Impact of PET in the Radiation Therapy Planning for Pediatric Cancer

Purpose: The purpose of this study was to evaluate the impact of including PET (Positron Emission Tomography) in the process of pediatric radiation therapy planning. The aim was to study the effect on target volumes and how this in turn affects the resulting treatment plans. This study also aims to compare the estimated risks of various long-term complications and how the use of PET influences these risks. Methods: Eleven pediatric sarcoma, NSCLC (non-small-cell lung cancer) and nasopharyngeal cancer patients, treated at Rigshospitalet in 2005-2011, were included in this study. The target volumes (GTV:s and CTV:s) were delineated by senior clinicians specialized in nuclear medicine, diagnostic radiology and radiation oncology. The delineation was performed without and subsequently with access to the PET scan information, on separate CT-sets. A margin of 6 mm was added to each CTV to render the PTV. Treatment plans were generated for three different photon therapy modalities and intensity-modulated proton therapy. Dose-effects models based on published studies were derived to evaluate and compare the risks of complications. Moreover, a rough estimation of the effective dose from the PET/CT scan and the associated risk was made. Results: The target volumes were evaluated with respect to volume size as well as shape. For the patients in this study, there was no significant change of the size of the target volumes: the average CTV size was 257 cm3 (range: 71.10-462.40 cm3, median 259.60 cm3) and 254 cm3 (range: 63.43-497.30 cm3, median 211.90 cm3) without and with PET respectively. Using PET did in some cases alter the shape of the treatment volumes, resulting in a mean Dice index of 0.91 (range: 0.86-0.95). The radiation therapy plans based on PET data were not significantly different from the noPET-plans, in neither the risk of normal tissue complications, nor the risk of secondary cancers. The impact of PET did not differ between the four treatment modalities. Conclusions: The main conclusion from this study is that while including PET in the radiation therapy treatment planning process of pediatric cancer patients may change the shape and size of the target volume, it does not significantly impact the risks attributable to the radiation therapy, neither of normal tissue complications nor secondary cancers. The risk of radiation-induced complications from the PET/CT scan is very small. The target volume may be decreased by including PET, when areas dubious on the CT are FDG-negative. This gives the potential to reduce the irradiated normal tissue volume. The PET-based target volume may be expanded for FDG-positive areas that are undistinguishable on the CT-images. This will likely decrease the risk of leaving malignant tissue untreated. The results suggest that there will be no significant differences between radiation therapy plans made with or without PET-data. The plans appear to be of comparable quality (provided that the therapeutic efficacy is maintained) and the risk of long-term complications is not changed. A vast amount of published results from more than a decade of research and clinical experience, indicate the usefulness and diagnostic value of including PET-data into the care of cancer patients – adults as well as children. Taking these factors into consideration, along with the very low risks of radiation-induced side effects from the PET/CT scan itself, the conclusion is that PET should be used as a complementary tool in target volume delineation for radiation therapy planning of pediatric patients.