[Collar-button choroidal melanoma. Anatomo-radiologic correlations]

Malignant melanomas of the choroid are the most frequent symptomatic eye tumours in adults. They often have a pathognomonic appearance, being collar-button or mushroom shaped due to rupture of Bruch's membrane by the tumoral mass. The ultrasonographic image of collar-button melanoma is well known: the head of the tumour is hyperechogenic and its base hypoechogenic. According to some authors, this is caused by difference in blood supply between the two parts. At MRI strongly pigmented melanomas emit a high-intensity signal on T1-weighted sequences and a low-intensity signal on T2-weighted sequences, but these characteristic features are inconstant. We present a case of collar-button melanoma explored by ultrasonography, colour Doppler Flow Imaging (CDFI) ultrasound and MRI, then enucleated. Flows and signals were different in front of, or behind the rupture of Bruch's membrane: ultrasounds showed a hyperechogenic image at the head and a hypoechogenic image at the base; on T2-weighted MRI sections intensity was greater in the head than in the base (head: 69 ms, base 180 ms) on CDFI, no flow was detectable in the head and very high flows were seen in the base of the tumour. Comparisons of these images with pathological findings, where there was no difference between head and base in melanin concentration and in cellular type (mixed or mainly epithelioid), led us to believe that the differences observed in images were essentially due to differences in blood supply between the two parts of the tumour constricted by the sides of the ruptured Bruch's membrane.