Context: Although minimally invasive (microinvasive) carcinoma (< or =0.1 cm) of the breast is a well-known and well-characterized entity in excision specimens, the significance of small foci of invasion in breast core needle biopsies has not been well described.
Objective: To define the significance of minimally invasive carcinoma in breast core needle biopsies.
Design: Review of a large series of core needle biopsies for invasive carcinomas measuring 0.1 cm or less and correlation of the results with those of subsequent excision.
Setting: Large community hospital.
Results: From approximately 8500 biopsies, a total of 18 cases of minimally invasive carcinoma from 16 women aged 42 to 80 years were identified. All were present on only 1 of 8 slides made from the block. Overall, the incidence was approximately 0.1% of all biopsies and 1% of all invasive carcinomas. Six cases were invasive lobular carcinomas, 1 was tubulolobular carcinoma, 3 were tubular carcinomas, and the remaining 8 were ductal carcinomas. Eight cases were associated with high-grade comedo ductal carcinomas, 2 with low-grade ductal carcinoma in situ, 3 with atypical ductal hyperplasia, 3 with atypical ductal hyperplasia and lobular carcinoma in situ, and 2 with no other lesion. From a total of 8 sections done entirely through the block, the lesion was present on the first level in 4 cases and the fifth level in 5 cases. No cases were identified in the last 3 levels. Subsequent pathology was available for 16 of the 18 cases. Invasive carcinomas measuring more than 1 cm were present in 9 cases (64%; along with 2 positive lymph nodes), invasive carcinomas less than 1 cm in 2 cases (14%), ductal carcinoma alone in 4 cases (29%), and no carcinoma in 1 case (7%). No pathologic or radiologic features were associated with the finding of invasive carcinoma at excision.
Conclusion: Invasive carcinoma measuring 0.1 cm or less is a rare finding in breast core needle biopsies, is commonly associated with in situ carcinomas and atypical hyperplasias, and is often associated with larger invasive foci at excision. However, invasive carcinomas smaller than 0.1 cm can occur without any other significant findings and may require relatively extensive sampling to identify.