Eccentric target sign in cerebral toxoplasmosis: neuropathological correlate to the imaging feature

Abstract

Cerebral toxoplasmosis remains one of the most common focal brain lesions in patients with acquired immune deficiency syndrome (AIDS). Diagnosis is a challenge because on cranial imaging it closely mimics central nervous system lymphoma, primary and metastatic central nervous system (CNS) tumors, or other intracranial infections like tuberculoma or abscesses. A magnetic resonance imaging (MRI) feature on postcontrast T1-weighted sequences considered pathognomonic of toxoplasmosis is the "eccentric target sign." The pathological correlate of this imaging sign has been speculative. Herein we correlate the underlying histopathology to the MR feature of eccentric target sign in a patient with autopsy-proven HIV/AIDS-related cerebral toxoplasmosis. The central enhancing core of the target seen on MRI was produced by a leash of inflamed vessels extending down the length of the sulcus that was surrounded by concentric zones of necrosis and a wall composed of histiocytes and proliferating blood vessels, with impaired permeability producing the peripheral enhancing rim.

Fig.1

Post-contrast T1-weighted coronal image shows characteristic eccentric target sign of cerebral toxoplasmosis in right superior frontal gyrus (a, arrow). T2 weighted image highlights alternating hyper and hypointense zones (b, arrow) with marked perilesional edema. Corresponding coronal slice of the brain highlights organizing abscess enclosing a sulcus (c, arrow) with surrounding edema. Note vessels traversing the sulcus. Another similar lesion is noted in superior parietal lobule (d, arrow) with close up view and whole mount preparation (e) showing a curved vessel (e, arrow) passing through the sulcus encircled by the Toxoplasma abscess. [e: Masson trichrome x 8]

Fig.2

Lesion in right superior frontal gyrus (a) on histology shows an organizing abscess surrounding the depth of sulcus with concentric zones (b). The center has a vessel traversing the sulcus (asterix) bordered by zone of necrosis with surrounding edematous inflamed wall reflected in whole mount preparation (b,c). Thickened sclerosed vessels are evident traversing the sulcus (d, asterix) and thrombosed vessels in the necrotic center (e). Concentric bands of histiocytic aggregates are seen bordering the necrotic zone and the inflamed wall of the abscess highlighted by CD68 immunostaining (f,g). Leash of vessels forming the wall (h, arrowheads) corresponding to ring enhancement on imaging is highlighted by immunostaining to factor VIII related antigen of vascular endothelium. These vessels are inflamed and thickened (i) with tachyzoites of T gondii surrounding it (j, arrow head). [b:HEx4, c:Masson trichromex4, d:Masson trichrome xObj.10, e:HExObj.20, f:CD68x4, g: CD68xObj.20, h:Immunostain to Factor VIII related antigenx4, i:HExObj.20, j:Immunostain for T gondii xObj.20] (n-necrosis, w-wall, v-vessel)