Assessment of response to induction therapy in acute myeloid leukemia using F-18 FLT PET

Objectives Assessment of treatment response in acute myeloid leukemia (AML) is very important to determine the next treatment. NCCN guideline recommends assessing the treatment response via bone marrow examination approximately 7-10 days after completion of induction chemotherapy. However, the possibility of sampling error and risks of infection and bleeding are the limitation of bone marrow biopsy. F-18 fluorodeoxythymidine (FLT) is a radiopharmaceutical for PET, reflecting the proliferation of the cell. There have been several studies that reported F-18 FLT PET could measure health and proliferation of the bone marrow. The aim of this study is to evaluate if F-18 FLT PET/CT is suitable for early response assessment of induction chemotherapy in AML patients.

Methods Ten AML patients (7 male, 3 female, age 19-73 years) who received induction chemotherapy with the intent of achieving a complete remission were enrolled. All patients performed F-18 FLT PET/CT 1-4 days after completion of induction therapy. FLT PET scans were visually analyzed, according to the intensity of bone marrow uptake as well as the degree of bone marrow expansion. Cases with FLT uptake in the proximal fermora and humeri or with higher bone marrow activity than liver activity were interpreted as positive for residual disease. Four to six weeks after induction chemotherapy, a follow-up bone marrow biopsy was performed to establish clinical response. FLT PET response assessment results were correlated with clinical response.

Results Nine out of 10 AML patients achieved clinical complete remission or showed no evidence of leukemia. Of 5 patients who showed no significant FLT uptake in the proximal fermora and humeri, 4 patients were clinically classified as complete remission and a patient showed marrow hypoplasia without evidence of leukemia. In the other 5 patients, FLT PET scans showed intense bone marrow uptake and bone marrow expansion to proximal femoral and humeri. Among these patients, only one patient exhibited residual disease. Of the 4 false positive cases, one presented histiocytic hyperplasia with hemophagocytosis in the bone marrow.

Conclusions Although there were some false positive cases, FLT PET imaging after induction chemotherapy showed high negative predictive values, and may serve as an early biomarker of treatment response in AML. Further study will be needed to evaluate the clinical utility of FLT PET in predicting AML relapse.