axial and sagittal T2-weighted images are useful for depicting tumor invasion of the muscular wall of the bladder, vagina, and pelvic floor. In females, the tumor demonstrates increased T2-weighted signal with disruption of the target-like appearance of the urethra. In contrast in males the tumor, regardless of histiologic type, is hypointense on T2-weighted images in males.

References:

1. Semelka, Richard C. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc, 2002.
2. Grainer, R.G.; Allison, D.; Adam, A., Dixon, A. (ed). Grainger & Allison’s Diagnositc Radiology: A textbook of Medical Imaging, 4th edition. London: Churchhill Livingstone Inc, 2001.

Location and Date of Scan: New York University Medical Center, New York, Sep 2003.