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case_images: Initial MRI and plane CT examination. T2-weighted Dixon MR sagittal image (A) and T2-weighted axial image (B) showing an intraosseous tumour at the S1-3 level of the right sacrum with an anterior extraosseous extension of the tumour. Planar CT showing an intraosseous tumour in the right sacrum with an anterior extraosseous tumour extension (C).
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case_images: Sacral magnetic resonance imaging revealed a tumour in the right wing of the sacrum (Figure 1A-C).
case_images: Histologic findings of the tumour in the sacral vertebrae. Haematoxylin-eosin stain demonstrated scattered lymphocyte clusters in the stromal connective tissue with necrosis (A). Medium-to-large clusters of lymphocytes with mild nuclear irregularities are visible in areas of marked necrosis, and round cells having mildly nuclear atypia, accompanied by massive necrosis with inflammation (B). Medium-to-slightly larger lymphocytes showed CD20(+) on immunostaining (C). EBER-positive cells in a small number of large lymphocytes (D).
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case_images: Pathological findings revealed the proliferation of atypical medial to large-sized lymphoid round cells, with mildly nuclear atypia and massive necrosis with inflammation (Figure 2A and B). Immunohistochemically, atypical cells were positive for Cluster Designation (CD) 20 (Figure 2C) and CD10 (focal), but negative for CD79a, CD3, CD5, bcl2, bcl6, MUM1, and c-Myc. A few large, atypical cells were positive for EBV-encoded small RNA (EBER; Figure 2D).
case_images: MRI following MTX withdrawal. (A, C) The tumour, which had penetrated intra- and extra-anteriorly up to the S1-3 level of the right sacral bone, reduced in size 3 months after MTX withdrawal. (B, D) MRI after 1 year from MTX withdrawal. The tumour, which was primarily intraosseous at the S1-3 level of the right sacrum and had developed extraosseously in the anterior direction, was even further reduced in size.
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case_images: Three months after MTX withdrawal (Figure 3A and C), we observed a decreased CRP and shrinkage of the tumour.