18F-FDG PET Definition of Gross Tumor Volume for Radiotherapy of Non-Small Cell Lung Cancer: Is a Single Standardized Uptake Value Threshold Approach Appropriate?

摘要

id="p-1">PET with 18F-FDG has been used in radiation treatment planning for non-small cell lung cancer (NSCLC). Thresholds of 15%-50% the maximum standardized uptake value (SUV_max) have been used for gross tumor volume (GTV) delineation by PET (PET_GTV), with 40% being the most commonly used value. Recent studies indicated that 15%-20% may be more appropriate. The purposes of this study were to determine which threshold generates the best volumetric match to GTV delineation by CT (CT_GTV) for peripheral NSCLC and to determine whether that threshold can be generalized to tumors of various sizes. >Methods: Data for patients who had peripheral NSCLC with well-defined borders on CT and SUV_max of greater than 2.5 were reviewed. PET/CT datasets were reviewed, and a volume of interest was determined to represent the GTV. The CT_GTV was delineated by using standard lung windows and reviewed by a radiation oncologist. The PET_GTV was delineated automatically by use of various percentages of the SUV_max. The PET_GTV-to-CT_GTV ratios were compared at various thresholds, and a ratio of 1 was considered the best match, or the optimal threshold. >Results: Twenty peripheral NSCLCs with volumes easily defined on CT were evaluated. The SUV_max (mean ?± SD) was 12 ?± 8, and the mean CT_GTV was 198 cm3 (97.5% confidence interval, 5-1,008). The SUV_max were 16 ?± 5, 13 ?± 9, and 3.0 ?± 0.4 for tumors measuring greater than 5 cm, 3-5 cm, and less than 3 cm, respectively. The optimal thresholds (mean ?± SD) for the best match were 15% ?± 6% for tumors measuring greater than 5 cm, 24% ?± 9% for tumors measuring 3-5 cm, 42% ?± 2% for tumors measuring less than 3 cm, and 24% ?± 13% for all tumors. The PET_GTV at the 40% and 20% thresholds underestimated the CT_GTV for 16 of 20 and 14 of 20 lesions, respectively. The mean difference in the volumes (PET_GTV minus CT_GTV [PET_GTV - CT_GTV]) at the 20% threshold was 79 cm3 (97.5% confidence interval, a?’922 to 178). The PET_GTV at the 20% threshold overestimated the CT_GTV for all 4 tumors measuring less than 3 cm and underestimated the CT_GTV for all 6 tumors measuring greater than 5 cm. The CT_GTV was inversely correlated with the PET_GTV - CT_GTV at the 20% threshold (R2 = 0.90, P 0.0001). The optimal threshold was inversely correlated with the CT_GTV (R2 = 0.79, P 0.0001). >Conclusion: No single threshold delineating the PET_GTV provides accurate volume definition, compared with that provided by the CT_GTV, for the majority of NSCLCs. The strong correlation of the optimal threshold with the CT_GTV warrants further investigation.