Megavoltage cone-beam CT in the evaluation of set-up errors in head and neck cancers treated with precision radiotherapy / 中华放射肿瘤学杂志

ABSTRACT

Objective To evaluate set-up errors by megavoltage cone-beam CT in head and neck cancers treated with precision radiotherapy. Methods From April 2007 to March 2008, 22 patients with nasopharyngeal carcinoma (15 patients) ,parotid carcinoma (4 patients) and brain glioma (3 patients) were enrolled, among whom 7 patients underwent three-dimensional conformal radiotherapy (3DCRT) and 15 received intensity modulated radiotherapy (IMRT). The radiation dose was 56. 0 -70.4 Gy in 28 -32 fractions within 6 -7 weeks. Megavoltage cone-beam scan was performed weekly before treatment. The isocenter displacement was calculated by comparing megavoltage cone-beam CT and planning CT in left-right (LR), cranio-caudal (CC), and anterior-posterior (AP) directions. Results Totally 129 sets of megavoltage cone-beam CT images were obtained for 22 patients. The frequency of isocenter shift more than 0. 3 cm,0. 4 cm and 0. 5 cm were 28,15 and 9, respectively. The maximum mean set-up error was found in CC, which was about 0. 1 cm more than that in LR and AP. The frequency of isocenter shift direction was almost identical in LR and CC, which was more frequent (about 75%) in the posterior direction. Conclusions During the course of radiation of brain tumor and head and neck cancer, the enlarging tendency of set-up errors has been found in all the three directions. The isocenter shift in AP was more frequent to the posterior direction.