Role of 18F-FDG PET/CT in detection of disease burden and response assessment in patients with myeloid sarcoma

Objectives: Bone marrow or medullary involvement is the most common presentation of acute myeloid leukemia (AML). As per an estimate , extramedullary involvement is seen in around 27% of patients. Though bone marrow biopsy is the investigation of choice for the diagnosis as well as response assessment in patients with AML, extra-medullary involvement will be missed by this

Methods: For extra-medullary involvement, NCCN guidelines recommend 18F-Fluorodeoxyglucose(FDG) Positron Emission Tomography/Computed Tomography (PET/CT). Myeloid sarcomas(MS) are a type of extra-medullary involvement, which are seen in 2-8% of patients with AML and are associated with poorer prognosis and shorter survival. In this study, we evaluated the role of 18F-FDG PET/CT in quantification of disease burden and in the assessment of response to chemotherapy in this particular sub-group. Material and Methods Twelve patients with histopathologically proven MS were included in the study with the mean age of 9.6+5.2 years and female/male ratio of 1/5. Patients underwent baseline whole body 18F-FDG PET/CT scan on Biograph mCT 64 slice PET/CT, after 60 minutes of injecting 5 MBq/kg of 18F-FDG intravenously. The scans were analyzed by 2 experienced nuclear medicine physicians. Lesions which were histopathologically proven MS were counted separately as primary lesions and other lesions were grouped separately as lymphnodal involvement, soft tissue lesions, bony lesions, CNS, spleen, liver, sinus and lung parenchyma involvement. Maximum standardized uptake values (SUVmax) were evaluated for all lesions in both scans using isocontour region of interest (ROI) with threshold 2.5 SUVmax. Results There were total 69 lesions in 12 patients out of which 14 were histopathologically proven primary lesions. Mean SUVmax for these 14 primary lesions was 6.44±3.89. Of these 14 lesions 10 were in orbital region which was the most common site of involvement. Other sites of involvement were breast (n=1), left maxillary sinus (n=1), right infra-temporal mass (n=1) and mandible (n=1). Fifty five additional lesions were identified in the initial scan with mean SUVmax 4.61±3.11. These included 28 lymphnodes, 14 soft tissue thickening(in costal pleura, paravertible, prevertible and presacral soft tissue thickening) , 3 bone lesions, 3 CNS involvement, 2 spleen, 2 sinuses, 2 lung parenchyma and 1 liver lesion with mean SUVmax 4.26±2.67, 5.61±4.31, 4.50±1.21, 4.78±2.36, 6.58±5.76, 3.33±0.7, 1.87±0.40 and 4.16 respectively. Follow up scan was done in 6 patients after completion of chemotherapy. Four patients had complete metabolic response while in 2 patients orbital lesions remained. 1 patient died due to brain hemorrhage post baseline scan. Conclusion MS are usually associated with significant undiagnosed extra-medullary disease. Thus, 18F-FDG PET/CT should be routinely considered for estimation of actual disease burden in these patients. Also, this modality can serve as a basis for the treatment response assessment in these patients after completion of therapy.