Microinvasive breast carcinoma (MIC) has a good prognosis but specific definitions have varied in the past, making the clinical significance of MIC a subject of debate.

Microscopic slides of 59 cases of breast carcinoma originally diagnosed as MIC were reviewed retrospectively. Histologic parameters were correlated with clinical findings and outcome to define diagnostic criteria better.

On review, the 59 cases were recategorized as follows: pure DCIS (N = 16), DCIS with foci equivocal for microinvasion (N = 7), DCIS with > or =1 focus of microinvasion (N = 11), T1 invasive carcinomas with > or =90% DCIS (N = 18), and T1 tumors with <90% DCIS (N = 7). The MIC cases in the current study averaged 3 separate foci of early infiltration outside the basement membrane, each one not >1.0 mm. The mean follow-up was 95 months. Six patients (10%) had only local recurrence: 1 case each in patients with equivocal microinvasion, microinvasion, and T1 tumors with <90% DCIS and 3 cases among the patients with T1 tumors with > or = 90% DCIS. Four patients, all with T1 tumors with > or =90% DCIS, had distant failure (7%). In the MIC group, only one patient developed a local recurrence after breast conservation. No patient had axillary lymph node metastasis. For the entire series, factors associated with local recurrence were younger age, breast conservation versus mastectomy, and close surgical margins. The only factor associated with distant failure was the size of the DCIS component. Seven patients with T1 tumors with > or =90% DCIS experienced local or distant failure and 5 of these (71%) developed progressive disease or died of disease. All other patients who developed a recurrence were disease free at last follow-up. In a retrospective series, poorer outcome in carcinomas with > or =90% DCIS may be related to the greater likelihood of missed larger areas of invasive carcinoma. Therefore, meticulous and extensive sampling of these carcinomas is required.

MIC as defined has a good prognosis. It has a different biology than T1 invasive carcinoma with > or =90% DCIS, which may progress and cause death. Large tumors with multiple foci of microinvasion may have metastatic potential.