CT and MR imaging of the "target sign" in metastatic brain disease

Brain targets: can you believe your own eyes?

The unquestionable advantages provided by modern neuroimaging techniques have recently led some to question the duty of the neurologist, traditionally struggling first and foremost to establish the semeiotic localization of brain lesions and only then to interpret them. The present brief report of six clinical patients who came recently to our attention aims to emphasize that the interpretation of neuroimaging results always requires integration with anamnestic, clinical and laboratory data, together with knowledge of nosography and the literature. The solutions of the reported cases always originated from close interaction between the neurologist and the neuroradiologist, based on the initial diagnostic uncertainty linked to the finding of isolated or multiple brain target or ring lesions, too often considered paradigmatic examples of the pathognomonic role of neuroimaging.

Metastatic adenocarcinoma mimicking ‘target sign’ of cerebral tuberculosis

We present a 74-year-old male ex-smoker presenting with a 6-week history of personality change, confusion and headache. Magnetic resonance imaging revealed multiple supratentorial and infratentorial parenchymal masses, predominately in the frontal and parietal lobe white matter. A thin enhancing halo was demonstrated with central low signal intensity on T(1)-and T(2)-weighted imaging compatible with calcification. A tiny extra-axial lesion was also noted near the right cerebellopontine angle. Computed tomography (CT) scan confirmed the finding of a 'target' lesion with a central core of calcification and a ring of enhancement. The 'target sign' of intracerebral tuberculomata was first described in 1979 and reported to be pathognomic for this diagnosis in 1988. However, cerebral tuberculosis was considered unlikely clinically because the patient had recently completed a 12-month course of therapy for Mycobacterium avium complex respiratory infection with agents also active against Mycobacterium tuberculosis. He was afebrile and blood tests did not support an inflammatory process. Subsequent histopathology demonstrated metastatic papillary adenocarcinoma and immunohistochemical studies revealed the origin to be that of primary lung carcinoma. A spiculated pulmonary nodule was seen on CT scan but previous bronchoscopy failed to demonstrate malignant cells. In summary, the 'target sign' is a non-specific radiologic finding but most commonly indicates cerebral tuberculoma or metastatic adenocarcinoma in the appropriate clinical context.