**4. Primary central nervous system lymphoma**

Primary central nervous system lymphoma (PCNSL) is a rare variant of extranodal non‐ Hodgkin lymphoma and affects about 1,000 people in the United States each year. Non‐ invasive diagnosis of PCNSL is of paramount importance given the dramatic benefits of chemotherapy in this tumor. Typical MR imaging features of PCNSLs are frequent periven‐ tricular locations, perilesional edema, well‐defined margin, and homogeneous and intense nodular enhancement.

#### **4.1. Diffusion‐weighted imaging**

PCNSLs tend to have a low ADC value because of high cellularity (**Figure 5**). Brain abscess has higher ADC values than lymphomas; thus, this feature can help in differentiating these two entities [15].

**Figure 5.** A case of lymphoma (A, B) revealing a low ADC value (C), low perfusion (D), and elevated levels of Lipids (E).

#### **4.2. Perfusion imaging**

delineation between white and gray. Contrast enhancement is absent [13]. In DWI, there is usually no restriction. Perfusion MR shows low or normal rCBV values correlating with the absence of vascular hyperplasia. MR spectroscopy reveals elevated Cho/NAA ratios and

**Figure 4.** Short‐echo MR proton spectrum shows an elevated myoinositol (mI) peak (at 3.55 ppm) in a case of gliomato‐

Primary central nervous system lymphoma (PCNSL) is a rare variant of extranodal non‐ Hodgkin lymphoma and affects about 1,000 people in the United States each year. Non‐ invasive diagnosis of PCNSL is of paramount importance given the dramatic benefits of chemotherapy in this tumor. Typical MR imaging features of PCNSLs are frequent periven‐ tricular locations, perilesional edema, well‐defined margin, and homogeneous and intense

PCNSLs tend to have a low ADC value because of high cellularity (**Figure 5**). Brain abscess has higher ADC values than lymphomas; thus, this feature can help in differentiating these

marked elevation of myoinositol [14] (**Figure 4**).

170 Neurooncology - Newer Developments

**4. Primary central nervous system lymphoma**

sis cerebri.

nodular enhancement.

two entities [15].

**4.1. Diffusion‐weighted imaging**

PCNSL has low rCBV compared with that of high‐grade gliomas and metastasis; however, overlapping values may exist (**Figure 5**). An important clue is that PCNSL demonstrates a significant increase in signal intensity above the baseline due to massive leakage of contrast media into the interstitial space contrary to high‐grade gliomas [16]. Lymphoma tends to demonstrate higher rCBV compared with toxoplasmosis [15].
