Pilot Study on the Impact of F18-Labeled Thymidine PET/CT on Gross Tumor Volume Identification and Definition for Pancreatic Cancer

Publication date: Available online 18 September 2017
Source:Practical Radiation Oncology
Author(s): Jennifer L. Pretz, Michael A. Blake, Joseph H. Killoran, Harvey J. Mamon, Jennifer Y. Wo, Andrew X. Zhu, Theodore S. Hong
PurposeAccurate target definition for radiotherapy planning in localized pancreatic cancer is critical, particularly when using strategies that omit elective coverage. Standard imaging modalities such as CT, MRI, and endoscopic ultrasound have limited concordance with pathologic evaluation. Biologic imaging with [F18]-fluorodeoxyglucose (FDG)-PET can also be difficult to interpret, as increased activity is indicative of increased glucose metabolism, rather than cellular proliferation. F18 – labeled thymidine (FLT) is a proliferative marker which exploits the expression of pyrimidine-metabolizing enzymes. We evaluate the impact of FLT-PET on pancreatic target definition for radiation planning.Methods and MaterialsPatients with biopsy proven, newly diagnosed, untreated pancreatic adenocarcinoma were enrolled on an IRB-approved prospective study. Patients were injected with FLT and scanned 20–30minutes later. Two physicians (referred to as observer 1 and observer 2) independently contoured the gross tumor volume (GTV) and involved nodes on CT scan only, and then again with the assistance of co-registered FLT-PET. Conformality index (CI), the ratio of the volumes of intersection and union, was used as the metric for volume comparison (where CI=0 represented no overlap and CI=1 represented perfect overlap).ResultsNine patients were enrolled on this study. FLT-avidity was discerned in 8 of 9 patients. Average CT-GTV volume for observers 1 and 2 was 38.1cc and 26.5cc respectively. Average FLT-GTV volume for observers 1 and 2 was 39.1cc and 25.0cc respectively. For the 8 patients with FLT-avid tumors, addition of FLT data improved concordance of GTV definition between physicians in 6 of 8 tumors. Average CI for inter-observer CT-GTV was 0.325. Addition of FLT-PET information improved average CI to 0.400.ConclusionFLT-PET improves inter-observer concordance in GTV definition. Further studies will focus on verification of these findings, pathologic verification of the FLT-PET signal, and optimization of the FLT-PET signal threshold for auto-segmentation.



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