B5-02: Radiotherapy volume delineation with PET in lung cancer may be less useful than foreseen

Abstract

PET may alter 50% of the radiation gross tumor volume (GTV) delineation in comparison with CT targeting in patients with NSCLC. Mathematical algorithms (such as the 40% threshold) have been used to guide the delineation of the GTV without correlation of PET with pathology. Our study had two aims: a) to evaluate/confirm the impact of FDG-PET/CT on the GTV delineation by comparison with CT without using any specific mathematical algorithm and b) to correlate the CT and PET/CT stages with pathological findings. Thirty-two patients seen between May/2004 and May/2005 with histologically proven NSCLC with tissue from both the mediastinum and primary lung tumor and with pre-treatment chest-CT and PET/CT were studied. For each patient two data sets with theoretical GTVs were contoured by the same radiation oncologist as if they were to receive curative radiation treatment. The 1st contour was done based on all available clinical information and the chest-CT only. The 2nd contour, done separately, included the fused PET/CT data. Two major differences between the GTV contours were defined: 1) when there was tumor and/or nodal regions that were included as GTV within one data but not the other, and 2) when the difference between the GTVs was > 30%. Each patient was staged three times using the TNM system: one based on the CT scan only, a second based on the PET/CT, and the third based on the pathologic findings. PET altered the TNM stage in 14/32 patients (44%) compared to CT, but only 6 of these 14 alterations were confirmed to be accurate by pathology. Pathology altered the CT and PET-TNM stages in 69% and 53% of the cases, respectively. Concerning the target delineation, PET altered the GTV in 18/32 (56%) cases compared to CT: 16 had differences in the GTV volume >30% (with or without changes in nodal status) and 2 had changes in the nodal status but not in the GTV volume. In these 18 cases with altered GTV, 12 had a decrease (range: 20%-100%) and 6 an increase (4%-237%) of the initial volume. Fused PET/CT altered the contour of GTV in over 50% of patients by comparison with CT targeting, in agreement with previously published data, even without any mathematical algorithm. However, correlation with pathologic findings showed that CT and PET TNM-stage were incorrect in 69% and 53% of these cases, respectively. As has been shown, PET is better than CT to evaluate the extent of the disease, but results of PET should ideally be confirmed by pathology. With the knowledge we have today it is impossible to accurately distinguish cancer from non-cancer soft tissue (such as atelectasis) based on PET. This factor and the lack of correlation with pathology make it difficult to determine the meaning of alterations in volume shown in most publications. Whether FDG-PET/CT-based radiation planning will improve outcome or quality of life for NSCLC patients remains to be determined. (For this study images speak better than words or numbers).