The differential diagnosis of ring-enhancing lesions largely depends on the immune status of the patient. In the immunocompetent host, tumors -- both primary and metastatic -- and pyogenic abscesses remain the most likely diagnoses; abscesses caused by atypical organisms and demyelinating disease must also be considered. In the immunocompromised host, the leading diagnoses are toxoplasmosis and primary CNS lymphoma. Furthermore, these patients are at risk for abscesses, from both pyogenic and atypical organisms, and tumors. Tuberculous brain abscess should be considered in endemic regions in both immunocompetent and immunocompromised hosts.
Attempts have been made to identify distinctive radiologic characteristics of ring-enhancing lesions. In general, abscesses are said to possess a thin, uniform ring, which is thinner on the medial border, and a smoother outer margin; satellite lesions are often present. By contrast, neoplasms are purported to have thicker, more irregular rims. Ring-enhancing lesions seen in demyelinating disease tend not to be perfect rings, but rather incomplete rings, hence the "open-ring sign." However, despite these attempts to correlate imaging features with specific underlying lesions, such lesions cannot be distinguished purely on the basis of the radiologic findings.
The different pathologies that can cause ring-enhancing lesions require very different treatment, the institution of which requires early diagnosis. This has led to the development of different strategies to permit noninvasive, early diagnosis.
Positron emission tomography (PET) can provide dynamic information regarding the metabolism of a lesion, which may be useful for differentiating tumors from abscesses, with a specificity and sensitivity above 90%. Tumors typically show increased metabolic activity in the center of the lesion, whereas abscesses do not. However, in high-grade neoplasms that often hold a necrotic center, the reduced metabolic activity in the center of the tumor can make it difficult to differentiate from the pattern found in abscesses.[3,4]
Since the 1980s, numerous reports have suggested that thallium-201 single photon emission computed tomography (Th-SPECT) is able to grade tumor malignancy.[5,6,7,8] In 1994, Ruiz and colleagues described 100% sensitivity and specificity of Th-SPECT in differentiating primary CNS lymphoma from toxoplasmosis. Despite initial enthusiasm, several studies since then have reported sensitivities and specificities as low as 60% and 71%.[6,7,8,9] Th-SPECT is still considered useful in this differentiation when in association with other tests, such as toxoplasmosis serology or CSF Epstein-Barr virus polymerase chain reaction.[6,7,8,9]
MR spectroscopy has led to identification of MR spectra patterns that appear specific to tumors and abscesses, and can help in the differentiation of these. Brain abscesses are characterized by reductions in choline, creatine, and N-acetyl-aspartate. Brain tumors, on the other hand, usually show elevated choline levels and reduced levels of creatine and N-acetyl-aspartate. These changes are not diagnostic, however. Abscesses also contain high lactate, succinate, acetate, alanine, valine, leucine, and isoleucine levels.[10,11,12] Amino acids and other small molecules, such as acetate, lactate, pyruvate, and succinate, are known metabolic end products of proteolysis of the microorganisms, or polymorphonuclear leukocytes in pus, or both. Among these, only lactate is a nonspecific marker, seen in many other neurologic diseases, including brain tumors. In fact, a succinate peak on MR spectroscopy, although not seen in all brain abscesses, is fairly specific for the diagnosis of intracranial infection rather than neoplasm because it was not seen in any brain tumors investigated. Acetate and pyruvate were only seen in conjunction with infection and not with tumors, as well. Thus, studies suggest that MR spectroscopy could accurately distinguish between brain tumors and bacterial abscesses; however, MR spectroscopy does not appear helpful in distinguishing parasitic or fungal infections from tumors.
Several reports have asserted that diffusion-weighted MRI imaging (DWI) can be helpful for distinguishing tumors and abscesses.[11,14,15] DWI is based on the random movement of water known as Brownian motion. Stationary water, unlike freely moving water, is depicted as high signal intensity on DWI, with a decreased signal on the corresponding apparent diffusion coefficient (ADC) maps. The more restricted the water motion is, the less is the value of the ADC. An abscess cavity usually demonstrates high signal on DWI with decreased ADC values, unlike necrotic tumor cavities, which demonstrate the opposite. The restricted diffusion is directly related to the presence of pus in the abscess cavity, likely associated with high cellularity and viscosity. Some studies have reported that abscesses are DWI bright/ADC dark, and that this is characteristic but not pathognomonic.[11,15]
MR perfusion is another technique that has been used in the noninvasive diagnosis of ring-enhancing lesions. The necrotic, avascular center of an abscess would be expected to have low flow, in contrast to a tumor, which is usually marked with hypervascularity and neovascularization. Several studies support the utility of this method, alone or in combination with others, for differentiating focal mass lesions.[16,17,18]
Notwithstanding these findings, the general consensus is that no noninvasive approach has emerged as being able to reliably discern whether a ring-enhancing lesion is tumor or abscess, and therefore early biopsy is generally considered essential for definitive diagnosis. The only exception to this rule is stated in the AAN guidelines for the management of patients with AIDS. In this case, only an isolated ring-enhancing lesion in the setting of negative toxoplasmosis serology requires early biopsy. Otherwise, a trial of pyrimethamine and sulfadiazine is considered appropriate for treatment of presumptive toxoplasmosis.
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Cite this: Case 11: A Young Woman With Ring-Enhancing Brain Lesions - Medscape - Jan 05, 2006.