Unexpected malignancy and phyllodes tumors among presumed benign breast masses: a single-center retrospective cohort study

Aims: We aimed to determine the rate of unexpected malignancy and phyllodes tumor, and factors associated with malignancy, among females undergoing surgery for a presumed benign breast mass. Methods: This single-center retrospective cohort study included females who underwent excision of a presumed benign breast mass in 2015-2025, identified from operative records of benign breast excision. Masses with a preoperative malignant (B5) biopsy were excluded; because a preoperative diagnosis of malignancy would have mandated oncological surgery, every malignancy on final histopathology was unexpected. Associations with malignancy were assessed by Firth penalized-likelihood regression; sensitivity analyses were restricted to masses without any documented suspicious preoperative finding. Results: Among 513 breast masses in 508 females (median age 27 years), unexpected malignancy occurred in 15/513 (2.9%, 95% confidence interval [CI] 1.6-4.8) and phyllodes tumor in 14/513 (2.7%); none of the 14 was malignant. Malignancy was associated with age ≥40 years (9.8% vs 1.0%; odds ratio 10.0, 95% CI 3.3–30.5). In exploratory analyses with few events, every malignancy with available ultrasonography (US) had a suspicious feature (negative predictive value 100%, 95% CI 98.6-100). Malignancy occurred in 9/418 (2.2%, 95% CI 1.0-4.0) without documented suspicious findings (all nine without an available US report) and in 0/41 with concordant benign imaging and biopsy. No benign (B2) biopsy was upgraded to malignancy (0/85). Conclusion: Malignancy occurred in about 2% of masses without documented suspicious findings, largely those without imaging records; older age identified females at higher risk, and histopathological examination of all excised specimens remains warranted.