Actin expression in purely nodular versus nodular-infiltrative basal cell carcinoma

Background: The prognosis of basal cell carcinoma (BCC) correlates with its histological subtype. Actin is a microfilament that contributes to cell motility and invasiveness of cancer cells. Actin has been found to be more prominently expressed in the tumor cells and stroma of the more aggressive BCC subtypes. Here we compare actin expression in purely nodular (N-BCC) versus nodular-infiltrative (NI-BCC) tumors. Methods: We studied seven cases of N-BCC and 13 cases of NI-BCC with immunohistochemistry for alpha-smooth muscle actin (SMA) and common muscle actin (CMA) within the tumor cells and stroma. A semiquantitative method was used to determine the degree of actin present in the tumor aggregates (on a scale of 0-4). Results: Actin was present in the nodular component of 2/7 (28%) purely N-BCC and 11/13 (85%) mixed NI-BCC (p = 0.001). Actin was present in the infiltrative component of 13/13 (100%) NI-BCC. The average SMA score was 0.57 within the N-BCC compared with 1.77 within the nodular component of NI-BCC (p = 0.04); and 2.46 within the infiltrative component of the NI-BCC. The CMA score was 0.57 within the N-BCC, 1.54 within the nodular component of NI-BCC, and 1.92 within the infiltrative component of the NI-BCC. Actin was not found in the stroma of any of the N-BCC, while it was present in 8/13 (62%) of the NI-BCC (p = 0.0009). Conclusions: Actin expression is more prominent in the nodular component of mixed NI-BCC when compared with purely N-BCC. This suggests that the nodular components of NI-BCC and N-BCC are different, and that actin expression in the nodular component may be associated with potential invasiveness. This finding may be relevant when examining incompletely sampled nodular BCC.