Update in Cranio-cerebral Lymphomatous Imaging Diagnosis.

New MR techniques: diffusion weighted imaging (DWI) and MR spectroscopy (MRS) are increasingly used in clinical radiologic practice and may help to differentiate CNS lymphomas from other lesions of the brain (1,2). Diffusion-weighted MRI is a powerful tool in characterization of brain neoplasms. DWI measures the diffusion of water molecules in biologic tissues; diffusion within the tumor is considered a marker of tumor cellularity because intact cells constitute a barrier to water diffusion (2). Tumor cellularity and tumor grade have been correlated with ADC values (2). Because CNS lymphomas are highly cellular tumors, water diffusion is often restricted, making them appear hyperintense on DWI and hypointense on ADC maps (Figure1). This characteristic is shared by acute ischemic stroke, the central necrosis of brain abscesses, the solid portion of high-grade gliomas, and some metastases. However, PCNSL lesions have often more restricted diffusion and lower ADC values than high-grade gliomas and metastases (1). A recent study showed that pretherapeutic ADC tumor measurements within contrast-enhancing regions were predictive of clinical outcome in patients with PCNSL (3). Thus, repeated ADC measurements may be used as T he incidence of cranio-cerebral involvements by malignant lymphoma has increased significantly in last 20 years. Therefore, interest in the pretreatment diagnosis and post-treatment surveillance for residual disease using ima ging and imaging biomarkers is increasing. Lymphoma of the central nervous system (CNS) consists of 2 major subtypes: secondary CNS involvement by systemic lymphoma (the most common) and primary central nervous system lymphoma (PCNSL), in which the lymphoma is restricted to the brain, leptomeninges, spinal cord, or eyes, without evidence of it outside the CNS at primary diagnosis. Early diag nosis of CNS lymphoma is crucial for proper management in both immunocompetent and immunocompromised individuals (1). Computed tomography (CT) and magnetic resonance imaging (MRI) represent the most important imaging modalities in the evaluation of patients with brain tumors. Although CNS lymphomas may have characteristic imaging findings on traditional CT and MRI, none of these will unequivocally differentiate CNS lymphoma from other neoplasms (eg, metastases, malignant gliomas, meningiomas) or non-neoplastic diseases (eg, multiple sclerosis, stroke, cerebral toxoplasmosis, pyogenic abscess) (1).