Pap Tests Offer Best Solution for Early Cancer Detection
Edition: August 2001 - Vol 9 Number 08
Author: Laura Thill
Nearly 100 million pap smears are taken in the United States each year, according to estimates by the American Cancer Society. Of the 13,700 women diagnosed with cervical cancer this year, about 4,500 will die.
The conventional pap test relies on a sample of cells taken from in and around the cervix. The cells are collected with a wooden scraper, cotton swab or small cervical brush and set up on a slide for microscopic analysis. Magnified, the cells can be studied for abnormalities, which are linked to cancerous or precancerous conditions.
Because the abnormal cells exist in the squamous epithelial cells lining the surface of the cervix, a patch of these defective cells is referred to as a squamous intraepithelial lesion (SIL). A SIL is considered low-grade if the abnormal cells are a normal size. These SILs often revert to normal, according to the FDA. If the cells are less than normal size, however, the SIL is ranked high-grade. High-grade SILs more often than not advance to cervical cancer within 10 or 15 years, according to the National Cancer Institute (NCI). High-grade SIL pap results indicate a need for a colposcopy or biopsy to confirm the diagnosis.
Both low- and high-grade SILs can be removed by cryosurgery (freezing), cauterization (burning) or via laser surgery or electrosurgery.
New Slide Preparation Techniques Mean Increased Accuracy
Since pap tests have become routinely incorporated into gynecological exams, the number of women dying from cervical cancer has dropped dramatically, by 70 percent between 1950 and 1970, according to the NCI.
Still, the traditional pap tests have not been without error, and an abnormal pap result does not necessarily mean that cancer exists. Consider that a standard lab slide may be lined with as many as 50,000 to 300,000 cervical cells, and as few as a dozen of these may be abnormal. Cell abnormalities can be hidden by infection, may be difficult to detect, or may simply escape the attention of an exhausted cytotechnologist. Conventional pap tests have been found to miss abnormal cell detection in 10 percent to 30 percent of patients, according to the University of Virginia Health System. Some reports claim these tests fail to detect cell irregularities in up to 60 percent of patients.
Thin Prep Test
The Thin Prep Processor Model 2000 (Cytyc Corp., Boston) was FDA-approved in 1996 for supplementary screening. The ThinPrep is an automated slide preparation system for pap smears designed to permit easier screening of abnormal cells.
For the ThinPrep test, a specimen is collected using a spatula/endocervical brush combination or a broom device. The specimen is placed in a liquid collection unit, which in turn is sent to the lab for analysis.
At the lab, an instrument disperses the cells in the solution, arranging them in a thin, even layer on a specially designed microscope slide. Partly, the test's accuracy depends on its automated ability to concentrate the cellular material and filter out other unnecessary material, such as blood, says the FDA. ThinPrep sample preparation adds a cost of $15 or $20 to the pap test, according to pathologist Joyce Johnson of Vanderbilt University Medical Center.
The ThinPrep test offers several benefits, according to the manufacturer:
A uniform, reproducible and statistically representative sample can be obtained.
Blood and mucous are removed, permitting more accurate analysis.
Better cell preservation.
The test does not require any technical proficiency in smear preparation.
Only part of the sample is used, allowing for reflex testing for HPV.
The Papnet device, licensed in the United States by NetMed Inc. (Dublin, OH), also has been FDA-approved for back-up screening. This system relies on neural net computer technology, which according to the manufacturer, was originally designed to detect flying missiles in the ''Star Wars'' defense strategy.
The Papnet computer system essentially teaches itself to detect irregular cervical cells. A series of digitized images of pap slides is input to a computer. The computer uses these examples to develop guidelines for determining abnormal cell structures.
The system scans each pap slide that already has been classified as normal. From each slide, Papnet selects 128 cells or cell clusters that are most likely to be irregular, according to the FDA. Enlarged color images of these cells are returned to the cytotechnologist for further analysis.
Papnet has been used to reexamine pap slides taken from women with high-grade cervical cell irregularities or cervical cancers. As a rescreening tool, the system has been found to detect some cell abnormalities missed by conventional pap tests.
A third FDA-approved system the AutoPap 300, introduced by the University of Virginia Health System pathology department in conjunction with the University of Virginia Cancer Center is designed to rescreen all negative pap smears using several new technologies. The AutoPap 300 employs high-speed video microscopy, field-of-view computing and image interpretation software.
The AutoPap system relies on image interproduction and pattern recognition capabilities to classify abnormal cells. Parameters within the system's scanner detect abnormalities based on a wide range of cellular characteristics, such as cell diameter, shape of nucleus, and nuclear density, according to the University of Virginia Health System. Any such discoveries can be resent to the cytotechnologist for rechecking.
New Technologies Improve Diagnosis
The FDA is confident that newer screening technologies can help limit the chance of missing a diagnosis of cervical cancer. But, it warns against any misconception that automated or electronic systems are foolproof.
Even if rescreening systems ensure more accurate detection of cervical cancer, when a test sample has too few cells on the slide, or the cell samples are not extracted from both the inside and surface of the cervix, cancer may be masked. In addition, douching within a couple of days of a pap test can wash away abnormal cells.
It is critical, notes the FDA, that labs evaluate the newer systems and determine how they will be used.