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:The physical rationale of the treatment was tested at select institutes between the 1970s and 1990s.(9,10) At the same time, more reliable dose constraints for normal tissues at the skull base for protons rather than the doses known to be acceptable for photon treatment were explored.
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