Variation of FDG-PET tumor metabolism during external radiotherapy (ERT) in patients with non small cell lung cancer (NSCLC)

Objectives To assess the feasibility of serial FDG-PET imaging during ERT and to determine when the variation of FDG uptake is maximal.

Methods 9 NSCLC patients with a significant FDG-PET tumor activity (versus background level), candidate for curative ERT alone (RT; n=3) (60 to 70 Gy, 2 Gy per fraction, 5 fractions per week) or ERT associated to chemotherapy (CRT; n=6) were prospectively investigated. 5 or 6 PET/CT (PET_n, n=1-6) per patient were performed on Siemens Biograph. For each exam, a 15min thoracic respiratory gated (RG) acquisition was started 60±5mn after the injection of 3.5 MBq/kg of FDG followed by a standard 5min ungated (STD) thoracic acquisition. PET_2-6 were performed during RT every 14 Gy. Tumor metabolism was assessed by the variation of SUV_max,PETn compared to SUV_max,PET1. We also compared the tumor volumes on RG and STD acquisitions applying a 40% threshold of SUV_max.

Results Tumor volumes were not different between STD or RG acquisitions because the tumor localizations were slightly influenced by respiratory movements. At 14 Gy, SUV_max increased in 4/9 patients (2/3 RT and 2/6 CRT). At 28 and 42 Gy, mean decrease of SUV_max was 42±20% and 39±21%. An increase of SUV_max was observed in a single CRT patient.

Conclusions FDG-PET images can be analyzed during thoracic RT, either alone or associated to chemotherapy. A single FDG-PET acquisition after 42 Gy appears to be able to separate the patients according to FDG-PET response during RT. A longer follow-up is necessary to assess the potential predictive value of FDG-PET response during RT.