Objective  To investigate the clinical characteristics and prognostic factors of malignant phyllodes tumors (MPTs) of the breast in patients below the age of 40.

  Methods  We conducted a retrospective review of 188 MPT patients treated from January 2006 to October 2020 at the Tianjin Medical University Cancer Institute and Hospital. The patients were divided into two groups according to age: 54 patients were assigned to the young group (age ≤40 years) and 134 to the old group (age >40 years). The Chi-square test or Fisher’s exact test, the Kaplan-Meier method, and the Cox proportional hazard regression model were used for statistical analysis.

  Results  The 5-year and 10-year cumulative recurrence-free survival, (RFS) rates were 55.8% and 46.6%, respectively, in the young group. In 59.3% (32/54) of cases in the young group and 35.8% (48/134) of cases in the old group, the size of the tumors was greater than 5 cm, with the difference in tumor size between the two groups being statistically significant (P=0.031). The proportions of participants with surgical margin <1 cm (50.0% vs. 35.8%, P=0.048) and participants placed on postoperative chemotherapy (20.4% vs. 7.5%, P=0.024) in the young group were significantly higher than those in the old group. Multivariate analysis revealed that age (P=0.034) and surgical margins (P<0.001) were independent prognostic factors for predicting RFS, and there was no statistically significant difference in RFS in the young group between participants who underwent lumpectomy with surgical margins ≥1 cm and those who underwent mastectomy (P=0.410).

  Conclusion  Age and surgical margins are important prognostic factors of MPTs. Patients aged ≤40 years had worse prognoses than those aged >40 years. For young patients, lumpectomy might be feasible with a surgical margin of at least 1 cm.