Phyllodes Tumor of the Breast: Histomorphological Transformation to High-Grade Tumor during Pregnancy and Challenging Oncoplastic Surgery.

Introduction: Phyllodes tumors of the breast are a very rare tumor entity. There are two different classifications according to AFIP (low-grade and high-grade phyllodes tumors of the breast) and WHO (three subtypes: benign, borderline, and malignant phyllodes tumors). Phyllodes tumors have an increased risk of recurrence. Primary clinical treatment of phyllodes tumors is surgery, although the width of the required resection margins often presents a challenge for surgical management. Case Presentation: This case report deals with a 24-year-old woman with second recurrence of borderline phyllodes tumor. The challenge of this case lays first in surgical removal of the large tumor by breast-conserving therapy with tumor-free margins and favorable aesthetic outcome. Achieving this goal by tumor-adapted reduction mammoplasty, skin excision, and slide-swing skin flap was challenging. The second exciting aspect of this case is the histomorphology of the tumor and the transformation in vivo from fibroadenoma with stromal atypia in 2021 to the first recurrence with a low-grade phyllodes tumor in 2022 and the further transformation to a high-grade tumor with stromal overgrowth in the second recurrence of phyllodes tumor in 2023 during pregnancy and childbed. Conclusion: This case of a phyllodes tumor shows how quickly tumor characteristics can change in fast-growing tumors due to transformation from low-grade to high-grade tumor. The influence of pregnancy on this process remains unclear at present. Surgical removal of phyllodes tumors should be performed during pregnancy.