Placental Site Plaque of the Uterine Cervix Misinterpreted as Low-Grade Squamous Intraepithelial Lesion in Liquid-Based Cervicovaginal Cytology: Usefulness of Inhibin-α Immunocytochemistry

Liquid-based cytological features of placental site plaque have been seldom documented in the literature. We recently experienced an extremely rare case of placental site plaque occurring in the endocervix, cytologically mimicking a low-grade squamous intraepithelial lesion in liquid-based cytological preparation. We describe the liquid-based cytomorphology of the placental site plaque resembling that of low-grade squamous intraepithelial lesion and its immunocytochemical staining results. Our report will improve the understanding of this rare condition and offer insight to pathologists faced with diagnostic decisions.

A 32-year-old woman who gave birth 7 months previously was diagnosed with a cervical low-grade squamous intraepithelial lesion during cytological examination. Liquid-based cervicovaginal cytological preparation (Fig. 1) revealed atypical squamous cells with enlarged hyperchromatic nuclei and irregular membranes and occasional intranuclear grooves. The atypical cells showed mild-to-moderate nuclear pleomorphism and perinuclear cytoplasmic clearing. Dense peripheral cytoplasmic condensation, a characteristic of koilocytosis observed in low-grade squamous intraepithelial lesions, was readily identifiable. The diameter of the enlarged nuclei was approximately 3 times that of the adjacent bland-appearing round nuclei. Severe nuclear pleomorphism, mitotic activity, and atypical mitotic figure were absent. Taken together, the cytological diagnosis of low-grade squamous intraepithelial lesion was established [1, 2].

Colposcopy-directed cervical punch biopsy and endocervical curettage were performed. The biopsied specimens were fixed in 10% neutral-buffered formalin and embedded in paraffin blocks. From each formalin-fixed, paraffin-embedded block, 4-μm sections were cut and stained with hematoxylin and eosin. The most representative hematoxylin and eosin-stained slides, containing an appropriate volume of lesional tissue, were chosen for immunostaining. The punch biopsy specimen (Fig. 2) revealed that the lesion consisted of relatively well-circumscribed plaques with hyalinized extracellular matrix, in which intermediate trophoblasts were distributed as single cells or clusters. Small-to-medium-sized intermediate trophoblasts were embedded in the hyalinized stroma and exhibited enlarged hyperchromatic nuclei with mild pleomorphism. No severe nuclear pleomorphism or mitotic activity was observed.

Immunohistochemical staining was performed using a compact polymer method (Bond Polymer Refine Detection kit, Leica Biosystems, Newcastle, UK). Four-micrometer formalin-fixed, paraffin-embedded sections were incubated with primary antibodies against cytokeratin, p63, inhibin-α, and p16. Immunohistochemical staining revealed that the trophoblastic cells were positive for cytokeratin, p63, and inhibin-α but negative for p16. Both the cytological and biopsied specimens tested negative for human papillomavirus. Based on the history of recent delivery, these histological features were compatible with placental site plaque of the endocervix [3-6].

We thoroughly reviewed the cytology slide and performed cytohistological correlation (Fig. 3). The trophoblasts observed in the placental site plaque are morphologically concordant with the koilocyte-like atypical cells identified in the liquid-based cytological preparation. Immunocytochemical staining highlighted the trophoblasts that expressed cytoplasmic inhibin-α.

We found that intermediate trophoblasts of placental site plaques closely mimic low-grade squamous intraepithelial lesion in liquid-based cytological preparation. They showed nuclear enlargement, hyperchromasia, membrane irregularity, and perinuclear halos with peripheral condensation, resembling koilocytes. Trophoblasts may be mistaken for atypical squamous cells on liquid-based cytological preparation [7]. In fact, we cannot completely exclude the possible presence of cervical low-grade squamous intraepithelial lesions in other portions of the cervix that were not sampled during punch biopsy or endocervical curettage. Nevertheless, a history of recent delivery, inhibin-α immunoreactivity [8-11], cytohistological concordance, and lack of human papillomavirus infection strongly suggest that koilocyte-like cells are not squamous epithelial cells but trophoblasts obtained from placental site plaques.

A written informed consent for publication was obtained from the patient. This study (2017-3062-001) was reviewed and approved by the Institutional Review Board of Severance Hospital (Seoul, Republic of Korea).

The authors have no conflicts of interest to declare.

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science and ICT) (2018R1C1B5043725).

Hyunjin Kim: conceptualization, data collection, data analysis, manuscript drafting, and project administration; Ha Young Woo: data collection and manuscript editing; Sang Hwa Lee: manuscript editing; Sung-Im Do: conceptualization, funding acquisition, manuscript editing, and supervision; Hyun-Soo Kim: conceptualization, data collection, data analysis, manuscript drafting, manuscript editing, and supervision. All authors read and approved the final manuscript.