Timing of PET/CT Exam After Therapy

Special concern is given to the timing of an exam (when to do the study) for patients who have recently undergone chemotherapy, surgery and/or radiation therapy – as each of these treatments can cause significant FDG uptake, interfering in scan interpretation.

Chemotherapy:

Delaying the PET/CT scan for a few weeks after chemotherapy or radiation therapy decreases the potential false negative resulting from “Metabolic Stunning” of active malignancy [Case 1] that can occur in the first few weeks after these therapies. It also reduces the chance of the false positive “Flare Phenomenon” [Case 2] that can occur with some lesions immediately after chemotherapy.

Surgery:

Surgery typically results in an inflammatory reaction, which can demonstrate significant metabolic activity, resulting in false positive results. Most FDG-avid post-surgical inflammation resolves within 8 weeks. The duration of this inflammation, however, can be extremely variable (e.g. post-inguinal herniorrhaphy and post-pleurodesis uptake can be intensely-avid for many years). [Case 3] [Case 4]

Radiation Therapy:

Radiation therapy typically results in an acute inflammatory reaction that is often intensely FDG-avid.  These changes are most pronounced when they involve the pulmonary parenchyma.

Acute radiation pneumonitis typically presents as ill-defined parenchymal air-space disease, which may be intensely FDG-avid.  It typically occurs between 4 and 12 weeks after therapy and can persist for many months (discussed here).

As these radiation changes can persist for many months, it is generally recommended to delay post-radiation PET/CT scanning for “as long as possible” – with 3 months being a reasonable goal. [Case 5]

“The Rule of Three”:

As a general recommendation, we propose delaying post-therapy PET/CT imaging – when possible – according to “The Rule of Three”: