PMC9295721_01
(A) Magnetic Resonance Imaging (MRI) T2-weighted (w) transversal MRI sequence; (B,C) computed tomography (CT), CT transversal (B) and dorsal (C) region of the lumbar epaxial muscles arrows: bilateral mildly asymmetric, multilobulated, well-circumscribed mass lesions in the superficial erector spinae muscles that present hyper-, hypo-, and isointense areas in T2w in MRI without contrast enhancement. On CT scan, the preceding lesion presents a hypodense center surrounded by a hyperattenuating rim giving an "eggshell appearance".
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F1
Magnetic resonance imaging of the lumbar vertebral column showed bilateral well-demarcated lesions (11 x 5 x 4 cm), more severe on the left side, within the superficial erector spinae muscles (Figure 1). T1 to S2 vertebra was performed. The lesions were mostly iso- to mildly hypodense to surrounding musculature and revealed a hyperattenuating rim surrounding a hypoattenuating center (Figure 1).
Histopathology-biopsy of affected lumbar muscles, hematoxylin and eosin (HE) stain, magnification 100 x. Extensive proliferation of fibroblasts (black arrows) encasing multiple myofibers (asterisks) accompanied by marked myofiber diameter variations and boarding well-differentiated bony trabeculae (between dotted lines). Cell atypia and inflammation are missing.
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F2
Microscopic examination revealed a non-encapsulated, solid, highly cellular proliferation of fibroblasts lined by well-differentiated and organized bony trabeculae (osseous metaplasia) recreating bone marrow spaces filled with adipose tissue (Figure 2).