Spindle cell lesions in cytology: Risks of malignancy and application of the upcoming World Health Organization Reporting System for Soft Tissue Cytopathology

Background Spindle cell lesions can be challenging to diagnose on cytology alone. The World Health Organization (WHO) is publishing a new WHO Reporting System for Soft Tissue Cytopathology (WHORSSTC). The authors examined the performance characteristics of spindle cell lesion diagnoses at their institution when reclassifying according to the WHORSSTC. Methods Spindle cell fine‐needle aspirations or fine‐needle core biopsies diagnosed on cytology were collected from August 2021 through July 2024. Clinical information, procedural data, cytologic diagnoses, and surgical pathology diagnoses were obtained from the electronic medical records. Cases were reclassified according to the WHORSSTC, and relevant metrics were calculated. Results In total, 296 cases were identified from 164 women, 131 men, and one transgender female. The average age was 60 years. Biopsies were obtained as fine‐needle core biopsies ( n = 152), fine‐needle aspirations ( n = 130), or both ( n = 12); and 127 patients (43%) had subsequent surgical pathology follow‐up. Most lesions ( n = 94; 74%) were mesenchymal on resection. These were initially diagnosed as negative ( n = 3; 2%), atypical ( n = 14; 11%), suspicious ( n = 5; 4%), malignant ( n = 35; 28%), and descriptive ( n = 70; 55%), with absolute risks of malignancy of 0%, 43%, 80%, 100%, and 44%, respectively. To align with the WHORSSTC, these cases were recategorized as insufficient/nondiagnostic ( n = 7; 6%), benign ( n = 13; 10%), atypical ( n = 12; 9%), soft tissue neoplasm of uncertain malignant potential ( n = 54; 43%), suspicious for malignancy ( n = 5; 4%), and malignant ( n = 36; 28%), with absolute risks of malignancy of 0%, 0%, 42%, 57%, 80%, and 100%, respectively. Conclusions Although not all spindle cell lesions prove to be mesenchymal, they can be effectively categorized by the upcoming WHORSSTC with increasing risks of malignancy that better predict biologic potential.