Some doctors believe it is beneficial to do a PET early, 6-8 weeks, instead of later, in case surgery is needed, and or additional chemo for unresponsive tumors, and to keep in line with a "total treatment package" time frame of 12 weeks or less, which is more optimal, than doing it later. They say by 8 weeks, a PET is 90 percent accurate. Less than 6 weeks is unacceptable. Three months is more optional, and at 4 months is even better, forgot the numbers.