Real-world data on malignant and borderline phyllodes tumors of the breast: A population-based study of all 921 cases in the Netherlands (1989-2020)

Aim: The aim of our study is to analyze patterns in treatment and outcome in a population-based series of patients with borderline and malignant phyllodes tumors (PT). Material and methods: Data on all patients with a borderline or malignant PT (1989-2020) were extracted from the Netherlands Cancer Registry and the Dutch nationwide pathology databank (Palga) and retrospectively analyzed. Results: We included 921 patients (borderline PT n = 452 and malignant PT n = 469). Borderline PT patients more often had breast-conserving surgery (BCS) as final surgery (81 vs. 46%). BCS rates for borderline PT increased over time (OR 1.08 per year, 95%CI 1.04 - 1.13, P < 0.001). In malignant PT adjuvant radiotherapy was given in 14.7%; this rate increased over time (OR 1.07 per year, 95%CI 1.02 - 1.13, P = 0.012). Local recurrence rate (5-year estimate of cumulative incidence) was 8.7% (95%CI 6.0-11.4) for borderline PT and 11.7% (95%CI 8.6-14.8) for malignant PT (P = 0.187) and was related to tumor size >= 20 mm (HR 10.6 (95%CI 1.5-76.8) and positive margin (HR 3.0 (95%CI 1.6-5.6), p < 0.001), but not to negative margin width (HR 1.3 ( 95%CI 0.7-2.3), p = 0.350)). Distant metastasis occurred only in malignant PT with a 5-year cumulative incidence of 4.7% (95%CI 3.3 - 6.1). Conclusion: This population-based series showed an increase in BCS in borderline PT and an increase in adjuvant radiotherapy in malignant PT over time. We identified malignant PT, BCS, larger tumor size and positive final margins as possible risk factors for local recurrence. Small but negative margins can be accepted.