Abstract
933
Objectives: Routinely used 18F-FDG (flurodeoxyglucose) is a glucose analog metabolic tracer which is not tumor specific, and hence is prone to false-positive results in metastatic workup. 18F-FLT (flurothymidine), a tracer for cell proliferation, shows higher specificity for proliferating tumor cells. We conducted a pilot study of 10 patients to demonstrate utility of FLT PET/CT scans in cases of lymphoma and lung cancer for differentiating malignant lesions from benign lesions and compared the results with FDG PET/CT scans.
Methods: 10 patients (8 biopsy proven cases of lymphoma and 2 biopsy proven cases of NSCLC) underwent FDG PET/CT, as either a part of pretreatment work up or followup cases. All these cases also underwent FLT PET/CT scan, followed by chemotherapy. FDG PET/CT and FLT PET/CT scans were repeated for response evaluation in all cases. To interpret FLT PET scan, lesions demonstrating radiotracer uptake substantially more than the mediastinal uptake were considered positive. Standardised uptake values of FDG and FLT were also used to compare pre and post therapy scans for positive lesions.
Results: Lymphoma (8 cases): 6/8 were pre-treatment lymphoma cases who demonstrated FDG uptake in 48 nodes, 1 stomach lesion, 2 splenic lesions and 2 brain lesions. FLT PET scan showed positive uptake in 35 nodes. Stomach, splenic and brain lesions were also positive on FLT scan. Post chemotherapy FDG scan and FLT scan showed response in stomach, brain and splenic lesions, however only prior FLT positive 35 nodes demonstrated response to treatment. FLT negative 13 nodes were status quo in post chemotherapy FDG and FLT scans suggesting benign etiology. 2/8 were follow up lymphoma cases. 1/2 had 2 FDG positive lung lesions which were negative on FLT scan. A follow up scan after 3 months of anti Koch’s treatment demonstrated overall decreased activity in lung lesions on FDG scan suggesting benign lesions. Another patient had a new FLT positive lesion in brain which did not show FDG avidity. Followup scans after 2 months demonstrated extensive recurrence in this patient on both FLT and FDG scans. 2 pre-treatment lung cancer cases showed positive primary lesions, mediastinal nodes (in both cases) and axillary nodes (in one case) on FDG PET scan. FLT PET scan was positive for all lesions in the first case, which showed partial response to chemotherapy on follow up scans. However in another case, FLT PET scan was positive only for the lung primary. The mediastinal and axillary nodes did not show FLT uptake. This case demonstrated good resonse post chemotherapy in the primary lesion on both FDG and FLT PET scans with no change in the mediastinal and axillary nodes suggesting possible benign etiology.
Conclusion: In this initial study, 18F-FLT radiotracer uptake correlated well with clinical outcome of the patients, successfully differentiating proliferative malignant lesions from other benign concomitant etiologies. Scan results demonstrated low grade to absent FLT tracer uptake in benign inflammatory/infective cells as compared to higher grade concentration in tumor cells.