The difference between gross tumor volume and biological target volume in postoperative local recurrent rectal cancer: A preliminary study of PET/CT radiation therapy planning

Purpose: To reveal the difference of gross tumor volume (GTV) in CT with biological target volume (BTV) in positron emission tomography using 18F-fluorodeoxyglucose with computed tomography (FDG-PET/CT). Materials and Methods: In 7 patients with postoperative local recurrence rectal cancer, three independent radiation oncologists delineated target volume on CT and PET, respectively, and compared the volume of GTV with that of BTV using the two-factor repeated measures ANOVA. FDG-PET/CT was performed in the same position as radiotherapy. The images by CT and PET were sent to the radiation therapy planning system as the DICOM datum, respectively, and fused those images using DICOM information. Each radiation oncologist delineated the abnormal shadow on CT as GTV and delineated macroscopically the region with high FDG accumulation using the tumor/muscle ratio. Results: In those seven patients, mean GTV was 77.2±53.6 cm³ and mean BTV was 58.0±48.0 cm³, and BTV was statistically smaller than GTV (p<0.001). There was a strong correlation between GTV and BTV (r=0.952, p<0.001). In patients with local recurrent rectal cancer, there were no significant differences among radiation oncologists (F (2, 18)=0.654, MSe=5220.046, p=0.532). Conclusion: There was a significant difference between the target volume on functional image (PET/CT) and the target volume on morphologic image (CT) in the patients with local recurrent rectal cancer.