Background: Extant information reveals inconsistencies concerning the natural history, pathologic features, and treatment of lobular carcinoma in situ (LCIS) of the breast. It is uncertain whether these are related to the methods of study, diagnostic criteria employed, relative paucity of cases, or varying lengths of follow-up.

Methods: The cohort was comprised of 182 women with LCIS who were enrolled in National Surgical Adjuvant Breast Project (NSABP) Protocol B-17 but received no treatment other than lumpectomy. Nineteen pathologic features were assessed and related to ipsilateral breast tumor recurrence (IBTR) and contralateral breast tumor recurrence (CBTR) at a mean time on study of 5 years.

Results: Thirteen IBTR and 4 CBTR, including 1 instance of bilateral recurrence, were observed. All IBTR occurred in the same quadrant as the index LCIS. All 4 (2.2%) IBTR that were invasive cancers were of the lobular type, as was 1 of the 2 (1.1%) CBTR that were invasive. The other was a mucinous carcinoma. Three (1.6%) IBTR were pure ductal carcinoma in situ (DCIS) and another was accompanied by LCIS. One instance of CBTR was also comprised of DCIS and LCIS. The remaining five IBTR and one CBTR were LCIS only. The only pathologic parameter found to be significantly predictive for invasive IBTR and DCIS was type 3 and, to a lesser extent, type 2 LCIS. Some heretofore unrecognized or little appreciated pathologic features of LCIS are noted. Ancillary histochemical findings strongly implicate the derivation of LCIS from ductal or secretory cells rather than "new cells" or myoepithelial elements. All examples tested were found to be c-erb B-2 negative, universally diploid with normoproliferative DNA content, and estrogen receptor and progesterone receptor positive. No other events related to the breast were encountered.

Conclusions: The number of events observed in this large cohort of patients with LCIS is markedly less than that noted by others after a comparable period of follow-up. Possible reasons for this dichotomy, including differences in patient characteristics, diagnostic criteria, and status of resection margins, are discussed. Considerations are also offered to support the view that LCIS may exhibit precursor activity as well as represent a risk factor (the term marker is literally inaccurate). In this light, the designation LCIS rather than lobular neoplasia is preferred. These preliminary findings and historical information presented in this study fail to provide any reason to perform mastectomy on patients with LCIS.