Metaplastic Breast Cancer Characteristics and Patterns of Recurrence: A Single Institution Experience.

Background Metaplastic breast cancer (MtBC) is a rare type of aggressive breast cancer. MtBC has multiple pathological subtypes, including squamous, sarcomatoid, spindle, adenosquamous, and mixed metaplastic. Given the rare nature of the disease, there is limited information available on the behaviour of this cancer and no well-defined recommendations for treatment. We aim to investigate the clinicopathological characteristics of MtBC, including overall survival and patterns of recurrence.  Methods  We conducted a single-institution retrospective review of 91 patients with MtBC who presented from 2009 to 2024. We collected patient-specific, clinicopathological, and survival data. Recurrence-free interval, medical and surgical treatment, follow-up, and time to death were also collected. Data is described with frequency counts and percentages. Product-limit survival estimates were calculated from the time from diagnosis to last follow-up or death.  Results  Our population consisted of 91 patients; the mean age at diagnosis was 60 years. All patients were female; the majority were Caucasian (59, 67.1%), followed by African American (24, 27.3%). Sixty-six patients (72.5%) had a BMI >25, and 69 (78.4%) were postmenopausal. MtBC was estrogen, progesterone, and human epidermal growth factor (Her2) negative in 82 (90.1%), 88 (96.7%), and 80 (88.9%) of the patients, respectively. Forty-five patients (49.5%) underwent a lumpectomy, 41 (45.1%) a mastectomy, and one (1.1%) no surgery. Sixty-four patients (73.6%) had undergone sentinel lymph node biopsy (SLNB), 16 (18.4%) had axillary lymph node dissection (ALND), three (3.4%) had targeted axillary surgery, and four (4.6%) had no axillary surgery. Sixty-six patients (76.7%) underwent chemotherapy, of which 27 (40.9%) were in the neoadjuvant chemotherapy (NACT) setting. Of the patients who underwent NACT, seven had a complete pathological response. Two of the pathologic complete response (pCR) group received Keynote-522. In our population, 29 (36.3%) patients had a recurrence, of which 12 (41.4%) had distant recurrence, with a median time to recurrence of 12 months. The 12-month, 36-month, and 60-month overall survival rates were 93.9% (Standard Error (SE): 2.7%), 81.3% (SE: 4.8%), and 79.2% (SE: 5.1%), respectively. There was no significant difference in the survival rates (p= 0.88) or recurrence rates (p=0.19) for NACT versus adjuvant cases. Subgroup analysis comparing patients with and without recurrence who received NACT revealed no statistically significant differences in clinicopathologic features such as grade, ER, PR, HER2 status, or metaplastic subtype. Moreover, 20 patients in this study received both chemotherapy and immunotherapy, and six (33.3%) experienced recurrences, with no significant differences in recurrence rates, recurrence pattern (p=0.40), or timing (p=0.32) between NACT and adjuvant groups.  Conclusion  We demonstrated that MtBC exhibits aggressive oncologic behaviour, often with triple-negative receptor status and high Ki-67. The most common pattern of recurrence is distant, mainly involving the lungs; hence, a chest tomography was essential during the one-year follow-up to detect these lesions. Seven patients (25.9%) who received NACT achieved a pCR, indicating that NACT can be highly beneficial for a subset of patients. Further studies investigating the effects of NACT, especially keynote-522, in a larger population are necessary to create standardized treatment.