Efficient approach for determining four-dimensional computed tomography-based internal target volume in stereotactic radiotherapy of lung cancer

PURPOSE: This study aimed to investigate efficient approaches for deter(min)ing internal target volume (ITV) from four-dimensional computed tomography (4D CT) images used in stereotactic body radiotherapy (SBRT) for patients with early-stage non-small cell lung cancer (NSCLC). MATERIALS AND METHODS: 4D CT images were analyzed for 15 patients who received SBRT for stage I NSCLC. Three different ITVs were determined as follows: combining clinical target volume (CTV) from all 10 respiratory phases (ITV(10Phases)); combining CTV from four respiratory phases, including two extreme phases (0% and 50%) plus two intermediate phases (20% and 70%) (ITV(4Phases)); and combining CTV from two extreme phases (ITV(2Phases)). The matching index (MI) of ITV(4Phases) and ITV(2Phases) was defined as the ratio of ITV(4Phases) and ITV(2Phases), respectively, to the ITV(10Phases). The tumor motion index (TMI) was defined as the ratio of ITV(10Phases) to CTV(mean), which was the mean of 10 CTVs delineated on 10 respiratory phases. RESULTS: The ITVs were significantly different in the order of ITV(10Phases), ITV(4Phases), and ITV(2Phases) (all p < 0.05). The MI of ITV(4Phases) was significantly higher than that of ITV(2Phases) (p < 0.001). The MI of ITV(4Phases) was inversely related to TMI (r = -0.569, p = 0.034). In a subgroup with low TMI (n = 7), ITV(4Phases) was not statistically different from ITV(10Phases) (p = 0.192) and its MI was significantly higher than that of ITV(2Phases) (p = 0.016). CONCLUSION: The ITV(4Phases) may be an efficient approach alternative to optimal ITV(10Phases) in SBRT for early-stage NSCLC with less tumor motion.