Papanicolaou classification

British Society for Clinical Cytology classification

The Bethesda System

In the USA, the National Cancer Institute devised a system called the “The Bethesda System” (TBS) for reporting cervical/vaginal smears (National Cancer Workshop 1988; Table 5.1).

The most important issues addressed in the first National Cancer Workshop in 1988 (and subsequently modified in 1991 and 2001) were: recommending a statement of adequacy; incorporating recommendations by the cytopathologist regarding follow-up; and changing the reporting of cytologic abnormality from the numeric Papanicolaou class system to a descriptive system that incorporates specific terms for non-diagnostic atypia (atypical squamous cells of undetermined significance (ASCUS)) and low- and high-grade squamous intraepithelial lesions (LSILs and HSILs, respectively).

The most recent revision (in 2001) redefined the previously described system by:

1. eliminating the adequacy category
   
2. combining the category previously termed “negative and benign cellular changes” into a single “negative” category
   
3. renaming ASCUS as “atypical squamous cells” (ASCs) and contracting its subtypes to ASCUS and “atypical squamous cells, cannot exclude high-grade lesion” (ASC-H)
   
4. renaming “atypical glandular cells of undetermined significance” (AGCUS) as atypical glandular cells and contracting its subtypes to “atypical glandular cells, not otherwise specified” (AGC-NOS) and “atypical glandular cells favor neoplasia” (AGC favor neoplasia).

This system has essentially three categories of findings:

1. Normal cytology.
   
2. ASCUS.
   
3. Cellular changes suggesting:
   
   
   
   1. LSIL
      
   2. HSIL.

The fundamental differences between TBS and the BSCC classification are:

1. At the low-grade end of the spectrum in TBS, koilocytotic atypia is seen as indistinguishable from mild dyskaryosis and incorporated seamlessly in LSIL.
   
2. The high-grade end incorporates predictions for CIN2 and CIN3.

The terminology recognizes the frequency of regression and refers to predicted “lesions,” not neoplasia. In use, the new system appears to have seen a major expansion of the inconclusive smear result, namely ASCUS or AGCUS (see section 5.5). However, this is probably more a consequence of the recent frequency of litigation for false-negative assessment in the USA rather than any criticism of the classification.

Basic cytologic pattern

Normal cytology

This list is routine but not exhaustive. If no dyskaryosis is present, the smear report is normal or negative (Figure 5.1).

Smear adequacy (Figure 5.2)

Evaluation of specimen adequacy by the screener is considered by many to be the single most important quality assurance procedure that can be provided by the laboratory. However, there has been much debate in the past regarding characterizing an adequate smear. Definitions of adequate smear that prescribe the number of cells are not practical. The total squamous cellularity is extremely variable. Endocervical and metaplastic squamous cells do not reliably indicate direct endocervical canal sampling. Provided that the smear-taker is confident that the cervix has been fully visualized and a full 360° sweep has been made while taking the smear, the conventional smear is considered adequate if:

1. the smear is not obscured by blood or overlying neutrophilic exudate or artefact (Figure 5.2a)
   
2. the smear is not purely composed of endocervical cells
   
3. the smear is properly fixed in alcohol without air drying
   
4. the smear is not overly thick as a result of poor spreading technique.

The introduction of liquid-based cytology (LBC), including the ThinPrep and SurePath methods, has dramatically improved the smear adequacy rates by providing clearer samples to read. Excess blood, mucus, and neutrophilic exudate is washed out and the LBC samples have more uniformly distributed cells. In Figure 5.2b, the conventional smear (Figure 5.2bi) is compared with an LBC-acquired specimen (Figure 5.2bii).

Atrophic smears (Figure 5.3)

Atrophic smears may present particular difficulties for cytologic assessment, whether postmenopausal or postnatal. The lack of estrogen produces thin epithelia, which yields rafts of cohesive parabasal cells (Figure 5.3a) or trails of “stripped” parabasal nuclei mimicking dyskaryosis (Figure 5.3b).

The cellularity of the smear is often low, and the retracted transformation zone may be poorly or not represented. This is the basis for treating such patients briefly with estrogens and retaking a smear to ensure adequacy by increasing the smear yield and decreasing the cytologic difficulties introduced by immature and inflamed cell populations.

Borderline abnormalities (Figure 5.4)

In the BSCC nomenclature, the category of “borderline nuclear changes” (BNC) appears largely to refer to “HPV-associated nuclear atypia” (Figure 5.4a–c). TBS 1988 created the ASCUS category to identify the subset of squamous atypias that cannot be readily designated as benign or preinvasive. The category was renamed simply as ASCs in the 2001 revision with its further subclassification into ASCUS and ASC-H; “ASCs” cannot rule out HSIL.

The more recent approach to incorporate HPV testing for the BNC or ASC category will help in further defining this uncertain category. However, for HPV testing to be effective it is critical that this category does not include cases that are a clear example of inflammatory atypia or reactive atypia.

Low-grade squamous intraepithelial lesions/mild dyskaryosis (Figures 5.5, 5.6)

This pattern is evident when intermediate or superficial squamous cells with nuclear enlargement (usually threefold) and hyperchromasia are seen alongside normal cells (Figure 5.5). The nuclei have a smooth nuclear membrane and small irregularities in nuclear shape and contour. Hyperchromasia is present and may take the form of a finely granular chromatin pattern or uniformly increased nuclear density with opaque or smudged appearance (Figure 5.6). Mimics of LSIL/mild dyskaryosis in-clude trichomonas infections with mild perinuclear halos, postmenopausal nucleomegaly, and non-specific reactive nuclear changes leading to mild superficial nuclear atypia.

High-grade squamous intraepithelial lesions/moderate and severe dyskaryosis (Figures 5.7, 5.8, 5.9)

On cytologic preparations the cells of HSIL/moderate and severe dyskaryosis are characteristically less mature and exhibit a higher nuclear to cytoplasmic ratio than those of LSIL (Figure 5.7a–c). The nuclear enlargement is generally in the same range as LSIL but, because the nuclear to cytoplasmic ratio is increased, the cells appear smaller (Figure 5.8a,b). Hyperchromasia, coarse chromatin, and nuclear membrane irregularities are all more severe than in LSIL (Figure 5.7c). These cells are arranged either as cohesive groups or as single cells (Figure 5.9a,b). Mimics of HSIL/moderate and severe dyskaryosis include: atrophic changes, which are evident by a high nuclear to cytoplasmic ratio but greater regularity in nuclear contour and absence of coarse chromatin; sampling of the lower uterine segment, in which syncytial groups of the lower uterine segment may be confused with HSIL; and adenocarcinoma in situ (AIS), which may be difficult to distinguish from HSIL.

Cytologic indications of invasion (Figure 5.10)

The principal and most reliable pattern of possible invasion in a cervical smear is the presence of an ulcerative or malignant “diathesis” (Figures 5.10, 5.11). This is the presence of pleomorphic, keratinized, and parabasal squamous cells with often pale, degenerate nuclei, together with inflammatory cell exudate, necrotic (ghost) tumor cells, and fresh blood (Figures 5.12, 5.13a,b). However, comedo-like crypt CIN3 lesions can mimic these appearances in smears.