Squamous intraepithelial lesions (SIL: LSIL, HSIL, ASCUS, ASC-H, LSIL-H) of Uterine Cervix and Bethesda System

For every 100,000 women in the United States, eight new cervical cancer cases and two deaths are reported as per the most recent (2017) Center of Disease Control and Prevention statistics. Of all the gynecologic cancers (ovary, uterus, cervix, vagina, and vulva), only cervical cancer has a screening test. Cervical Pap test (or Pap smear) is the best screening method for cervical precancerous lesions and is best reported using a unified and a well-established reporting system like The Bethesda System. In this system, “Epithelial cell abnormality: Squamous” includes squamous intraepithelial lesion (SIL) category which encompasses a spectrum of squamous cell lesions starting from the precancerous lesions of low-grade SIL (LSIL) to high-grade SIL (HSIL), and ultimately invasive squamous cell carcinoma. However, depending on the qualitative and quantitative limitations with the specimen, some equivocal morphological features suggestive of squamous cell abnormality may fall under equivocal category: “Atypical Squamous Cells” (ASCs), which are subdivided into two categories; “Atypical Squamous Cells of Undetermined Significance” (ASC-US) or “Atypical Squamous Cells, HSIL cannot be excluded” (ASC-H), based on the suspected underlying lesion LSIL versus HSIL, respectively. This review provides the key cytologic features that distinguish Bethesda squamous categories from other important entities, using algorithmic approach and illustrations of common cytomorphologic patterns for clear identification of those entities in practice. The important mimickers which may be considered during the differential interpretation of SIL are discussed and presented here in a brief cytomorphologic review.