Carcinoma, female urethra

Clinical diagnosis of urethral carcinoma is difficult. Patients often present with symptoms mimicking those of chronic urinary tract infection, and a mass lesion may not be appreciated, even by an experienced clinician. Patients, therefore often have advanced disease by the time of diagnosis. Important prognostic variables include tumour location, size and stage. Although no statistical correlations have been made between staging and location of urethral tumours, lesions confined to the distal urethra are generally of a lower stage. Once the location is determined, urethral carcinoma may be classified as either anterior (distal) - tumour confined to the distal third of the urethra, or of posterior (proximal)  tumour involving the portion of the urethra close to the bladder neck. Some authors classify tumours as "entire" when any portion of the urethra other than the distal third is involved. Tumour size (maximum tumour diameter) is another important factor in patient prognosis. Classification of urethral tumours into those less than 2 cm, those 24 cm, and those greater than 4 cm has been suggested. Spread of urethral carcinoma is usually orderly and in centrifugal fashion, progressing from local extension to lymphatic spread and, finally, to distant metastasis. Local extension is to the periurethral adipose tissue, vagina, and bladder; with more extensive spread to the levator ani muscle, pubic symphysis and vulva. Sites of lymphatic spread depend on the site of the primary urethral disease. Tumours in the distal third of the urethra spread to the nodes and then proceed to pelvic nodes. Tumours of the proximal urethra, however, are more likely to initially involve the pelvic nodes, including the obturator and external inguinal nodes. Distant metastases are uncommon at presentation and are reported to occur in 1015% of patients.

Imaging

Although carcinoma of the urethra can be detected on MR scans, it is not possible to differentiate among the various malignant tumour types or between malignancy and benign granulation tissue. On unenhanced T1-weighted images, the urethral lesions demonstrate low signal intensity and cannot be differentiated from the periurethral muscular layer or from the adjacent low-signal-intensity anterior wall of the vagina. On T2-weighted images, however, tumours image with increased signal intensity and disrupt the target-like appearance of the normal urethra (Fig.1). Urethral tumours demonstrate signal enhancement following injection of gadolinium DTPA.

Location, size, and tumour spread can be identified by MRI. Because the prognosis for carcinoma of the female urethra depends mainly on the stage of disease rather than on tumour histology, accurate assessment by MRI is clinically useful. MR offers a new and promising imaging approach which can provide information valuable in planning surgical and/or radiation therapy. An MRI tumour classification has been developed from the pathological staging system proposed by Grabstald. When the inner muscular ring is preserved, the tumour does not extend beyond the muscular wall (stage B or less). Early (stage B) disease is sometimes overstaged by MRI because the complete low-signal-intensity ring of the periurethral muscular layer cannot be detected, and infiltration of the vaginal wall cannot be excluded.

Stage C disease is diagnosed when anterior spread through the periurethral muscular layer (and eventual obliteration of fat posterior to the pubic symphysis) is detected. Tumour spread and T2-weighted axial images, but the degree of lymphadenopathy may be underestimated.

Present radiological studies provide rather poor evaluation of urethral carcinoma. Primary lesions may be detected by urethrography, but this method fails to evaluate local spread of disease and does not add to the information gained at cystourethroscopy. CT has been used to evaluate local tumour spread, but the difficulty in differentiating urethra from vagina has been a limitation. CT is further limited in the evaluation of the bladder base, both because the tumour and bladder wall have similar CT density and because the bladder base is difficult to evaluate in the axial plane. Few data are available on ultrasound evaluation for urethral carcinoma, probably because of the difficulty in transabdominal scanning of a structure located just posterior to the pubic symphysis. Endovaginal scanning may circumvent this problem. Evaluation of lymphadenopathy has been attempted by both CT and lymphangiography, but again, limited data are available in the literature.

MRI can play an important role in the valuation of urethral carcinoma, especially in staging. The value of MRI in detecting urethral tumours is limited because benign granuloma and malignant tumour have similar MRI appearances. Once urethral carcinoma is diagnosed, however, MRI is useful in demonstrating tumour size and location, and the assessment of tumour spread. The excellent tissue contrast offered by MRI allows signal intensity changes (not just anatomic distortion caused by mass effect) to be used in determining the extent of lesions. Secondly, the ability to image in multiple planes makes MRI particularly useful in evaluating craniocaudal or anteroposterior tumour spread.

MRI is highly accurate (90%) in the evaluation of direct tumour extension with a positive predictive value of 80% and a negative predictive value of 83% in the detection of tumour spread into periurethral adipose tissue. In the evaluation of vaginal wall invasion, MRI accuracy is 73%, with a positive predictive value of only 25%. Negative predictive value is 100%. The negative predictive value for bladder invasion is also 100%, and the positive predictive value is 75%.

In summary, MRI evaluation of tumour extent has excellent negative predictive values, but overestimation of tumour extent and lower positive predictive values result from the inability to differentiate among tumour, oedema and inflammation.

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