Discussion

Atypical parathyroid tumors are variants of parathyroid adenomas that exhibit concerning histologic features suggestive of malignant potential but lack definitive evidence of invasion or metastasis, and therefore do not meet criteria for parathyroid carcinoma. Parathyroid carcinoma can present similarly, with severe hypercalcemia and significantly elevated PTH, making preoperative differentiation challenging. This distinction is critical, as en bloc resection during the initial operation is associated with improved outcomes in parathyroid carcinoma [3].

Histopathologic evaluation revealed features atypical for parathyroid adenomas, including nuclear atypia and broad fibrous bands. However, there was no evidence of capsular or vascular invasion to support a diagnosis of parathyroid carcinoma. Therefore, according to the 2022 WHO Classification of Endocrine and Neuroendocrine Tumors, the lesion was classified as an atypical parathyroid tumor [4].

Atypical parathyroid tumors have uncertain malignant potential and require closer postoperative surveillance. The 2022 WHO classification recommends immunohistochemical analysis for risk stratification for future recurrence [4]. This patient’s parathyroid mass had a Ki-67 proliferation index of five percent and retained nuclear parafibromin expression. Elevated Ki-67 indices of greater than 5% and loss of parafibromin staining are more commonly associated with parathyroid carcinoma and increased recurrence risk [4]. Thus, this patient’s immunohistochemical profile suggests a lower risk of recurrence compared with tumors demonstrating these high-risk features, although continued surveillance remains warranted given the tumor’s atypical nature.

An alternate imaging modality, such as four-dimensional computed tomography (4D CT), may better visualize findings suggestive of local invasion than traditional sestamibi imaging [5, 6]. Preoperative use of advanced imaging could improve localization, refine surgical strategy, and potentially impact outcomes in cases with concern for atypical or malignant pathology [7]. In this case, lack of 4D CT limited the ability to fully characterize the lesion, contributing to increased operative complexity and time.

Atypical parathyroid tumors are rare and can be challenging to diagnose preoperatively. In this case, a 75-year-old female initially suspected to have a large parathyroid adenoma was ultimately found to have an atypical parathyroid tumor. This case highlights the importance of maintaining suspicion for atypical or malignant pathology in patients with severe hyperparathyroidism and suggests that advanced preoperative imaging, such as 4D CT, may improve lesion characterization, facilitate operative planning, and help anticipate increased surgical complexity.