Dosimetric analysis of respiratory-gated radiotherapy for hepatocellular carcinoma

The purpose of this study was to define individualized internal target volume (ITV) for hepatocellular carcinoma (HCC) using 4D computed tomography (4DCT), and to determine the geometric and dosimetric benefits of respiratory gating. Gross tumor volumes (GTVs) were contoured on 10 respiratory phases of 4DCT images for 12 patients with HCC. Three treatment plans were prepared using different planning target volumes (PTVs): (1) PTV(3D), derived from a single helical clinical target volume (CTV) plus conventional margins; (2) PTV(10 phases), derived from ITV(10 phases), which encompassed all 10 CTVs plus an isotropic margin of 0.8 cm; (3) PTV(gating), derived from ITV(gating), which encompassed three CTVs within gating-window at end-expiration plus an isotropic margin of 0.8 cm. The PTV(3D) was the largest volume for all patients. The ITV-based plans and gating plans spared more normal tissues than 3D plans, especially the liver. Without increasing normal tissue complication probability of the 3D plans, the ITV-based plans allowed for increasing the calculated dose from 50.8 Gy to 54.7 Gy on average, and the gating plans could further escalate the dose to 58.5 Gy. Compared with ITV-based plans, the dosimetric gains with gating plan strongly correlated with GTV mobility in the craniocaudal direction. The ITV-based plans can ensure target coverage with less irradiation of normal tissues compared with 3D plans. Respiratory-gated radiotherapy can further reduce the target volumes to spare more surrounding tissues and allow dose escalation, especially for patients with tumor mobility >1 cm.