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Neuroradiology Case of the Week

Case 72

Ravinder Sidhu MD, Jeevak Almast MD, Leena Ketonen MD, PhD
and P-L Westesson MD, PhD, DDS

Clinical Presentation: A 45-year-old male, status post-op for melanoma of the right forearm 13 years ago, presented with headache and mental status changes.

Radiological Findings:  Non-enhanced head CT revealed well-defined iso to hypodense lesions of varying size involving the left frontal and temporoparietal region. No evidence of calcification was seen within these lesions (Fig. 1A). Contrast enhanced CT showed mild enhancement of these lesions (Fig. 1B). Contrast enhanced CT diagnosed two more lesions, one each in the left temporoparietal and right frontal lobe (Fig. 1C). Axial T1-weighted MR images showed the lesions to be of high-signal intensity with mild heterogeneous intensity in the larger lesions (Fig. 2A&B). T2-weighted MR images showed a hypointense nature of the lesions associated with surrounding edema (Fig. 3). Post-gadolinium enhanced MR images showed heterogeneous enhancement of the lesions. The lesion in left temporoparietal showed enhancing peripheral rim (Fig.4 A&B). Diffusion-weighted MR image revealed low-signal intensity within the lesions thereby indicating melanin content within them (Fig. 5).

Diagnosis: Metastatic malignant melanoma

Discussion:  Malignant melanoma is the third most common neoplasm to metastasize to the brain. Although the brain is the most common site of metastases to the head from melanoma, melanoma can metastasize to almost any intracranial or extracranial structure such as bone, muscle, nasopharynx, parotid gland, internal auditory canal or orbit. In the brain parenchyma, cerebral lesions are more common than cerebellar lesions. Most patients with brain metastases have multiple, large lesions located at gray white matter junction. However, melanoma may rarely show military, subependymal, and cystic pattern. Cystic pattern is most likely to represent the final stage of hematoma evolution [1,2].
    Melanoma metastases exhibit a variable MR pattern that is dependent on factors such as amount of melanin and associated hemorrhage. Metastases are considered highly melanotic when more than 10% of tumor cells contain melanin, and intermediate when melanin is less than 10% in tumor cells [3]. Stable free radicals within the melanin pigment are paramagnetic which cause shortening of T1 and T2 relaxation times [4]. The melanotic pattern consists of high-signal intensity on T1-weighted images and low-signal intensity on T2-weighted images. In the amelanotic pattern, the lesion is hypointense or isointense to the cortex on T1-weighted images and hyperintense on T2-weighted images [5].
    These tumors are also among the most likely to hemorrhage. The presence of hemorrhage within many melanoma metastases may contribute blood products that alter signal patterns. The appearance of hemorrhagic lesions on MR images depends on the age of hematoma [6]. On contrast administration, most lesions show inhomogeneous and peripheral rim enhancement. Rarely, nodular enhancement may also be seen [3].
    To conclude, MR appearances of malignant melanoma are variable depending upon the melanin content associated with hemorrhage and age of hematoma.

References:

1. Escott EJ. A variety of appearances of malignant melanoma in the head: a review. Radiographics 2001; 21:625-39.
2. Takahashi I, Sugimoto S, Nunomura M et al. A case of cystic metastatic intracranial amelanotic melanoma - analysis of findings in CT and MRI. No To Shinkei 1990; 42:1031-34.
3. Isiklar I, Leeds NE, Fuller GN, Kumar AJ. Intracranial metastatic melanoma: correlation between MR imaging characteristics and melanin content. AJR Am J Roentgenol. 1995; 165:1503-12.
4. Sealey RC. Radicals in melanin biochemistry. Methods Enzymol 1984; 105:479-483.
5. Atlas SW, Grossman RI, Gomori JM, et al. MR imaging of intracranial metastatic melanoma. J Comput Assist Tomogr. 1987; 11:577-82.
6. Bradley WG. MR appearance of hemorrhage in the brain. Radiology 1993; 189:15-26.