Journal of Thoracic Oncology:
Proffered Paper Abstracts: Session D7: Novel Therapeutics II, Thursday, September 6: Novel Therapeutics II, Thu, 12:30 - 14:15
1Department of Radiation Oncology (MAASTRO), Maastricht, The Netherlands 2Department of Radiation Oncology (MAASTRO), GROW, University Hospital, Maastricht, The Netherlands
In recent years, it has become clear that tumors are not homogeneous, also for radioresistance. The identification of areas of radioresistance within the same tumor is therefore of great interest, for it may allow selective boosting of these zones. This study therefore addressed two critical questions needed for escalating dose to specific areas within the tumor: 1. Where does the residual disease occur after treatment based on FDG PET imaging? 2. Does the high FDG uptake zone remain stable within the tumor during a course of radiotherapy?
1. Location of residual disease within the tumor. Six patients with inoperable NSCLC, stage I-III, treated with radical radiotherapy, showing residual disease on FDG-CT-PET scan 70 days post-treatment, were studied. The volume of residual disease was defined by the area with a Standardized Uptake Value (SUV) higher than mediastinal uptake. Overlap fractions (OF) were calculated between this residual disease volume and the original tumor, with several SUV threshold based auto-delineations on a FDG-CT-PET scan before radiotherapy.
2. Stability of the high FDG-uptake areas during radiotherapy. Twenty-three patients underwent repeated FDG-CT-PET scans before and one and two weeks after the start of radiotherapy. Tumors with volumes >70 cm3 (n=9) were used to assess heterogeneity in FDG-uptake. OF’s were used to demonstrate the stability of the high FDG uptake regions (50% SUV volume) during treatment.
1. Location of residual disease within the tumor. The location of the residual disease was nearly completely (OF > 95%) within the contour of the original tumor, defined as the 34% threshold SUV contour, based on the source-to-background ratio (van Baardwijk et al Int J Radiat Oncol Biol Phys, 2007). As depicted in figure 1, on average (bold black line) the residual had an OF of more than 80% with the 50% threshold of the pre-treatment PET scan. This indicates that residual disease occurs within the high-uptake region of the original tumor and therefore, escalating dose to this specific area is likely to improve local control.
2. Stability of the high FDG-uptake areas during radiotherapy The mean OF’s of the high uptake zone during therapy were 75.7 ± 10.9% at day 7 and 67.5 ± 16.4% at day 14 during treatment. So, the location of the high FDG uptake region within the tumor during treatment remained the same.
Residual disease within the primary tumor is not due to geographic miss. The location of residual disease corresponds with the original 50% highest FDG uptake before radiotherapy.
This location of the high FDG uptake within the tumor remained stable during radiotherapy. Boosting of tumor sub-volumes may thus be feasible and potentially beneficial, using only a FDG-PET scan before therapy.
Copyright © 2007 by the European Lung Cancer Conference and the International Association for the Study of Lung Cancer.