e23522 Background: Malignant phyllodes tumors of the breast (B-MPT) are supposed to arise de novo or to derive from a fibroadenoma/benign phyllodes tumor ("secondary"), through a MED12-dependent pathway. Secondary B-MPTs may be associated to better outcomes: both the history of previous fibroadenomas and the presence of MED12 mutations were demonstrated to be associated to improved recurrence-free survival, in small case series. We assessed the prevalence and the prognostic value of fibroadenoma-like areas (FLA) in the context of B-MPTs, which were described as potentially associated with MED12 mutations (Pareja, 2017). Methods: Single-center, retrospective, translational study conducted at European Institute of Oncology. Consecutive patients aged 18 years old or more, with non-metastatic B-MPT, who underwent surgery from Jan 2000 to Dec 2021, were included. FLA were defined as intracanalicular, pericanalicular or myxoid areas of low stromal cellularity, lacking cytologic atypia and mitotic activity. All specimens were reviewed by three pathologists. The endpoints were the cumulative incidences of distant and local recurrences. Results: 89 patients were included (Table). After a median follow-up of 6.4 years, 18 and 14 local and distant recurrences were registered, respectively. The cumulative incidences at 5-years of local and distant recurrences were 19% (95% Confidence Interval [CI], 11–28%) and 11% (95%CI, 5–18%). 47% of tumors had FLA, which were associated with lower ( &lt; 15/10HPF) mitoses (p = 0.05). No pathologic feature was associated with distant or local relapse. The presence of FLA areas was associated with fewer local recurrences, without reaching the statistical significance (p = 0.14); no difference in terms of distant recurrences emerged (p = 0.61). Conclusions: 47% of B-MPTs are characterized by FLAs, which are associated with a lower mitotic count. The association between FLAs and fewer local recurrences did not reach the threshold for statistical significance, probably due to the small number of events. The prognostic role of FLAs needs to be assessed in a larger cohort and may potentially enhance the selection of patients deserving postoperative treatment intensification. [Table: see text]

The prevalence of fibroadenoma-like areas in malignant phyllodes tumors of the breast: A retrospective analysis from a referral center.