The World Health Organization (WHO) diagnostic criteria for malignant phyllodes tumor (MPT) may miss a significant number of MPTs with metastatic potential. New refined diagnostic criteria (Refined Criteria) for MPT were recently proposed. The aim of this study is to validate the Refined Criteria. This validation study included 136 borderline (borderline phyllodes tumor [BoPT]) and MPT cases that were not included in the initial study. We evaluated tumor classifications based on both the Refined Criteria and the WHO criteria. The Refined Criteria defines MPT when these criteria are met (1) stromal overgrowth with ≥ 1 feature(s) of marked stromal cellularity, marked stromal cytologic atypia, or ≥10 mitoses per 10 high-power fields (10 mitoses/10 HPFs) or (2) marked stromal cellularity with ≥1 feature(s) of marked stromal cytologic atypia, ≥10 mitoses/10 HPFs or permeative border. The WHO criteria require all 5 morphologic features (stromal overgrowth, permeative border, marked stromal cellularity, marked stromal cytologic atypia, and ≥10 mitoses/10 HPFs) for an MPT diagnosis. Using the Refined Criteria, none of the 61 BoPTs developed metastasis and 40.0% of the 75 MPTs developed metastases; local recurrence was seen in 11.5% BoPTs and 25.3% MPTs. Using the WHO criteria, 9.6% of the 94 BoPTs developed metastases and 50.0% of the 42 MPTs developed metastases; 14.9% of the BoPTs had local recurrence and 28.6% of the MPTs had local recurrence. Nine (30.0%) of the 30 tumors that developed distant metastases were diagnosed as BoPTs by the WHO criteria. When we combined the 75 MPTs from this validation cohort with the 65 MPT cases from the published data using the Refined Criteria, 50 (35.7%) of the 140 MPTs developed metastases, whereas 8 cases with metastases were <5 cm. In the univariate analysis with log-rank test, stromal overgrowth, marked stromal cellularity, marked stromal cytologic atypia, ≥10 mitoses/10 HPFs, presence of heterologous components other than liposarcomatous component, and presence of stromal necrosis were significantly associated with the risk of metastasis (all with P < 0.05). In multivariate analysis with Cox proportional hazard regression, stromal overgrowth and marked stromal cellularity were significantly associated with metastasis (both with P < 0.001). The Refined Criteria are superior to the WHO criteria in predicting the clinical outcomes of BoPTs and MPTs. Using the Refined Criteria, 35.7% of 140 patients with MPT developed metastases, whereas none (0%) of the patients with BoPT developed metastases. Patients with MPT have a high metastatic rate; these patients may benefit from systemic chemotherapy or targeted therapies. In contrast, patients with BoPT may be managed with complete local excision alone without chemotherapy.Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.