Pleomorphic adenoma as a clinicopathological continuum: borderline/atypical lesions, surgical control, and malignant...

Pleomorphic adenoma (PA) of the salivary gland, particularly of the parotid gland, is generally considered benign but exhibits heterogeneous biological behavior, including recurrence and malignant transformation. While benign pleomorphic adenoma (BPA) and carcinoma ex pleomorphic adenoma (CXPA) are well defined, lesions with borderline or atypical features represent a clinically challenging gray zone with unclear surgical implications. This study aimed to characterize the clinicopathological features of borderline/atypical PA (BaPA) within the PA spectrum and to evaluate its recurrence parttern and malignant transformation risk.

We conducted a single-center retrospective cohort study of surgically treated PAs diagnosed between 2008 and 2023. Cases were categorized as BPA, BaPA, and CXPA based on standardized histopathological re-review. BaPA and CXPA were considered higher-risk lesions within the PA spectrum. Preoperative fine-needle aspiration (FNA) concordance, Ki-67 proliferation index, surgical approach, and recurrence were analyzed, with follow-up focused on pathology-confirmed recurrence.

A total of 1,306 cases were included, of which 87.9% arose in the parotid gland. FNA concordance with final diagnosis was high in BPA (84.0%) but substantially lower in BaPA (54.4%) and CXPA (32.1%). Ki-67 increased stepwise across tumor categories and showed good discriminative ability for malignant transformation (area under the curve =0.841), with an optimal cutoff value of 10%. During follow-up, recurrence occurred in 15.5% of BaPA and 27.0% of CXPA cases. In BaPA, recurrence was associated with surgical approach; in sensitivity analysis excluding enucleation cases, recurrence remained significantly lower after total parotidectomy than after superficial parotidectomy (7.0% vs. 19.2%, P=0.01). Among recurrent BaPA cases, malignant transformation occurred in 41.7% and was associated with longer tumor duration. In CXPA, recurrence was primarily associated with advanced T category and stage.

These findings support PA as a clinicopathological continuum and suggest that BaPA may represent an intermediate-risk subgroup within this spectrum. Adequate local surgical control, awareness of the limitations of FNA, and incorporation of Ki-67 (≥10%) may aid individualized risk stratification and postoperative surveillance, while further validation of this category is warranted.