Malignant phyllodes tumors of the breast: the malignancy grading and associations with prognosis.

Purpose: This study aimed to correlate clinicopathological parameters with survival outcomes in a cohort of patients diagnosed with malignant phyllodes tumors (MPTs). We also analyzed the malignancy grade of MPTs and investigated the prognostic significance of the malignancy grading system.

Methods: Clinicopathological parameters, malignancy grades, and clinical follow-up data of 188 women diagnosed with MPTs in a single-institution were analyzed. MPTs of the breast were grouped according to stromal atypia, stromal overgrowth, mitotic count, tumor differentiation, and necrosis. A Fleiss' kappa statistic was calculated to test the agreement between the pathologists for the grading of MPTs. Disease-free survival (DFS), distant metastasis-free survival (DMFS) and overall survival (OS) were estimated using the Kaplan-Meier method and compared between groups using the log-rank test. Cox regression was carried out to identify factors predictive of locoregional recurrence (LRR), distant metastasis (DM) and death.

Results: A total of 188 MPTs were classified according to the malignancy grading system: 88 (46.8%) as low grade, 77 (41%) as an intermediate grade, and 23 (12.2%) as high grade. Excellent agreement between pathologists for the grading of MPTs (Fleiss' kappa 0.807). In our study population, the occurrence of DM and death were associated with the malignancy grade of MPTs (P < 0.001). Based on the DFS curves, the presence of heterologous elements (P = 0.025) and younger age (P = 0.014) were independent prognostic indicators. Additionally, the malignancy grade retained independent prognostic significance for predicting DMFS and OS (P < 0.001 and P = 0.009).

Conclusions: Higher malignancy grade, presence of heterologous elements, younger age, larger tumor size, and recent rapid tumor growth are poor prognostic factors for MPTs of the breast. The malignancy grading system may be generalized in the future.