Yamada et al. Journal of Medical Case Reports 2012, 6:104 http://www.jmedicalcasereports.com/content/6/1/104 Page 2 of 4 Figure 1 Pre-operative magnetic resonance imaging. High- intensity areas in bilateral occipital forceps majors, the left caudate head, and the left semicentral ovale are shown in both fluid- attenuated inversion recovery (FLAIR) and T2-weighted (T2W) imaging. These lesions are enhanced indistinctly by gadolinium- dimeglumin (Gd). upper frames). These lesions were enhanced by gadoli- nium-dimeglumin with expansion along the corpus cal- losum (Figure 1, lower frame). But these enhancements were not as strong as those recognized in malignant brain tumors. Definite uptake of thallium was identified by 201Tl scintigraphy in both early and delay phases. However, the retention index (ratio of thallium uptake between delay and early phases) was 0.88, which was too low for malignant brain tumors to be suspected (Figure 2). tMS was suspected on the basis of the obser- vations of MRI and scintigraphy. However, oligoclonal bands were not detected by CSF examinations. For further investigation, MRS and PET were planned for our patient. A few days later, he was admitted to our hospital because of progression of his left leg motor weakness (4 +/5 on the MRC scale). The day after admission, he exhibited mental confusion and left hemiparesis (2/5 on the MRC scale), and three-day consecutive steroid pulse therapy (1000 mg of methylprednisolone sodium succi- nate and 8 mg of dexamethasone per day) was applied. Despite the treatment, his symptoms did not improve, and MRI showed further enlargement of the lesions (Figure 3). Therefore, without MRS and PET studies, an Figure 2 Uptake of thallium in the lesions on 201Tl scintigraphy. Definite uptake of thallium is identified in the lesions. However, the retention index (ratio of thallium uptake between late and early phases) is 0.88, suggesting a low possibility of malignancy. emergent open biopsy under a navigation system was performed for definite diagnosis. Histological investigation showed perivascular prolif- eration of lymphocytes without atypical features, and Kluver-Barrera staining displayed sporadic absence of the myelin sheath, suggesting demyelinating disease (Figure 4a-c). The majority of the proliferating lympho- cytes were T-cell type (Figure 4d), and the existence of lymphocytes positive for both cluster of differentiation (CD) 4 and CD8 (Figures 4e and 4f) ruled out monoclo- nal increase of the lymphocytes. Furthermore, the pre- sence of macrophages (CD68) strongly implied inflammatory changes in the lesion (Figure 4g). After one month of prednisolone treatment, MRI revealed remarkable shrinking of the lesions (Figure 5), but mild disorientation and left hemiparesis (4+/5 on the MRC scale) remained as sequelae. Discussion The term ‘tumefactive MS’ is used when the clinical presentation and MRI findings are indistinguishable