Micronodular basal cell carcinoma. A deceptive histologic subtype with frequent clinically undetected tumor extension.

BACKGROUND AND DESIGN Micronodular basal cell carcinoma (BCC) is thought to have a greater potential for clinically surreptitious tumor spread compared with the majority of BCCs that are nodular. However, most supporting data are anecdotal. This study gives objective evidence that micronodular BCCs have wider and deeper tumor extensions than nodular BCCs of similar clinical size. In this retrospective study, 69 cases of micronodular BCC excised by Mohs' micrographic surgery (MMS) were matched to a control group of 69 cases of nodular BCC that were similarly excised. They were paired by site, size, number of recurrences, age, gender, and previous treatment type. The cases were selected and paired by computer from 1070 consecutive BCCs (primary and recurrent) referred for MMS over a 4-year period. The MMS technique allowed us to quantitate and compare the extent of tumor spread using three measurements: the number of surgical stages required for complete removal of the tumor, the width of tissue required to remove subclinical extension of tumor, and the depth of defect at completion of MMS. RESULTS Analysis showed the micronodular BCC to have significantly more covert tumor extension, making it more difficult to detect and to eradicate than the nodular BCC. The number of surgical stages required for complete removal of tumor, the width of tissue required to remove subclinical extension of tumor, and the depth of defect at completion of MMS were all greater with micronodular BCCs when compared with nodular BCCs regardless of whether cases were primary or recurrent. These differences were all statistically significant. CONCLUSIONS Micronodular BCCs can be significantly more destructive than nodular BCCs because tumor extension is difficult to detect clinically. When treating micronodular BCC, clinicians should keep in mind its potential for clandestine invasion.