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Neuroradiology Case of the Week

Case 86

Terry Chun, MD and P-L Westesson, MD, PhD, DDS

Clinical Presentation: This is a presentation of an unusual DWI appearance of a brain metastasis. A 45-year-old woman with a history of right lung adenocarcinoma and known brain metastasis had an MRI for follow-up. The patient was afebrile.

Radiological Findings:  There was a large enhancing mass in the posterior right temporal lobe (Fig. 1) with a moderate amount of surrounding edema (Fig. 2). The diffusion weighted image showed increased signal intensity (Fig. 3) with decreased ADC, indicating true restricted diffusion (Figs. 4 & 5). There is increased perfusion to the mass (Fig. 6).

Diagnosis: Metastatic lung adenocarcinoma that demonstrates restricted diffusion

Discussion: Diffusion imaging is principally bright in acute infarcts and abscesses. There are a few other much rarer situations where the diffusion also might be bright such as this case of a metastasis [1]. The reason for the high DWI signal is thought to be due to high cellular density [2]. The apparent diffusion constant (ADC) for metastatic disease, including those from the lungs, have a wide range of values [2]. One study demonstrated measurements from 0.35 to 1.39 x 10-3 mm2/s [2]. Because the ADC of metastatic lesions can be low, these masses often can be mistaken for abscesses, which usually demonstrate low ADC values [1]. Clinical correlation with patient symptoms and white blood cell count are necessary to understand the etiology of the brain lesion. Another method that could help in distinguishing between abscess and tumor is perfusion imaging. Tumors may show increased perfusion, whereas abscesses generally have decreased perfusion.
     The most common primary tumors that metastasize to the brain are lung carcinomas followed by breast, melanoma, and GI/GU tumors [3]. Parenchymal metastases rarely occur in children but represent between one quarter to one third of all brain tumors in adults [1].
     Metastases to the brain appear as well-defined nodules of varying size on MRI. The masses often have both solid and cystic components, being filled with mucinous, necrotic, or hemorrhagic material [3]. There is often accompanying perifocal edema that is more extensive than the mass.

References:

1. Holtas S, Geijer B, Stromblad LG, et al. A ring enhancing metastasis with central high signal on diffusion-weighted imaging and low apparent diffusion coefficients. Neuroradiology 2000; 42:824-827.
2. Kono K, Inoue Y, Nakayama K, et al. The role of diffusion-weighted imaging in patients with brain tumors. AJNR 2001; 22:1081-1088.
3. Osborn AG. Diagnostic Neuroradiology. Mosby, St. Louis, 1994.