Fibroadenoma vs Phyllodes

A 32-year-old Indian woman presents with a painless, firm, mobile breast mass in the left upper outer quadrant that she first noticed 18 months ago. The mass has remained stable in size at approximately 2 cm. On examination, the mass is well-circumscribed, rubbery, and freely mobile. Ultrasound shows a homogeneous, hypoechoic lesion with no vascularity on Doppler. Fine-needle aspiration cytology shows benign epithelial cells and stromal fibroblasts. What is the most likely diagnosis?

A 28-year-old woman presents with a 3-month history of a rapidly enlarging left breast mass. On examination, a firm, mobile, well-circumscribed mass measuring 4 cm is palpated in the upper outer quadrant. Mammography shows a large mass with lobulated contours and heterogeneous density. Core needle biopsy reveals a biphasic lesion with prominent stromal cellularity, increased mitotic activity (8 mitoses per 10 high-power fields), and areas of stromal overgrowth. What is the most likely diagnosis?

Which of the following histological features is most characteristic of phyllodes tumor and distinguishes it from fibroadenoma?