Fan-Shaped Complete Block on Helical Tomotherapy for Esophageal Cancer: A Phantom Study

Radiation pneumonitis (RP) is a common complication for radiotherapy of esophageal cancer and is associated with the low dose irradiated lung volume. This study aims to reduce the mean lung dose (MLD) and the relative lung volume at 20 Gy (V-20) and at low dose region using various designs of the fan-shaped complete block (FSCB) in helical tomotherapy. Hypothetical esophageal tumor was delineated on an anthropomorphic phantom. The FSCB was defined as the fan-shaped radiation restricted area located in both lungs. Seven treatment plans were performed with nonblock design and FSCB with different fan angles, that is, from 90 degrees to 140 degrees, with increment of 10 degrees. The homogeneous index, conformation number, MLD, and the relative lung volume receiving more than 5, 10, 15, and 20Gy (V-5, V-10, V-15, and V-20) were determined for each treatment scheme. There was a substantial reduction in the MLD, V-5, V-10, V-15, and V-20 when using different types of FSCB as compared to the nonblock design. The reduction of V-20, V-15, V-10, and V-5 was 6.3%-8.6%, 16%-23%, 42%-57%, and 42%-66% for FSCB 90 degrees-140 degrees, respectively. The use of FSCB in helical tomotherapy is a promising method to reduce the MLD, V-20, and relative lung volume in low dose region, especially in V-5 and V-10 for esophageal cancer.