Application of Current Pathologic Criteria for Atypical Placental Site Nodule Suggests That Refined Criteria Are Needed.

Atypical placental site nodules (APSNs) are histologically intermediate between placental site nodules (PSNs) and epithelioid trophoblastic tumors (ETTs). Little data exists to characterize these lesions and the risk of transformation from PSN to ETT. Recent World Health Organization (WHO) criteria for distinction of APSN are vague and not objectively defined. We identified cases signed out as PSN (n=33) and APSN (n=11) and aimed to characterize, statistically compare, and assess the risk of transformation in PSNs using data including size, location, mitotic rate, Ki-67 proliferation index, trophoblastic cells per high-power field, presence of severe cytologic atypia, beta-human chorionic gonadotropin levels, time since last pregnancy, presence of calcification, necrosis, or apoptosis, and follow-up results. All cases were confirmed to be positive for p63, and a Ki-67/AE1/AE3 dual stain was used to evaluate the Ki-67 proliferation index in the trophoblastic cells. In our cohort, slight changes in the interpretation of WHO criteria for PSN and APSN led to marked differences in the proportion of PSNs flagged as "atypical." There was no statistically significant difference in the persistence of APSN versus non-APSN. None of the PSNs transformed to ETT. Current criteria for distinction between PSN and APSN are largely subjective. More objective, clearly defined, and clinically meaningful criteria are needed to distinguish between PSN and APSN, thus aiding in assessing the rare risk of transformation to ETT.