Volumetric and Dosimetric Influences of Internal Target Delineation Based on 4D CT Measured Tumor Motion in the Treatment of Lung Cancer with Stereotactic Body Radiotherapy

Purpose: To investigate the volumetric and dosimetric impact of Internal Target Volume (ITV) delineations with tumor motions measured from 4D CT in the Stereotactic Body Radiotherapy (SBRT) treatment of lung cancers. Materials and Methods: Three ITV definition methods for six lung cancer patients were investigated. The first one is combining all the Gross Tumor Volumes ( GTVs) in the 10 respiratory phases of 4D CT data sets (ITV all); the second is adding the measured tumor motions in Anterior-Posterior (AP), Left-Right (LR) and Cranial-Caudal (CC) directions from 4D CT to the GTV in free-breathing CT (ITV motion); the third is adding 5 mm in the AP, LR direction and 10 mm in the CC direction to the GTV in free-breathing CT (ITV general). The target volumes and dosimetric differences resulted from different ITVs were analyzed. Results: The average ratios of ITV motion and ITV general to ITV all were 2.36 and 3.51, respectively. The volume of ITV motion and ITV general was close to each other, except for case two and case five with the largest and smallest dimension. The PTV coverage was very close for these three ITV definitions. The Lung Mean Dose (LMD) was decreased as the TIV decrease. The average LMD of ITV all was 77.4 cGy and 106.1 cGy less than those of ITV motion and ITV general, respectively. Conclusion: Due to the relative small dimensions of tumor for SBRT treatment, no significant dosimetric differences were observed between these two ITV generation methods. Considering the workload and time required to generate ITV all, ITV motion may be a good option.