Trends in the Diagnosis of Phyllodes Tumors and Fibroadenomas Before and After Release of WHO Classification Standards

Background: In 2012, the World Health Organization (WHO) released diagnostic criteria for grading phyllodes tumors based on histologic features. This study sought to examine the application of the WHO criteria and the changing epidemiology of fibroepithelial tumors. Methods: A retrospective review of surgically excised fibroepithelial lesions from 2007 to 2017 at a single tertiary care institution was conducted. Data regarding the WHO criteria (tumor border, stromal cellularity, stromal cell atypia, stromal overgrowth, mitotic activity) and traditional descriptors (leaf-like architecture, periductal stromal condensation) were collected. Clinical and pathologic characteristics of cases with diagnoses determined before and after 2012 were compared. Results: During the study period, 305 fibroepithelial tumors were identified. No significant differences were observed in terms of mean age, race/ethnicity, presenting symptoms, or method of diagnosis between cases diagnosed before and after 2012. After 2012, the findings showed statistically significant increases in reporting of WHO and traditional histologic features, a decrease in diagnoses of fibroadenomas (85.9% [116/135] before vs 70.0% [119/170] after 2012), and an increase in benign phyllodes tumors (0% [0/135] before vs 12.9% [22/170] after 2012). Patients with a diagnosis of benign phyllodes tumors were significantly younger than those with a diagnosis of borderline, malignant, or non-graded phyllodes tumors (mean age, 25.7 ± 10.6 vs 52.8 ± 9.9, 40.7 ± 24, 46.3 ± 1.5 years, respectively; p = 0.006). Conclusions: The expanding use of the 2012 WHO criteria has been accompanied by an increased diagnostic frequency of benign phyllodes tumors and a decrease in fibroadenomas. As fibroepithelial diagnoses become more distinct, evidence-based management recommendations for less virulent phyllodes diagnoses should be developed.