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:In many studies, red marrow dosimetry is approached with great caution, and a maximum absorbed dose of 2 Gy to the blood (as a red marrow surrogate) is generally accepted [20] for radioiodine treatment of diferentiated thyroid cancer.
Category: