Importance of timing of PET scan after therapy in oncology

13014 Background: Growth factors are routine after systemic chemotherapy both as primary and secondary prophylaxis. Similarly PET scan is increasingly used for initial staging and follow-up of response. Timing of both is important since false positive results are noted due to high sensitivity of the test. Methods: Records of four patients with super positive PET scan with diffuse uptake in the axial skeleton were analyzed for the timing of administration of white cell growth factor before imaging and confirmation of bone finding with a follow- up bone scan to rule out mets. Results: Two patients were treated for stage IIIB/IV non-small cell lung cancer, one patient with breast cancer was receiving adjuvant treatment for stage IIIA breast cancer, and the last patient was treated for metastatic pancreatic cancer. All had negative scans for bone mets before the start of treatment. PET scan was done for staging in one patient and evaluation of abnormal CT scan in another, and follow-up of response in two patients. It was inadvertently obtained 10–14 days after the filgrastim/pefilgrastim administration. Based on the unusual uptake in bony skeleton, bone scan was ordered to rule out bone mets in all four patients. Since their bone scans were negative for metastatic disease they were continued on their treatment plan as scheduled. Conclusions: PET scan has been sparingly used until 1990’s due to high cost. Currently Medicare has approved it for wider indications. PET scan in general has high sensitivity and low specificity and false positive results are more common from metabolically active infection and inflammation. Granulocyte colony stimulating factor used with cancer treatments can increase the FDG uptake in PET scan. Diffuse increased uptake in bone marrow by PET scan can be seen in reactive marrow following growth factor therapy. Usually FDG uptake is modest with SUV of less than 3, uniform and diffuse if due to growth factor stimulation in contrast to greater intensity with SUV around 6 with non uniform distribution. Awareness of this pitfalls associated with PET scan allows image interpretation accurately. PET-CT fusion may increase the diagnostic specificity. However timing of the scan in relation to growth factor administration may also help to avoid the unnecessary anxiety and further follow up testing which adds to the health care costs. No significant financial relationships to disclose.