Comparison of biological target volume metrics based on FDG PET-CT and 4DCT for primary non-small-cell lung cancer

Fluorodeoxyglucose positron emission tomography-computed tomography (PET-CT) and four-dimensional CT (4DCT) are used in several methods for defining the biological target volume (BTV) in primary non-small cell lung cancer (NSCLC). Disagreements between the assessments using these methodologies make the use of BTV for radiotherapy planning controversial. In this study, we compared existing methods with our proposed internal biological target volume (IBTV) metric, derived by combining internal target volume (ITV) and BTV metrics. We defined the IBTV from ITV (IBTVi) or BTV (IBTVb) based on ITV or BTV with symmetrical margin expansion. We detected large differences between IBTV, IBTVi and IBTVb (p < 0.001), but no difference between ITV and BTV. A margin expansion of about 13 mm was necessary for ITV or BTV to encompass > 95% IBTV. The conformity index correlated negatively with IBTV/ITV, IBTV/BTV, IBTVi/ITV, and IBTVb/BTV volume ratios (p < 0.05). VR also increased the margins of IBTVi and IBTVb. Indeed, IBTV was much smaller than IBTVi or IBTVb, suggesting that using IBTV for radiotherapy planning could improve treatment by minimizing the radiation exposure of healthy tissue and organs surrounding tumors.