Alcian blue-positive stromal phenotype in basal cell carcinoma is associated with progression on first-line hedgehog inhibitors.

Basal cell carcinoma (BCC) is the most frequent malignancy in fair-skinned populations. Although curable in most cases, approximately 4% of patients develop locally advanced or metastatic disease (advBCC) requiring systemic therapy. Hedgehog pathway inhibitors (HHIs; vismodegib/sonidegib) constitute standard first-line treatment, yet individual responses vary and no histopathological biomarker predicting therapeutic outcome exists. We conducted a retrospective, multicenter analysis of 70 BCCs encompassing clinically common and advanced stages. Routine hematoxylin and eosin and Alcian blue (AB; pH 2.5) staining was evaluated using a 17-parameter, numerically encoded histopathology matrix spanning tumor morphology, stromal composition, and immune contexture. Data were mapped by unsupervised hierarchical clustering. Distinct AB staining patterns were observed: superficial and nodular BCCs typically exhibited an AB-positive peritumoral border, whereas infiltrative and sclerosing subtypes displayed a diffuse AB-positive desmoplastic stroma. The latter also correlated with advanced EADO clinical stages (correlation coefficients 0.46-0.48; p < 0.001). In a subset of 30 advBCCs obtained before or during HHI therapy, AB-positive stroma was the only parameter independently associated with shorter progression-free survival (multivariable hazard ratio = 23.8; 95% CI 4.02-141.3; p < 0.001). Established clinical or histological features failed to associate with outcome. Our findings identify diffuse AB-positive stroma as a readily detectable feature of histologically aggressive BCC and as a candidate biomarker associated with progression under HHI treatment. Because AB staining is routine, inexpensive, and easily standardized, this phenotype represents an immediately implementable readout for prospective validation and a potential link between extracellular-matrix remodeling and therapy resistance in BCC.