Primary Central Nervous System Lymphomas

CNS lymphomas may be either primary or secondary Secondary involvement occurs in 25-30% of non-Hodgkin's lymphomas and is exceptional in Hodgkin's disease. Secondary lymphomas tend to infiltrate the leptomeninges and spare the parenchyma, while primary lymphoma typically presents deep in the brain parenchyma, sparing the leptomeninges [49, 50]. In immunocompetent patients, primary CNS lymphomas (PCNSL) are generally non-Hodgkin's lymphomas of germinal B-cell origin [51], and arise and remain restricted to the CNS in more than 85 % of cases. They constitute between 1 % and 6% of all malignant tumours of the CNS and 3-5% of all extranodal non-Hodgkin's lymphomas [49,50]. PCNSL extensively infiltrate the brain, and individual tumour cells are seen in otherwise normal brain with intact blood-brain barrier because no neovascularization accompanies this infil-trative disease [49, 50]. The most frequent location of PCNSL is the cerebral hemispheres, basal ganglia and thalamus, and less frequently the corpus callosum and choroid plexus [52]. On conventional MRI, lymphomas are characterized by prominent contrast enhancement, and reappearance of contrast-enhancing lesion after treatment is usually a sign of recurrence. T2 and FLAIR images reveal homogeneous areas iso- or hypointense to cortex [52]. However, radiological diagnosis of PCNL remains a challenge because MR imaging findings can be similar to those of other intracranial tumours or demyelinating lesions [52, 53]. 'H-MRSI, DWI and PWI have been applied in combination with conventional neurimaging to ameliorate the differential diagnosis. In contrast-enhancing areas, 1H-MRSI generally reveals spectra with a marked increase of Cho and LL, and a conspicuous decrease of Cr and NAA (Fig. 18.8), or an absence of metabolite signals [53]. DWI generally shows an ADC lower than or similar to normal white matter [53], and rCBV is higher than contralateral normal ROIs [18, 54]. The ability of PWI to detect tumour angiogenesis can be useful in differentiating high-grade gliomas from PCNL. Hartmann et al. [54] reported a maximum rCBV in PCNSL significantly lower than in glioblastomas. These results are in agree ment with histopathological evidence showing low neovascularization in lymphomas, although vascular abnormalities and tumour invasion of endothelial cells and vessel lumen are often seen [55]. In two cases of PCNSL, in addition to ROIs with a „tumour" pattern, we could identify ROIs with an „oedema" multipara-

metric pattern. In one case, according to the neuro-pathological findings showing diffuse tumour infiltration in tissues surrounding the PCNSL mass [55], we identified ROIs with an „oedema/tumour" multipara-metric pattern (Fig. 18.8).

Fig. 18.8. Contrast-enhanced T1-weighted (a), T2-weighted (b) and FLAIR (e) images, ADC (c), rCBV (d), Cho (f ),NAA (g), Cr (h) and LL (i) maps, and proton MR spectra (1 -9) from selected ROIs in a 73-year-old man with a primary CNS lymphoma in the left parietal lobe. ROIs are tumour mass (1,2), ROIs with „tumour/oedema" (3) and „oedema" (4,5) pattern, homolateral (6) and contralateral (7-9) normal ROIs. Note: the high levels of Cho and LL in the tumour mass; the abnormal Cho/NAA ratio and the presence of LL in the „tumour/oedema" ROI; the normal Cho/NAA ratio in the „oedema" ROIs; the values of ADC and rCBV in the „tumour/oedema" ROI, which are respectively, lower and higher than in the „oedema" ROIs

Fig. 18.8. Contrast-enhanced T1-weighted (a), T2-weighted (b) and FLAIR (e) images, ADC (c), rCBV (d), Cho (f ),NAA (g), Cr (h) and LL (i) maps, and proton MR spectra (1 -9) from selected ROIs in a 73-year-old man with a primary CNS lymphoma in the left parietal lobe. ROIs are tumour mass (1,2), ROIs with „tumour/oedema" (3) and „oedema" (4,5) pattern, homolateral (6) and contralateral (7-9) normal ROIs. Note: the high levels of Cho and LL in the tumour mass; the abnormal Cho/NAA ratio and the presence of LL in the „tumour/oedema" ROI; the normal Cho/NAA ratio in the „oedema" ROIs; the values of ADC and rCBV in the „tumour/oedema" ROI, which are respectively, lower and higher than in the „oedema" ROIs

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