Effects of setup error on brainstem dose distribution in patients with nasopharyngeal carcinoma and weight loss

Objective: To investigate the effects of setup error acquired by cone-beam computed tomography (CBCT) on the dose to the brainstem. Methods: Setup errors were collected from 4 patients with locally advanced nasopharyngeal carcinoma (NPC) undergoing three-dimensional radiotherapy. In the 33 radiotherapy plans, the upper limit of the dose to the brainstem was 54 Gy in total radiation and 163.3 cGy in a single radiation. Treatment was re-planed using a corrected center based on setup error. For each patient,areas exposed to radiation higher than 163.64 cGy were recorded in each layer of the brainstem after single treatment. The CorelDraw software was used to superpose all overdose areas in each layer of the brainstem. The Photoshop software was used to calculate the volume of the overdose areas. The changes in weight after radiotherapy were recorded and analyzed using paired t test. Results: In 116 CBCT scans without correction, the setup errors were (0.90 ± 0.85), (0.98 ± 0.6), and (0.98 ± 1.05) mm in x, y, and z directions, and (0.25 ± 0.49)° in rotation. According to the images of a total of 2502 layers of the brainstem acquired by 116 CBCT scans, 2040 layers of the brainstem had doses over the upper limit to different degrees. The patient with the highest frequency of overdose in the brainstem had a total overdose volume up to 3.83% in 20 scans. Another patient had the overdose volume only 0.13% of the total volume. There was no significant relationship between weight loss and setup error (P > 0.05). Conclusions: Setup error acquired by CBCT has a substantial impact on the dose to the brainstem. It is an effective approach for verification of target volume delineation. (authors)