Comparison of planning target volumes based on three-dimensional and four-dimensional CT imaging of thoracic esophageal cancer

BACKGROUND AND PURPOSE: To investigate the definition of planning target volumes (PTVs) based on four-dimensional computed tomography (4DCT) compared with conventional PTV definition and PTV definition using asymmetrical margins for thoracic primary esophageal cancer. MATERIALS AND METHODS: Forty-three patients with esophageal cancer underwent 3DCT and 4DCT simulation scans during free breathing. The motions of primary tumors located in the proximal (group A), middle (group B), and distal (group C) thoracic esophagus were obtained from the 4DCT scans. PTV(3D) was defined on 3DCT using the tumor motion measured based on 4DCT, PTV conventional (PTV(conv)) was defined on 3DCT by adding a 1.0 cm margin to the clinical target volume, and PTV(4D) was defined as the union of the target volumes contoured on the ten phases of the 4DCT images. The centroid positions, volumetric differences, and dice similarity coefficients were evaluated for all PTVs. RESULTS: The median centroid shifts between PTV(3D) and PTV(4D) and between PTV(conv) and PTV(4D) in all three dimensions were <0.3 cm for the three groups. The median size ratios of PTV(4D) to PTV(3D) were 0.80, 0.88, and 0.71, and PTV(4D) to PTV(conv) were 0.67, 0.73, and 0.76 (χ(2)=−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The dice similarity coefficients were 0.87, 0.90, and 0.81 between PTV(4D) and PTV(3D) and 0.80, 0.84, and 0.83 between PTV(4D) and PTV(conv) (χ(2) =−3.18, −2.98, and −3.06; P=0.001, 0.003, and 0.002) for groups A, B, and C, respectively. The difference between the degree of inclusion of PTV(4D) in PTV(3D) and that of PTV(4D) in PTV(conv) was <2% for all groups. Compared with PTV(conv), the amount of irradiated normal tissue for PTV(3D) was decreased by 11.81% and 11.86% in groups A and B, respectively, but was increased by 2.93% in group C. CONCLUSION: For proximal and middle esophageal cancer, 3DCT-based PTV using asymmetrical margins provides good coverage of PTV(4D); however, for distal esophageal cancer, 3DCT-based PTV using conventional margins provides ideal conformity with PTV(4D).