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?

Abstract

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 (SUVmax) have been used for gross tumor volume (GTV) delineation by PET (PETGTV), 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 (CTGTV) 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 SUVmax of greater than 2.5 were reviewed. PET/CT datasets were reviewed, and a volume of interest was determined to represent the GTV. The CTGTV was delineated by using standard lung windows and reviewed by a radiation oncologist. The PETGTV was delineated automatically by use of various percentages of the SUVmax. The PETGTV-to-CTGTV 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 SUVmax (mean ± SD) was 12 ± 8, and the mean CTGTV was 198 cm3 (97.5% confidence interval, 5-1,008). The SUVmax 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 PETGTV at the 40% and 20% thresholds underestimated the CTGTV for 16 of 20 and 14 of 20 lesions, respectively. The mean difference in the volumes (PETGTV minus CTGTV [PETGTV - CTGTV]) at the 20% threshold was 79 cm3 (97.5% confidence interval, 2922 to 178). The PETGTV at the 20% threshold overestimated the CTGTV for all 4 tumors measuring less than 3 cm and underestimated the CTGTV for all 6 tumors measuring greater than 5 cm. The CTGTV was inversely correlated with the PETGTV - CTGTV at the 20% threshold (R2= 0.90, P < 0.0001). The optimal threshold was inversely correlated with the CTGTV (R2 = 0.79, P < 0.0001). Conclusion: No single threshold delineating the PETGTV provides accurate volume definition, compared with that provided by the CTGTV, for the majority of NSCLCs. The strong correlation of the optimal threshold with the CTGTV warrants further investigation. Copyright © 2006 by the Society of Nuclear Medicine, Inc.