SU-GG-I-135: Average CT in Colorectal Cancer PET Studies

Abstract

Purpose: To assess the effect of average CT (ACT) used for attenuation correction of PET data on SUVmax and gross tumor volume (GTV) in liver lesions of colorectal cancer patients. Method and Materials: We studied 34 colorectal cancer patients with metastasis in the liver. A routine PET/CT scan was immediately followed by a cine CT scan of the liver region for ACT. Patients were free‐breathing during the helical CT (HCT) and ACT acquisitions. Both HCT and ACT were used for attenuation correction of the PET data. 59 liver lesions were included in the study and SUVmax for each lesion was measured on PETHCT and PETACT. Both PETdata sets were imported to Pinnacle3 7.6 treatment planning system. GTVs for PETimages were generated based on the SUV thresholds, normally used for assessing tumor response to therapy. Percentage volume change, centroid shift and concordance index were defined for each lesion. Results: 67% of the studies showed respiratory artifacts in PETHCT while 15% demonstrated a difference in SUVmax above 20%. The effect of misregistration between HCT and PET data was more evident for smaller lesions with the most pronounced case showing a 29% increase in SUVmax for PETACT, 246% increase in GTV, 2mm shift in GTV centroid location and concordance index value of 0.34. We analyzed correlation between the variations and the lesion size with a non‐paired t‐test at 0.05 p‐value level. The means of concordance indexes in two distributions of the lesions normalized to their size (less and greater than 5cm3) were shown to be significantly different (p=0.007). Conclusion: ACT is effective in improving the registration between the CT and PET data in PET/CT, which is important when using PETimages for treatment planning. Misregistration is more pronounced for small lesions and should be considered in the generation of the GTVs.