In the past, PCNSL has been reported to represent about 1-2% of all NHL and 5% of all brain tumors.1 However, these numbers do not reflect the rising incidence of this condition in the past three decades. Much of this rise has been due to the AIDS epidemic, but a two- to threefold rise has also been seen in the immunocompetent population.2 Some population-based studies suggest this increase is a result of improved detection due to the widespread availability of computed tomographic (CT) scans and magnetic resonance imaging (MRI) and due to better diagnostic techniques, such as stereotactic biopsy. However, a recent Surveillance, Epidemiology and End Results (SEER) analysis refuted this hypothesis.3 It is reported that the age-adjusted incidence increased from 0.15 to 0.48 per 100,000 person-years from 1973 to 1997. This increase was seen in all age groups and both genders and outpaced the increased incidence of systemic NHL for this period sixfold. Furthermore, if this increase were due to ascertainment bias, there would have been an increased rate of glioma over time, which was not seen. Although the rate of increase of PCNSL has lev eled off in the last decade, it appears that the true incidence is increasing.

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