Are pet scans accurate 8 weeks after radiation therapy to see if cancer has spread?

Generally, yes: Radiation can cause temporary increases in uptake on a pet scan (especially if the radiation is to the lung). Thus, it can sometimes be difficult to tell whether the primary tumor has changed much. That said, the radiation does not affect the ability to determine whether cancer has actually metastasized, or spread to different parts of the body like lymph nodes or liver.

Yes for spread: Since radiation does cause an inflammatory response that can increase the uptake on a pet scan a period of time must go by to allow that inflammation to decrease. This would not affect its ability to see tumors that have spread to other parts of the body. Depending upon the type of cancer eight weeks may not be enough time to determine if the tumor that was treated with radiation has been entirely destroyed. For head and neck cancers we often wait 12 weeks to determine its success.

No: I assume your doctor is scanning you to establish a baseline for future comparisons.