An additional dermoscopic clue in basal cell carcinoma of the lower limb: the purpura-like pattern

Dermoscopy is a critical tool for diagnosing basal cell carcinoma (BCC) and differentiating its histopathological subtypes. However, BCC on the lower limbs may present with atypical dermoscopic features, making its diagnosis challenging.

To identify specific dermoscopic patterns of BCC on the lower limbs, focusing on the newly observed purpura-like pattern, and to assess how these features differ from BCCs in other anatomical sites.

A retrospective, single-centre study reviewed standardized polarized dermoscopic images of 478 BCCs collected between January 2011 and December 2022. Two independent dermatologists evaluated the dermoscopic images, blinded to histopathological diagnoses and anatomical sites. Statistical analysis was performed to determine correlations between dermoscopic features and BCC locations. A matched test set with dermoscopic images of skin conditions of the lower limb (control group) was then evaluated.

A new dermoscopic pattern, a purpura-like pattern, was identified in lower-limb BCCs, in addition to other features such as dotted-glomerular vessels, shiny-white structures and small erosions. The purpura-like pattern, which appeared as peripheral, out-of-focus dots or globules on a coppery red background, was significantly associated with lower-limb BCCs (odds ratio 7.13, 95% confidence interval 5.01-10.16, P < 0.001). Dotted-glomerular vessels were also more frequent in lower-limb BCCs. In the control group, composed of 239 histologically confirmed non-BCC lower-limb lesions, the purpura-like pattern was identified in 3.8% (9/239) of lesions, including Kaposi sarcoma, acroangiodermatitis/stasis dermatitis, mycosis fungoides, viral wart and basosquamous carcinoma. Interrater agreement for pattern recognition was high (Cohen's kappa = 0.81, P < 0.001).

The purpura-like pattern is an additional dermoscopic feature significantly associated with BCC on the lower limbs. Although not exclusive to BCCs, its presence, especially if combined with other dermoscopic criteria, may enhance diagnostic accuracy in this diagnostically challenging anatomical site. Further validation in larger, prospective studies is warranted.