Atypical notochordal cell tumors: an alternative diagnosis when diagnostic criteria are not straightforward : Short title: atypical notochordal cell tumor.

Tumors with notochordal differentiation are categorized into two distinct entities in the latest WHO classification: benign notochordal cell tumor (BNCT) and chordoma. Previously, atypical notochordal cell tumor (ANCT) has been described for lesions that exhibit radiological and morphological features that deviate from BNCT and do not meet the criteria for chordoma. We present the clinicopathological and radiological findings of eight ANCT cases along with follow-up data. M: F was 2:6, with a mean age of 37 years. Tumors were located in the clivus (n = 3), coccyx (n = 3), thoracic vertebra (n = 1), and cervical vertebra (n = 1). The mean tumor size was 2 cm. Radiologically, features such as cortical erosion (n = 5), minimal soft-tissue extension (n = 3), and a predominant soft-tissue location (n = 2) were atypical. Morphologically, none of the tumors showed prominent cytological atypia or lobular growth. Only two cases exhibited focal minimal myxoid change, and one case showed focal necrosis. Previous diagnoses included chordoma (n = 3), ANCT (n = 3), and BNCT (n = 2). All patients are alive, with follow-up periods ranging from 2 months to 20 years (median 7.5 years), and there is no evidence of disease progression. We recommend using the term "atypical notochordal cell tumor" for tumors that do not fully meet the morphological and radiological criteria for BNCT or chordoma. This approach aims to ensure close patient follow-up and prevent unnecessary, extensive medical interventions.