Sacral benign notochordal cell tumor with atypical imaging features and 16-year spontaneous partial regression: A case report.

Based on the 2020 World Health Organization (WHO) Classification of Soft Tissue and Bone Tumours (5th edition), notochordal tumors are classified as benign notochordal cell tumor (BNCT) and conventional chordoma, a malignant bone tumor. Differentiation between these entities may be challenging because of overlapping imaging, histopathologic, and molecular features. Atypical notochordal cell tumor (ANCT), although not formally recognized in the WHO Classification of Tumours, has been proposed as a descriptive term for notochordal lesions that cannot be confidently classified as either BNCT or chordoma. We report a 26-year-old man who presented with a symptomatic sacral mass demonstrating mild osseous destruction and an extraosseous soft tissue component. Three computed tomography (CT)-guided biopsies yielded differing diagnostic impressions, although repeated sampling ultimately favored BNCT. Given the substantial anticipated morbidity of surgical resection, interval symptomatic improvement, repeated benign-appearing biopsy findings, and minimal enhancement, an individualized multidisciplinary decision was made to pursue imaging surveillance. Over 16 years without definitive therapy, both the intraosseous and extraosseous soft tissue components of the lesion progressively decreased in size, consistent with spontaneous partial regression. This case highlights the diagnostic challenges posed by notochordal tumors with discordant imaging and histopathologic findings and illustrates the complementary value of longitudinal imaging in selected diagnostically indeterminate cases.