Cervical Cancer Screening Guidelines

The Pap Smear

The Pap smear, named for pathologist George Papanicolaou, was introduced in 1949 and represented the first use of cytology for cervical cancer screening. The purpose and benefit of the Pap smear is the early detection (and thus treatment) of abnormal cervical cells, cervical precancers, and cervical cancer. In fact, cervical cancer is almost entirely preventable and curable if detected early.

Pap smears look for abnormalities in the epithelial cells of the transformation zone of the cervix. While many abnormalities noted on a Pap smear are due to HPV, some abnormalities may be due to other infections such as Herpes simplex virus, Trichomonas vaginalis infection, fungal organisms (e.g., Candida species), cellular changes associated with inflammation or intrauterine contraceptive devices, or atrophy. The cells in the superficial layer of the transformation zone normally exfoliate from the epithelial surface. It is these cells that form the basis for a Pap smear. It is important to note that Pap smears detect cervical dysplasia caused by HPV but do not detect the presence of HPV per se. HPV testing such as Hybrid Capture^® II detects the presence of HPV DNA.

Types of Pap Smears

Two general types of cytology specimen collection are currently utilized when performing Pap smears: the traditional Pap smear and liquid-based Pap smear (also known as fluid-based or thin-layer-based Pap). Regardless of the type of Pap smear performed, the general principle is to obtain an adequate sample of exocervical and endocervical cells and to assess those cells for any abnormalities. Depending upon patient age and anatomy, cervical cells may be removed with a swab, brush, or spatula (and any visible lesions noted during the examination are typically biopsied). For the traditional Pap smear, the cells are manually smeared onto a glass slide, immediately set in place with a spray fixative, stained with Papanicolaou stain and sent for cytologic examination.

For the liquid-based Pap smear the cells are removed from the cervix as above and the sampling device is immediately placed into a vial containing an alcohol-based fixative. The sealed container is sent to the cytology laboratory, where a specialized machine puts the cells onto a slide. The slides are stained and examined like a traditional Pap smear, however, unlike traditional Pap smears, these cells are displayed in a homogenous monolayer of cells [Wiley, 2004]. The two FDA-approved liquid-based monolayer cytology methods are the PrepStain system (TriPath^® Imaging Inc.) and the ThinPrep^® Pap smear method (Cytyc Corp.). While there are slight differences in slide preparation between the two systems, the underlying principles of cell collection and examination described above are very similar.

Abnormalities characteristic of precancerous or cancerous cells include a nucleus several times larger than normal, an increased ratio of nuclear to cytoplasmic area, irregularity in chromatin distribution or nuclear shape, or chromatin being coarsely granular or appearing smudged or densely opaque. Examples of cytology are shown in Figure 44.

Figure 44. Pap Cytology

Sensitivity and Specificity of Pap Smear Types

There is a relatively wide range of sensitivity, specificity, and positive predictive values reported with both traditional and liquid-based Pap smears. Published sensitivity estimates for Pap smears have ranged from 11–99% (with an average sensitivity between 55–80%) and specificity from 14–97%. Like all laboratory tests, traditional Pap smears have certain limitations [Wiley, 2004].

• Approximately 8% of specimens received are inadequate, meaning the sample does not have enough visualized squamous cells from the transformation zone.
• A false-negative result in a Pap smear can occur when cells are not effectively transferred to the slide or are not spread evenly and uniformly resulting in cell clumping.
• Exposure to air and contaminants such as blood, bacteria, or yeast limits the detection of abnormal cells in the cervical specimen.
• Finally, in a small percentage of cases, precancerous or cancer cells may not adequately shed from lesions and are missed on sampling.
• Abnormal cells may be missed among the much larger number (50,000–300,000) of normal cells

Despite the fact that each traditional Pap smear is not particularly sensitive, cervical cancer incidence and mortality have been markedly reduced through their use. Because the development of cervical cancer has a long natural history (i.e., years to decades), lesions that may be missed by a single Pap smear are usually detected by subsequent Pap smears at a point prior to the development of invasive cancer.

Liquid-based Pap smears are utilized to increase sensitivity and reduce the false-negative rate by helping avoid some of the conditions associated with a false-negative result in traditional Pap smears. While more costly than a traditional Pap smear, monolayer cytology improves specimen adequacy, detection of epithelial cell abnormalities and when correlated with pathology on biopsies is significantly better at predicting the presence of dysplasia. An additional benefit of the liquid-based Pap smear is that a sample of the specimen can be reserved in the event the clinician wants to screen for HPV DNA.

Although a liquid-based Pap smear is more sensitive than traditional Pap smears in detecting precancerous lesions, the specificity of these two methods has not been adequately compared. Consequently, both the American Cancer Society (ACS) and American College of Obstetricians and Gynecologists (ACOG) endorse either methodology in their screening recommendations and the United States Preventive Services Task Force (USPSTF) cannot recommend for or against the use of new screening methods. Therefore, a single screening method has not become the standard for specimen collection and examination.

U.S. Guidelines for Cervical Cancer Screening

When women are appropriately screened for cervical cancer in a timely and consistent manner, cervical abnormalities are likely to be detected at a stage when the disease can almost certainly be cured. There are several organizations in the U.S. that publish cervical cancer screening guidelines (addressing age at initiation, frequency, and age at discontinuation) including the ACS, ACOG, and USPSTF. Their guidelines are summarized below in Table 12 [Wiley, 2004].

Table 12. Comparison of the Guidelines for Cervical Cancer Screening

In general, it is agreed that patients have an initial cytology screen approximately three years after they begin having sexual intercourse or when they reach the age of 21 years (whichever comes first) followed by regular cytology screenings. ACOG and USPSTF recommend that women under the age of 30 test annually whereas the ACS recommends testing every two years.

The overall consensus for women 30 years of age or older with three consecutive normal Pap tests is testing every 2-3 years, with women with abnormal cytology screened more frequently. ACOG also recommends women who have had a normal Pap test and a negative HPV DNA test be screened at intervals of 3 years or more. Discontinuation of cervical cancer screening in older women is appropriate, provided women have had adequate recent screening with normal Pap results. With regard to screening after a hysterectomy, all three guidelines note that if the hysterectomy was performed for reasons other than those classified as high-risk for cervical cancer (e.g., HIV infection, prior history of CIN 2/3) screening can be discontinued.

Management of Cytologic Abnormalities

A standardized system for reporting the results of Pap testing is needed to ensure the results are interpreted properly and consistently. Previously, utilization of several different classification systems and inconsistent grading scales led to a great deal of confusion among clinicians and laboratories. The Bethesda System, established in 1988 and revised in 1991 and 2001, classifies the terminology used in reporting the results of cervical cytology.

General Concepts in the Triage of Abnormal Pap Testing Results

Once the results of an abnormal Pap test have been reported they are triaged for further evaluation and appropriate patient management. This helps to stratify patients according to the risk of developing more severe disease. Epithelial cell abnormalities are classified using the following terminology:

• Atypical Squamous Cells (ASC)
• Atypical Squamous Cells of Undetermined Significance (ASC-US)
• Atypical Squamous Cells, cannot exclude HSIL (ASC-H)
• Squamous Intraepithelial Lesion (SIL)
• Low-Grade Squamous Intraepithelial Lesion (LSIL)
• High-Grade Squamous Intraepithelial Lesion (HSIL)
• Squamous Cell Carcinoma (SCC)
• Atypical Glandular Cells (AGC)
• Atypical Glandular Cells, Favor Neoplastic (AGC, favor neoplastic)
• Adenocarcinoma in situ (AIS)
• Adenocarcinoma

LSIL is generally caused by a transient HPV infection and/or is associated with mild dysplasia or CIN 1. In contrast, HSIL is more often associated with persistent HPV infection, and has a higher risk for progression to moderate (CIN 2) or severe dysplasia and/or carcinoma in situ (CIN 3). As might be expected, HSIL is most often the result of infection with a high-risk HPV type. However, perhaps surprising, >75% of LSIL are also caused by high-risk rather than low-risk HPV types [Harper, 2004]. A considerable number of women are classified as having abnormal and/or equivocal [e.g., atypical squamous cells of undetermined significance (ASC-US)] Pap smears that require further evaluation. Figure 45 shows the annual breakdown of cervical cancer screening in the U.S. [Solomon, 2003; Schiffman, 2003]

Figure 45. Breakdown of the Results of Annual Cervical Cancer Screening in the U.S.

It is important to note that cytology results are based on the degree of the abnormality observed (e.g., from ASC-US to cancer), not a positive or negative result. Equivocal results can result in significant anxiety, further tests and medical procedures, multiple physician visits, and extra costs. The American Society for Colposcopy and Cervical Pathology (ASCCP) has developed consensus guidelines for the management of women with the cervical cytological abnormalities described in the Bethesda System [Wright, 2002]. The initial triage of cytological abnormalities and the tests utilized in their management as shown in Table 13 are based on these guidelines.

Table 13. The Bethesda Classification for Cytology Results and Recommended Triage

As summarized in Table 13, the triage of patients after Pap testing is dependent on the severity of the abnormality. For example, a cytologic result of HSIL has a high specificity that does not require further testing to clarify the result; consequently, a patient would be referred directly for colposcopy. In the case of ASC-US, however, triage may involve a repeat Pap smear, HPV DNA testing (for high-risk types of HPV), or colposcopy. All are acceptable alternatives, and all have their advantages and disadvantages.

•  Repeat cytology has the advantage of being routine and familiar to many women; its disadvantage is its relatively low sensitivity for detecting CIN 2/3.

•  Colposcopy has the advantage of immediately determining if a more serious condition, such as CIN 2 or CIN 3, is present; its disadvantages are that the procedure can be uncomfortable, expensive, and requires special training on the part of the clinician.

•  The ASCUS/LSIL Triage Study (see below) found that HPV testing was most useful in the triage of women with ASC-US.

Thus, referral for colposcopic evaluation and potential biopsy is an option in the management of women with ASC-US but is a recommendation in women with ASC-H, LSIL, HSIL, AGC, and AGC favor neoplastic.

A noted exception for referral to immediate colposcopy concerns LSIL in adolescents. Because LSIL is generally transient in this patient population, acceptable alternatives include repeat cytology at 6 and 12 months or HPV DNA testing at 12 months.

HPV DNA Testing

The utility of HPV DNA testing was examined in the ASC-US/LSIL Triage Study (ALTS) [ALTS Group, 2003a & 2003b]. This trial randomized women with ASC-US to 1 of 3 evaluation strategies: immediate colposcopy, triage-to-colposcopy based on HPV DNA results from HC2 testing, or repeat cytology. The study determined that HPV DNA testing of the large number of women with ASC-US is a more efficient strategy than immediate colposcopy or serial Pap tests. HPV DNA testing was not found to be useful in the management of women with LSIL since LSIL was highly associated (>80% positive) with high-risk types of HPV.

HPV DNA testing for high-risk types of HPV is primarily used in managing women with ASC-US. It is used either reflexively or adjunctively with Pap testing.

•  In reflexive testing, HPV DNA testing is not performed until after the results of the initial Pap smear are received.

•  In women ≥ 30 years of age, HPV DNA testing can be performed as an adjunct to a Pap smear. In this scenario, HPV DNA testing is performed at the same time as the initial Pap smear and supplements the cytologic result.

•  In women < 30 years of age, HPV DNA testing is not recommended due to the high incidence of transient HPV infection in this age group.

The results of combining cytology and HPV DNA testing from women ≥ 30 years of age can lead to various management scenarios [Wright, 2004b] (Table 14).

Table 14. Results of Adjunctive HPV DNA Testing and Cytology (in women ≥ 30 years of age)

Studies have indicated that patients with a negative Pap test combined with a negative HPV DNA test carry a negligible risk of subsequently developing CIN 3 or cancer during the next four years.

Colposcopy with Potential Biopsy

Colposcopy is the examination of the cervix, vagina, and, in some instances the vulva, with a light magnifying instrument (colposcope) after the application of a dilute (3-5%) solution of acetic acid (vinegar) to the cervix [Modern Colposcopy, 2004]. This acid treatment causes cervical cells to fill with water so light will not pass through them. Abnormal cervical changes are seen as white areas; the whiter the area, the worse the dysplasia. Thus, an acetowhite area on the cervix indicates a positive result. All suspicious lesions are biopsied and subjected to histologic examination.

Figure 46. Colposcopy of a Normal Cervix, a Cervix with Acetowhitening, and a Cervix with Invasive Carcinoma

Biopsy-confirmed CIN of any grade or cancer found after colposcopy for evaluation of ASC-H and LSIL is managed according to the ASCCP guidelines for histologic abnormalities. If CIN is not found, it is recommended that the patient have a repeat Pap smear in 6 and 12 months or HPV DNA testing in 12 months [Wright, 2002].

References

ALTS Group. A randomized trial on the management of low-grade squamous intraepithelial lesion cytology interpretations. Am J Obstet Gynecol. 2003b;188:1393-1400.

ALTS Group. Results of a randomized trial on the management of cytology interpretations of atypical squamous cells of undetermined significance. Am J Obstet Gynecol. 2003a;188:1383-1392.

Harper DM. Why am I scared of HPV? CA Cancer J Clin. 2004;54:245-247.

Modern Colposcopy Textbook and Atlas, 2nd Ed. Ferris DG, Cox JT, O’Connor DM, et al, eds. Kendall/Hunt Publishing Co., Dubuque IA. American Society for Colposcopy and Cervical Pathology; 2004.

Schiffman M and Solomon D. Findings to date from the ASCUS-LSIL Triage Study (ALTS). Arch Pathol Lab Med. 2003;127:946-949.

Solomon D. Role of triage testing in cervical cancer screening. J Natl Cancer Inst Monogr. 2003;31:97-101.

Wiley DJ, Monk BJ, Masongsong E, et al. Cervical cancer screening. Curr Oncology Rep. 2004;6:497-506.

Wright TC, Schiffman M, Solomon D, et al. Interim guidance for the use of human Papillomavirus DNA testing as an adjunct to cervical cytology for screening. Obstet Gynecol. 2004b;103:304-309.

Wright TC, Cox JT, Massad LS, et al. 2001 consensus guidelines for the management of women with cervical cytological abnormalities. JAMA. 2002;287(16):2120-2129.

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