In this case, we describe a phyllodes tumor that grew from the pregnancy period through childbirth to the lactation period. A systemic review assessed 43 cases of PT during pregnancy and lactation [3]. According to the publication, 79.5% of examined cases were characterized by a rapid growth before surgery. Notably, there was no relationship between rapid growth and malignancy, suggesting that its growth may be related to the hormonal milieu during gestation and/or lactation, rather than an indicator of malignancy [3]. The review included 26 cases (60.5%) of malignant PT. Malignant phyllodes tumors during pregnancy have also been reported numerously by other authors [4, 5] However, whether phyllodes tumors diagnosed during pregnancy are truly more malignant or not remains unclear, taking into account the publication bias. To draw a conclusion, a larger epidemiological investigation based on healthcare statistics data, including comparisons with non-pregnant women, is necessary.

The aforementioned review also discussed ultrasound findings, such as tumor shape, but there was no mention of intra-tumoral findings. In a recent case report that conducted serial ultrasound measurements of a benign phyllodes tumor during pregnancy and lactation, the largest growth was observed during early pregnancy and no obvious milk retention was noted upon removal. [6]. Meanwhile, Likhitmaskul et al. reported the case of a 36-year-old woman who was diagnosed with a 20 cm-sized benign phyllodes tumor at 32 week gestation, whose internal cystic cavity contained about 300 mL of milky liquid when the surgical specimen was resected [7]. Likewise, in our case, the surgical specimen had accumulated a milk-like liquid and presurgical MRI images revealed prominent fluid retention. Hence, the rapid enlargement during the lactation period was considered to be mainly due to the accumulation of breastmilk produced by the mammary duct epithelium inside the tumor.

In the current case, the biopsy specimen showed a marked proliferation of stromal components with high cell density. However, in the surgical specimen, the increase in stromal cell density was just mild, and fibrosis with associated degeneration was more prominent. We suspect that the observed difference in findings may be attributed to the rapid filling of the cystic cavity with milk-like components, leading to partial ischemia and detachment of stromal cells, resulting in fibrosis.

Reports of enlargement of a phyllodes tumor during pregnancy are prevalent, yet thorough considerations on the causes are lacking. Due to alterations in hormonal milieu such as serum estrogen, breasts undergo substantial physiologic changes during pregnancy including vascular hyperplasia and proliferation of alveoli and lobules [3, 8,9,10]. However, no evidence has been produced that a phyllodes tumor may be influenced by steroid hormones. A previous study suggested that stromal estrogen receptor (ER)-α promoted tumor growth via promoting angiogenesis [11] but stromal cells in mammary tissue are generally negative for ER by immunohistochemistry. We also confirmed that stromal cells in all phyllodes tumors and fibroadenoma examined were negative for ER in a recent study [12]. Moreover, because of the inherent tendency of phyllodes tumors to grow rapidly, a rigorous comparison with tumors in non-pregnant women is necessary to determine whether growth is accelerated during pregnancy.