Discrepancy between Tumor Size Assessed by Full-Field Digital Mammography or Ultrasonography in a Multicenter Series of Breast Metaplastic Carcinoma Patients

In this study on metaplastic breast cancer (BC-Mp), we assessed the accuracy of predicting tumor size measurements in pathology examinations (pT) using (1) preoperative breast ultrasound and (2) full-field digital mammography (cT). The retrospective multi-center cohort study included 45 females with BC-Mp who underwent upfront surgery. Results showed that tumor sizes were statistically assessed as significantly smaller compared to the pathological examination in both imaging methods. Neither of the techniques showed clear statistical superiority, but both were more accurate for smaller tumors. The risk of underestimating tumor size with these methods, especially in larger tumors, needs consideration in surgical planning for BC-Mp. Neither ultrasound nor full-field digital mammography can be recommended as superior in this diagnostic setting. Tumor size was found to be the sole predictive factor for BC-Mp, emphasizing the crucial role of its assessment. Metaplastic breast cancer (BC-Mp) presents diagnostic and therapeutic complexities, with scant literature available. Correct assessment of tumor size by ultrasound (US) and full-field digital mammography (FFDM) is crucial for treatment planning. Methods: A retrospective cohort study was conducted on databases encompassing records of BC patients (2012–2022) at the National Research Institutes of Oncology (Warsaw, Gliwice and Krakow Branches). Inclusion criteria comprised confirmed diagnosis in postsurgical pathology reports with tumor size details (pT) and availability of tumor size from preoperative US and/or FFDM. Patients subjected to neoadjuvant systemic treatment were excluded. Demographics and clinicopathological data were gathered. Results: Forty-five females were included. A total of 86.7% were triple-negative. The median age was 66 years (range: 33–89). The median pT was 41.63 mm (6–130), and eight patients were N-positive. Median tumor size assessed by US and FFDM was 31.81 mm (9–100) and 34.14 mm (0–120), respectively. Neither technique demonstrated superiority (p > 0.05), but they both underestimated the tumor size (p = 0.002 for US and p = 0.018 for FFDM). Smaller tumors (pT1-2) were statistically more accurately assessed by any technique (p < 0.001). Only pT correlated with overall survival. Conclusion: The risk of underestimation in tumor size assessment with US and FFDM has to be taken into consideration while planning surgical procedures for BC-Mp.