- Pathologic diagnosis of lymphoma in central nervous system (CNS) must be confirmed but no previous treatment is allowed. In participating institutions where CNS biopsies cannot be obtained, the patient may be considered eligible if space-occupying lesions have been demonstrated on computerized tomography or magnetic resonance imaging with negative titers for toxoplasmosis or negative response to empiric therapy for intracerebral toxoplasmosis and negative workup for other infectious etiology of CNS disease.