World Health Organization Reporting System for Soft Tissue Cytopathology: Risk of malignancy and reproducibility of categories among observers

Introduction

In 2024, the World Health Organization (WHO) is scheduled to publish the WHO Reporting System for Soft Tissue Cytopathology (WHORSSTC). This system establishes categories with well-defined definitions, criteria, and estimated risks of malignancy (ROMs) for soft tissue tumors. The estimates of ROM are based on a relatively small number of published studies. Interobserver reproducibility is not addressed in the reporting system even though reproducibility of a reporting system is highly important.

Methods

A manual search of one authors personal consultation files and teaching set (L.J.L.) was conducted for all cytologic specimens of soft tissue tumors accessioned between January 1, 1985 and December 31, 2022. Only cases with documented surgical pathology follow-up were included in the study. Slides from each case were evaluated independently by three cytopathologists with each case assigned to one of the WHORSSTC categories. A ROM for each of the WHORSSTC categories was calculated. Interobserver agreement was evaluated by the kappa and weighted kappa statistics.

Results

Risk for malignancy by category were: Category 1: 0%, Category 2: 28%, Category 3: 57%, Category 4: 47%, Category 5: 63%, and Category 6: 88%. Kappa statistics for agreement between raters varied from 0.2183 to 0.3465 and weighted kappa varied from 0.3778 to 0.5217.

Conclusions

The WHORSSTC showed a progression of malignancy risk from the category “benign” (28%) to the category “malignant” (88%). Interobserver agreement was only fair.