Basal cell carcinoma of the skin with mixed histomorphology: a comparative study

Basal cell carcinoma (BCC) of the skin exhibits a very heterogeneous histomorphology, on the basis of which it is classified into several subtypes and variants. In many cases, however, a definite categorization remains difficult, because BCC may consist of more than one histopathological subtype. There are limited data exploring the characteristics of these mixed BCCs, since they have not been specifically analysed. The aim of this study was to estimate the prevalence of BCCs with mixed histomorphology observed in a set of primary BCCs and to compare their clinicopathological features with a single type BCC subgroup. A total of 911 histologically proven primary BCCs from 697 patients were investigated. Prevalence of single and mixed type BCCs was 64.9 % and 35.1 %, respectively. In mixed type BCC subgroup, a very heterogeneous histomorphology was found comprising a mixture of two to four different subtypes in various proportions. The most frequent combinations included nodular-infiltrative, superficial-nodular, nodular-trichoepithelial and nodular-micronodular subtype. Comparative analysis of the two given subgroups showed that mixed type BCCs were significantly more frequently localized on the extrafacial regions of the head (30.0 % vs. 20.0 %, p = 0.02) and less often on the face (37.2 % vs. 45.2 %, p = 0.03). There were not convincing differences in the occurrence of single vs mixed type BCCs in other parts of the body. Histologically, mixed type BCCs exhibited an aggressive-growth pattern more frequently (64.6 % vs. 13.0 %, p < 0.0001). Positive surgical margins were significantly more common in mixed type BCC subgroup (17.8 % vs. 12.6 %, p = 0.02). Cutaneous BCCs with mixed histomorphology represented about one third of the cases. It is a common finding in routine pathological practice, probably suggestive of evolution and phenotypic transformation of the cancer. Since mixed type BCCs are frequently composed of aggressive histological subtypes, regardless the personal habits in description or terminology among pathologists, the presence of aggressive-growth component in tumor tissue should always be mentioned in final biopsy report.