INTRACRANIAL METASTATIC MELANOMA - CORRELATION BETWEEN MR-IMAGING CHARACTERISTICS AND MELANIN CONTENT

OBJECTIVE. Preliminary reports based on limited numbers of cases have proposed that specific MR imaging patterns may permit a distinction be tween melanotic and amelanotic brain metastases in melanoma patients, The purpose of this study was to test this hypothesis by categorizing MR images obtained from a large series of patients and correlating the results with the percentage of melanin-containing cells in surgically resected metastases. SUBJECTS AND METHODS. The MR images of 30 patien ts with histologically proven intracerebral melanoma were reviewed ret rospectively. Precontrast MR images were obtained with T1-weighted spi n-echo sequences in axial and sagittal sections and with proton densit y-weighted and T2-weighted sequences in axial sections. After IV injec tion of gadopentetate dimeglumine (0.1 mmol/kg of body weight), T1-wei ghted images were obtained in axial and coronal sections. All patients had undergone gross total resection of the evaluated lesions, MR imag es of the metastases were reviewed and sorted into four groups on the basis of putative patterns: (1) melanotic pattern-hyperintense in rela tion to cortex on T1-weighted images, hypointense in relation to corte x on T2-weighted images, and isointense or hyperintense in relation to cortex on proton density-weighted images; (2) amelanotic pattern-hypo intense or isointense in relation to cortex on T1-weighted images and hyperintense or isointense in relation to cortex on T2-weighted and pr oton density-weighted images; (3) indeterminate, or mixed, pattern-MR imaging characteristics that did not conform to those of one of the fi rst two categories; and (4) hematoma pattern-MR imaging features that exhibited only hematoma characteristics. Tissue sections from all eval uated lesions were independently reviewed by a neuropathologist (G.N.F .), and the percentage of melanin-containing tumor cells in each resec ted metastatic lesion was estimated, The MR imaging data and histologi c data were then compared to assess the predictive value of the MR ima ging patterns. RESULTS. Forty-two metastatic lesions were identified a nd categorized by MR imaging pattern as follows: 10 melanotic, 11 inde terminate (mixed), 16 amelanotic, and five hematoma. Correlation with histologic findings revealed that a majority (7/10) of lesions that ex hibited a melanotic MR imaging pattern had more than 10% melanin-conta ining cells, over half (9/16) of lesions that exhibited an amelanotic MR imaging pattern contained histologically identifiable melanin (but always in less than 10% of cells), and lesions that exhibited a mixed MR imaging pattern were either amelanotic or contained less than 10% m elanotic cells, Conversely, a majority of lesions containing more than 10% melanotic cells (7/8) demonstrated the typical melanotic MR imagi ng pattern, lesions with less than 10% melanin-containing cells exhibi ted a variety of MR imaging patterns, and only about half of patients with amelanotic lesions (6/13) showed the characteristic amelanotic MR imaging pattern. For five lesions, potentially informative imaging da ta on melanin content was obscured by histologically documented hemato ma formation. CONCLUSION. Only a minority of melanoma metastases have the anticipated MR imaging findings of melanotic melanoma, which consi st of high signal intensity relative to that of cortex on T1-weighted images and low signal intensity relative to that of cortex on T2-weigh ted images. Of tumors that do exhibit this melanotic pattern, the majo rity have more than 10% melanin-containing cells, The putative MR imag ing pattern for amelanotic melanoma is nonspecific, as over half of tu mors with this pattern contain melanin.