The term dysplasia is a disorder of maturation and differentiation, in which there is an increase in population of immature cells which are restricted to the mucosal surface and have not invaded through the basement membrane to the deeper soft tissues. The term cervical dysplasia was originally coined in the late 1950s to designate the cervical epithelial atypia that is intermediate between the normal epithelium and CIS [6]. Thus, squamous metaplasia is the replacement of one type of normal epithelium by another type of normal epithelium, while dysplasia is the replacement of normal epithelium by abnormal epithelial cells. Dysplasia was further categorized into three groups – mild, moderate, and severe – depending on the degree of involvement of the epithelial thickness by the atypical cells. Subsequently, for many years, cervical precancerous lesions were reported using the categories of dysplasia and CIS and are still widely used in many countries.

A system of classification with separate classes for dysplasia and CIS was increasingly perceived as an arbitrary configuration, based upon the findings from a number of follow-up studies involving women with such lesions. It was observed that some cases of dysplasia regressed, some persisted, and others progressed to CIS. A direct correlation with progression and histological grade was observed. These observations led to the concept of a single, continuous disease process by which normal epithelium evolves into epithelial precursor lesions and onto invasive cancer. On the basis of the above observations, the term cervical intraepithelial neoplasia (CIN) was introduced in 1968 to denote the whole range of cellular atypia confined to the epithelium. CIN was divided into grades 1, 2, and 3 [7]. CIN 1 corresponded to mild dysplasia, CIN 2 corresponded to moderate dysplasia, and CIN 3 corresponded to both severe dysplasia and CIS.

In the 1980s, the pathological changes such as koilocytic or condylomatous atypia associated with human papillomavirus (HPV) infection were increasingly recognized. Koilocytes are atypical cells with a perinuclear cavitation or halo in the cytoplasm indicating the cytopathic changes due to HPV infection. This led to the development of a simplified two-grade histological system. Thus, in 1990, a histopathological terminology based on two grades of disease was proposed:

In CIN 1 there is good maturation with minimal nuclear abnormalities and few mitotic figures (Fig. 2.2). Undifferentiated cells are confined to the deeper layers (lower third) of the epithelium. Mitotic figures are present but not very numerous. Cytopathic changes due to HPV infection may be observed in the full thickness of the epithelium. CIN 2 is characterized by dysplastic cellular changes mostly restricted to the lower half or the lower two-thirds of the epithelium, with more marked nuclear abnormalities than in CIN 1 (Fig. 2.3). Mitotic figures may be seen throughout the lower half of the epithelium. In CIN 3, differentiation and stratification may be totally absent or present only in the superficial quarter of the epithelium with numerous mitotic figures (Fig. 2.4). Nuclear abnormalities extend throughout the thickness of the epithelium. Many mitotic figures have abnormal forms. The high-grade lesions were considered to be true precursors of invasive cancer [7].

In 1988, the US National Cancer Institute convened a workshop to propose a new scheme for reporting cervical cytology results [8, 10, 11]. The recommendations from this workshop and the subsequent revision in a second workshop held in 1991 became known as the Bethesda system (TBS) [9]. The main feature of TBS was the creation of the term squamous intraepithelial lesion (SIL) and a two-grade scheme consisting of low-grade squamous intraepithelial lesion (LSIL) and high-grade squamous intraepithelial lesion (HSIL). TBS classification combines flat condylomatous (HPV) changes and low-grade CIN (CIN 1) into LSIL, while the HSIL encompasses more advanced CIN such as CIN 2 and 3. TBS has become over a short period of time the internationally standardized method of cervical cytology reporting. The most recent review and revision of this terminology took place at the Bethesda Workshop in 2001.

In March 2012, the College of American Pathologists and American Society for Colposcopy and Cervical Pathology, in collaboration with 35 stakeholder organizations, convened a consensus conference called the Lower Anogenital Squamous Terminology (LAST) Project [12]. The recommendations of this project include using a uniform, two-tiered terminology to describe the histology of human papillomavirus-associated squamous disease across all anogenital tract tissues: vulva, vagina, cervix, penis, peri-anus, and anus. These important observations resulted in a number of changes in recommendations for reporting HPV-associated squamous histopathology of lower anogenital tract sites, including the cervix. The group recommended using terms familiar to clinicians and decided on a two-tiered system similar to that used for reporting cervical cytology. Lesions are now categorized as high grade or low grade followed by the phrase “squamous intraepithelial lesion.” Acronyms like the Bethesda system (LSIL and HSIL) are used. During transition to the new terminology and at the clinician’s request, the diagnosis may be further supplemented with existing “(-IN)” terminology for each lower anogenital site. If the -IN qualifier is used, it has to be reported in parentheses after the main diagnosis. For example, a cervical biopsy previously reported as “CIN 2” will now be reported as “HSIL” or “HSIL (CIN 2).” A prior “CIN 3” now would be reported as “HSIL” or as “HSIL (CIN 3).” Because the LAST Project terminology parallels cytology reporting, healthcare providers must ensure that the report received refers to either a cytologic or histopathologic specimen. The use of similar terminology was not intended to alter the role of cytology as a screening test or to imply that cytology can substitute for histologic diagnosis.

The advantages of the binary terminology consist in the fact that this type of terminology reflects our current understanding of the biology of precancerous lesions; it can be used both in cytopathologic diagnosis and in the histopathological one, while at the same time reflecting the current clinical attitude according to which patients with LSIL and satisfactory colposcopy may be simply monitored, while patients with HSIL must be treated [3]: