18-Fluorodeoxy-Glucose Positron Emission Tomography-Computed Tomography (18-FDG-PET/CT) for Gross Tumor Volume (GTV) Delineation in Gastric Cancer Radiotherapy

PURPOSE: Evaluation of the 18-fluorodeoxy-glucose positron emission tomography-computed tomography (18-FDG-PET/(CT)) for gross tumor volume (GTV) delineation in gastric cancer patients undergoing radiotherapy. METHODS: In this study, 29 gastric cancer patients (17 unresectable and 7 inoperable) were initially enrolled for radical chemoradiotherapy (45Gy/25 fractions + chemotherapy based on 5 fluorouracil) or radiotherapy alone (45Gy/25 fractions) with planning based on the 18-FDG-PET/CT images. Five patients were excluded due to excess blood glucose levels (1), false-negative positron emission tomography (1) and distant metastases revealed by 18-FDG-PET/CT (3). The analysis involved measurement of metabolic tumor volumes (MTVs) performed on PET/CT workstations. Different threshold levels of the standardized uptake value (SUV) and liver uptake were set to obtain MTVs. Secondly, GTV(PET) values were derived manually using the positron emission tomography (PET) dataset blinded to the computed tomography (CT) data. Subsequently, GTV(CT) values were delineated using a radiotherapy planning system based on the CT scans blinded to the PET data. The referenced GTV(CT) values were correlated with the GTV(PET) and were compared with a conformality index (CI). RESULTS: The mean CI was 0.52 (range, 0.12-0.85). In 13/24 patients (54%), the GTV(PET) was larger than GTV(CT), and in the remainder, GTV(PET) was smaller. Moreover, the cranio-caudal diameter of GTV(PET) in 16 cases (64%) was larger than that of GTV(CT), smaller in 7 cases (29%), and unchanged in one case. Manual PET delineation (GTVPET) achieved the best correlation with GTV(CT) (Pearson correlation = 0.76, p <0.0001). Among the analyzed MTVs, a statistically significant correlation with GTV(CT) was revealed for MTV(10%SUVmax) (r = 0.63; p = 0.0014), MTV(liv) (r = 0.60; p = 0.0021), MTV(SUV2.5) (r = 0.54; p = 0.0063); MTV(20%SUVmax) (r = 0.44; p = 0.0344); MTV(30%SUVmax) (r = 0.44; p = 0.0373). CONCLUSION: 18-FDG-PET/(CT) in gastric cancer radiotherapy planning may affect the GTV delineation.