High-resolution ultrasonography assists the differential diagnosis of blue naevi and cutaneous metastases of melanoma.

Background: Metastases of cutaneous melanoma may simulate benign blue naevi clinically.

Objectives: To investigate the value of ultrasonography in the differential diagnosis of lesions that look similar clinically, i.e. blue naevi and cutaneous metastases of melanoma.

Methods: Participants were invited for inclusion in the study if they had a cutaneous blue lesion clinically suggestive of a blue naevus or cutaneous metastasis of melanoma. After obtaining signed consent, the lesion was photographed and studied using dermoscopy and high-resolution ultrasonography before being removed for histological examination. Clinical, dermoscopic and ultrasonographic images were reviewed anonymously by four dermatologists to assign the diagnosis of blue naevus or metastasis of melanoma. The diagnostic performance of clinical examination, dermoscopy and sonography was assessed for the ability of each to differentiate between metastases of melanoma and blue naevi with reference to the histological diagnosis. Moreover, experts undertook a semeiological description of each ultrasonographic image according to seven items: location of the lesion, echogenicity, homogeneity, shape of the lesion, definition of margins, posterior acoustic shadow and increased posterior echogenicity.

Results:   Twenty-eight patients were included with a total of 39 blue skin lesions, and 17 of the 28 patients had a previous history of melanoma. Interobserver agreement in the semeiological description of the sonographic images was good (κ≥0·6) for five of seven items. Sonography was more specific (94%) than clinical examination (77%) and dermoscopy (74%). The sonographic features contributing to the differential diagnosis were: location of the lesion (P=0·027), shape of the lesion (P<0·001), homogeneity (P=0·001) and increased posterior echogenicity (P=0·007).

Conclusions: Ultrasonography is a reproducible and specific tool that can assist the differential diagnosis between blue naevi and metastases of melanoma. A blue naevus is a homogeneous, hypoechoic, 'dish-shaped' lesion, located in the superficial dermis, whereas metastases of melanoma are 'potato-shaped', hypoechoic, heterogeneous lesions, located in the hypodermis.