WE‐A‐217A‐02: The Impact of Fluorodeoxyglucose Positron Emission Tomography (FDG‐PET) Variability on the Assessment of Treatment Response in Head‐And‐Neck Cancer Patients Undergoing Chemoradiotherapy

Abstract

Purpose: To compare intrinsic, temporal variability of tumor glucose metabolism versus early treatment‐induced change. This assessment is necessary to use changes in imaging parameters during therapy to properly segregate metabolic responders from non‐responders Methods: As part of an IRB‐approved study, 12 patients underwent 2 baseline PET scans (mean separation: 11 days) prior to chemoradiotherapy (70 Gy, 2 Gy/fraction). The intra‐RT PET scan was acquired at 10‐20 Gy (mean 14 Gy) into therapy. For each patient, the two baseline scans were analyzed using Bland‐Altman analysis to determine the PET repeatability coefficient as a function of average baseline voxel SUV (repeatability coefficient for any voxel is the treatment‐induced SUV change from the average baseline value required for the change to be deemed significantly higher than baseline variation). Using the repeatability coefficient, the intra‐treatment PET nodal/GTV volume was divided into 3 groups of voxels: significantly reduced SUV, no significant difference, significantly increased SUV. A volume difference metric, defined as the difference in the percentages of node/GTV voxels with significantly decreased and increased SUV, was matched against each patient's post‐RT assessment of residual/no‐residual disease. Results: The PET repeatability coefficient decreased from 49–17% as the baseline voxel SUV increased from 2–12. The repeatability coefficient identified large volumes of the nodes (mean:52;std:35%) and GTV (mean:49;std:26%) as not having significant change. One patient had clinically‐assessed residual disease. The node/GTV volume metric for this patient was positive (22%/46%), compared to an average node/GTV volume metric of −36%/– 40% for patients without residual disease. Among the patients with no residual disease, only one had a positive nodal volume metric and one other had a positive GTV volume metric. Conclusions: The repeatability coefficient can be used in future head‐and‐neck studies with only one baseline PET. A positive volume metric on an early intra‐treatment scan may identify patients with residual disease post‐RT.