mercoledì 6 dicembre 2000
Melanoma of the ciliary body
Findings: MRI (axial T1 pre- and post -gadolinium, axial T2): There is increased T1 signal and decreased T2 signal with in the region of the medial ciliary body of the left orbit, which demonstrates enhancement after the administration of gadolinium.
Diagnosis: Melanoma of the ciliary body
The uvea (choroid, iris, and ciliary body) is derived from neuroectoderm and mesoderm and may develop tumors from both origins. Malignant melanoma is the most common primary intraocular tumor of adulthood. It arises from the choroid in 75% of the cases but the iris, the ciliary body and the optic nerve head are other sites of reported involvement. Those involving the ciliary body and choroid are thought to originate from preexisting nevi. Metastatic sites of primary uveal melanoma include the liver, lungs, bones and kidney in order of decreasing frequency. The demonstration of the tumor in relation to the sclera is important as the prognosis is worse in-patients with tumor extension beyond the sclera.
The most common malignancy found in the orbit is a metastatic deposit. The richly vascularized choroid layer is the most common site of involvement. Malignant cells gain access to the globe by way of the bloodstream via the posterior ciliary arteries explaining why most deposits are in the posterior portion of the eye. Breast, lung and kidney carcinomas have the highest incidence of ocular involvement.
Small choroidal melanomas (less than 10mm in diameter and 3mm in thickness) have a generally favorable prognosis with a 85-90% survival rate at 5 years. Clinically stable choroidal melanomas (no visible increase in size) are managed by serial observations by clinical exam, and CT or MR imaging. Distant metastatic disease and extraocular extension, however, has occurred in visibly stable lesions. With evidence of tumor growth, enucleation and wide local excision is performed.
Uveal melanomas can be detected by ophthalmologic exam, fluorescein angiography ,or sonography. Both MR and CT are accurate in determining the size and location of uveal melanomas but MR is superior in assessing for the presence of retinal detachment or vitreous change and for differentiating uveal melanoma from choroidal hemangioma or choroidal detachment.
Uveal melanoma usually appears as a well defined, solid mass but when hemorrhagic or necrotic foci are present, the heterogeneity can cause diagnostic problems. Organized subretinal hemorrhage may have MR characteristics similar to uveal melanoma.
MR: Uveal melanomas are unique among the malignant tumors in that both T1 and T2 are shortened secondary to the paramagnetic properties of melanin. Melanin produces a stable free radical signal under all known conditions which enhances proton relaxation. The degree of shortening corresponds to the melanin content. The lesion is hyperintense on T1 weighted images and hypointense on T2 weighted images. MR imaging can not differentiate amelanotic melanoma from other tumors.
CT: Elevated, sharply marginated, hyperdense lesions demonstrating mild to moderate enhancement following contrast administration