THURSDAY, Oct. 20 (HealthDay News) -- The number of American women with precancerous cells of the vulva increased more than fourfold between 1973 and 2000, and the increase has led to the release Thursday of new treatment guidelines by two major medical groups.

Cancer of the vulva -- the outside of the genital area -- is usually caused by infection with human papillomavirus (HPV), which also causes cervical cancer. The precancerous condition -- known as vulvar intraepithelial neoplasia (VIN) -- should be treated with surgery, laser removal or medical therapy, according to the new guidelines from the American College of Obstetricians and Gynecologists and the American Society for Colposcopy and Cervical Pathology.

VIN is divided into two main types. Usual-type VIN is often associated with cancer-causing strains of HPV and risk factors such as smoking and a weakened immune system. Differentiated VIN is often linked to dermatologic conditions of the vulva.

In most cases of VIN, which is increasingly evident among women in their 40s, there are visible raised lesions that may be white, gray, red, brown or black.

"Most women with VIN will not notice any symptoms, but some may have bleeding, discharge, or itching," Dr. Gerald F. Joseph, Jr., ACOG's vice president for practice activities, said in a college news release.

"It would be sensible for women to periodically examine their vulvar area for any unusual spots or lesions, and if they find something, make an appointment with their ob-gyn," he advised.

While VIN appears to be increasing in the United States, the risk of vulvar cancer is small when compared with cervical, ovarian and uterine cancers, Dr. L. Stewart Massad, a member of ACOG's Committee on Gynecologic Practice, said in the news release.

Much like precancerous cervical lesions, VIN is generally slow-growing, he said. "The quadrivalent HPV vaccine that helps prevent cervical cancer and genital warts has also been shown to decrease the risk of VIN," Massad noted.

Visual examination is the only way to diagnose VIN, and most lesions will need to be biopsied. If cancer is suspected, surgery is the preferred treatment. Laser removal is an acceptable treatment for VIN lesions that appear precancerous. Low-grade lesions can be monitored or treated with a topical cream (5 percent imiquimod) for 12 to 20 weeks, according to the guidelines.

The recurrence rate of VIN is high regardless of the treatment method, and women who've had VIN remain at risk for recurrent VIN and vulvar cancer for the remainder of their lives.