The WHO defines thin HSIL as a high-grade intraepithelial lesion of the cervix that is usually less than 10 cells thick. These lesions usually develop in early metaplastic squamous epithelium without anteceding LSIL. The prevalence of thin HSIL is not well documented. We evaluated different characteristics of thin HSIL at time of treatment.
We studied 25 formalin-fixed and paraffin-embedded conization specimens processed as step-serial sections. HSIL < 9 cells thick were classified as thin HSIL. HSIL >10 cells thick were classified as classic HSIL. Immunohistochemical p16ink4a staining was used to confirm lesions of thin HSIL.
Overall, 19 (76%) specimens contained both thin HSIL and classic HSIL; 4 (16%) contained thin HSIL only; 1 (4%) contained classic type HSIL only; and 1 (4%) contained thin HSIL and LSIL. Thin HSILs developed in both the columnar surface epithelium and deep cervical glandular epithelium. Most thin HSILs were 5 cells thick. All HSILs (thin and classic) were located inside the transformation zone including the squamocolumnar junction and had a median horizontal extension of 8 (0.3-21) mm.
Our findings suggest that thin HSILs are frequent findings in cone specimens, that they coexist with classic HSIL, and preferably arise in the exposed parts of the transformation zone including the glandular crypts.
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