To obtain the target center positioning errors and the maximum tumor displacement positioning errors within the target volume for lung cancer patients during radiotherapy through cone-beam computed tomography (CBCT) image registration, and to determine two corresponding expansion margin respectively. To explore the influence of these two expansion margins on radiotherapy planning and treatment delivery for lung cancer patients, thereby improving the accuracy of clinical target volume to planning target volume (CTV-PTV) margin delineation.

A retrospective analysis was conducted on CBCT images from 40 lung cancer patients who underwent itensity-modulated radiation therapy (IMRT) in the Radiotherapy Department of Southern Hospital, Southern Medical University, between March and September 2024. Displacement data were collected using two registration methods: target center registration and maximum tumor displacement registration within the target volume. The expansion margins for both registration methods were calculated based on the classic formula for MPTV (margin of planning target volume) expansion.

A total of 227 CBCT scans from the 40 lung cancer patients were analyzed. Statistics showed that under the target center registration method, the positioning errors in the anterior-posterior (AP), head-foot (HF), and left-right (LR) directions were (-0.019±0.218) cm, (0.028±0.239) cm, and (0.022±0.217) cm, respectively. Under the maximum tumor displacement registration method, the positioning errors in the AP, HF, and LR directions were (-0.017±0.205) cm, (0.038±0.290) cm, and (0.025±0.241) cm, respectively. The intra-class correlation coefficient (ICC) method confirmed a high consistency between the two datasets. Using the classic formula, the calculated expansion margins for the target center registration method in the AP, HF, and LR directions were 0.373 cm, 0.477 cm, and 0.466 cm, respectively. For the maximum tumor displacement registration method, the corresponding margins were 0.429 cm, 0.602 cm, and 0.506 cm.

The target center registration and maximum tumor displacement registration within the target volume were performed using CBCT images of lung cancer patients, two sets of positioning errors and expansion margins were obtained. The ICC consistency test demonstrated good agreement between the two datasets. These results can serve as a theoretical basis for determining CTV-PTV margins in lung cancer radiotherapy and possess practical clinical application value.