RESUMO
AIMS: Radiotherapy for non-small cell lung cancer (NSCLC) increasingly utilises fluorodeoxyglucose positron emission tomography/computed tomography (PET/CT) fusion. However, it is unknown whether a PET/CT scan conducted in the treatment position results in more accurate registration to the radiotherapy planning CT (rCT) than a diagnostic PET/CT scan. The aim of this study was to compare the accuracy of registration of the CT components of the planning PET/CT scan (pCT) and diagnostic PET/CT scan (dCT) scan with the rCT. MATERIALS AND METHODS: Ten patients with stage I-III NSCLC underwent an rCT immediately followed by a planning PET/CT scan, both carried out with arms placed above the head and immobilisation in the treatment position. All previously underwent a diagnostic FDG PET/CT, which was carried out with the arms above the head, but without custom immobilisation. dCT and pCT were registered to the rCT using a rigid body mutual information algorithm. Four observers identified 12 anatomical points on each scan and differences in their absolute location were analysed. RESULTS: At the carina, the mean absolute error (MAE) for pCT-rCT compared with dCT-rCT was 4.37 versus 5.73 mm (P=0.028). However, there was no significant difference in the root mean squared error (RMSE) for that point. There were no significant differences in MAE or RMSE for all other anatomical points. The MAE for all points was 4.11 versus 4.15 mm (P=NS) and RMSE was 4.40 versus 4.48 mm for pCT-rCT compared with dCT-rCT (P=NS). CONCLUSIONS: There is an average of 4mm of misregistration when registering the CT components of PET/CT scans to the rCT for NSCLC. Using a rigid registration technique, the registration of a diagnostic PET/CT is as good as the registration of a planning PET/CT.