Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Systemic nonHodgkin lymphoma is estimated to affect 43 000 persons per year in the United States; ocular involvement is uncommon, representing approximately 2% of all extranodal lymphoma cases.(1-20) Lymphoproliferative lesions include a wide range of diseases such as benign reactive lymphoid hyperplasia, atypical lymphoid hyperplasia, and lymphoma, based on varying degrees of histopathologic and immunophenotypic features.
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