Response Evaluation Following Radiation Therapy With ¹⁸F-FDG PET/CT: Common Variants of Radiation-Induced Changes and...

Radiation therapy (RT) is one of the cornerstones in cancer treatment and approximately half of all patients will receive some form of RT during the course of their cancer management. Response evaluation after RT and follow-up imaging with ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) can be complicated by RT-induced acute, chronic or consequential effects. There is a general consensus that ¹⁸F-FDG PET/CT for response evaluation should be delayed for 12 weeks after completing RT to minimize the risk of false-positive findings. Radiation-induced late side effects in normal tissue can take years to develop and eventually cause symptoms that on imaging can potentially mimic recurrent disease. Imaging findings in radiation induced injuries depend on the normal tissue included in the irradiated volume and the radiation therapy regime including the total dose delivered, dose per fraction and treatment schedule. The intent for radiation therapy should be taken in consideration when evaluating the response on imaging, that is palliative vs curative or neoadjuvant vs adjuvant RT. Imaging findings can further be distorted by altered anatomy and sequelae following surgery within the radiation field. An awareness of common PET/CT-induced changes/injuries is essential when interpreting ¹⁸F-FDG PET/CT as well as obtaining a complete medical history, as patients are occasionally scanned for an unrelated cause to previously RT treated malignancy. In addition, secondary malignancies due to carcinogenic effects of radiation exposure in long-term cancer survivors should not be overlooked. ¹⁸F-FDG PET/CT can be very useful in response evaluation and follow-up in patients treated with RT, however, variants and pitfalls are common and it is important to remember that radiation-induced injury is often a diagnosis of exclusion.