Rainbow pattern in Kaposi's sarcoma under polarized dermoscopy: A dermoscopic pathological study

Background We found previously that the features of Kaposi's sarcoma (KS) under polarized dermoscopy are characterized by a bluish-reddish coloration, a scaly surface, small brown globules and, most distinctively, the multicoloured 'rainbow pattern'. Objectives To evaluate the significance of the rainbow pattern on dermoscopy as a diagnostic feature in KS, and to demonstrate that it is associated with the unique vascular structure of the tumour. Methods More than 100 lesions from seven patients with histologically proven KS were examined with polarized light dermoscopy. Sixty-three patients with various other cutaneous vascular and nonvascular tumours were also examined. KS lesions exhibiting the rainbow pattern and KS lesions lacking the rainbow pattern on dermoscopy were excised, and dermoscopic features were compared with histopathological structures. The dermoscopic patterns of other vascular tumours were also compared with histological features. In addition, the changes in dermoscopic features and histological structures were assessed before and after surgical therapy in one patient with KS. Results On the basis of evaluations with polarized dermoscopy, the rainbow pattern was found to be a highly specific dermoscopic feature for KS. Histology of KS lesions showing the rainbow pattern under polarized light dermoscopy demonstrated a vascular lumen-rich pattern of closely arranged 'back-to-back' vascular structures, whereas histology of KS lesions without the rainbow pattern showed a vascular lumen-poor pattern with vascular lumina separated further apart by intervening stromal and cellular tissue. Other vascular tumours did not exhibit the rainbow pattern and were characterized histologically by variably sized vascular structures separated by substantial amounts of stromal and cellular tissue. In one patient with KS, disappearance of the rainbow pattern was associated with obliteration of the vascular structure following surgical ablation therapy. Conclusions The rainbow pattern in KS is associated with the vascular lumen-rich histological subtype, is not manifest in the vascular lumen-poor subtype and disappears following total tumour removal. Therefore, the underlying structural arrangement of the vessels in KS may determine whether or not the rainbow pattern can be seen on polarized dermoscopy.