'Rainbow Pattern': a Dermoscopic Sign of Invasive Melanoma

Background: The 'rainbow pattern' was initially described as a highly specific dermoscopic feature of Kaposi sarcoma. Since then, it has been reported in many benign and malignant cutaneous tumours, including a few malignant melanomas (MMs).

Aim: To determine the frequency and presentation of this dermoscopic pattern in primary cutaneous MMs in comparison to other cutaneous tumours.

Methods: The presence of a rainbow pattern was evaluated in a sample of 1100 dermoscopic images of different melanocytic and nonmelanocytic cutaneous neoplasms.

Results: The rainbow pattern was observed in 23 of 245 (9.4%) MM and 44 of 855 (5.1%) non-MM neoplasms. MMs presenting this feature were generally thicker: 82.6% > 1 mm and 43.0% > 2 mm. Compared with non-MMs, rainbow pattern in MMs was more commonly focal (82.7% vs. 36.4% nonfocal, P = 0.001) and associated with > 2 dermoscopic structures associated with MM (100% vs. 9% with fewer, P = 0.001).

Conclusion: The rainbow pattern is a dermoscopic sign that can occasionally be observed in invasive MMs. In MMs, this feature is usually associated with other dermoscopic criteria of MM and located in a focal and eccentric area, as opposed to a diffuse and isolated presentation in non-MM neoplasms.