We estimate that 30% to 40% of radiation treatment plans for cancer patients are changed when positron emission tomography (PET) scan findings are factored into the plan. The most frequent changes are upstaging of disease and the finding of new distant metastases. When a tumor demonstrates high tracer uptake, the PET scan has high accuracy in staging and in determining recurrent disease. However, the absence of tracer uptake cannot exclude residual dormant disease. Following radiation therapy, a PET scan should be delayed at least 6 weeks and preferably 3 to 4 months so that inflammation within the radiation field can dissipate. PET has resulted in changes in the gross tumor volume; however, the process remains subjective.
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