Comparison of Six‐ and Three‐Dimensional Error Corrections for the Planning Target Volume in Patients With Nasopharyngeal Cancer

INTRODUCTION: During radiation therapy, patients may experience positioning errors that can affect the irradiated volume and result in unnecessary radiation doses to normal tissues. Correcting for setup errors is essential for patients undergoing radiation therapy; however, it remains unclear whether six-dimensional (6D) correction is superior to conventional three-dimensional (3D) correction in nasopharyngeal cancer. Therefore, the purpose of this study was to compare the planning target volume (PTV) coverage between 3D and 6D cone-beam computed tomography (CBCT) corrections for patients with nasopharyngeal carcinoma. METHODS: We retrospectively enrolled 30 patients with nasopharyngeal cancer who underwent image-guided radiation therapy. The 3D and 6D corrections were performed using CBCT images registered to CT simulation images based on skeletal structures at 1, 3, and 5 weeks, respectively. The volume percentages of the PTV achieving the prescribed dose of 70, 63, and 56 Gy were calculated after both 3D and 6D corrections and were statistically compared using a repeated measures analysis of variance test. RESULTS: The results showed that the percentage coverage of the target volume was 94.1% ± 4.3% and 95.4% ± 3.4% for 70 Gy, 94.4% ± 2.6% and 95.3% ± 2.4% for 63 Gy, and 92.1% ± 7.7% and 95.6% ± 5.4% for 56 Gy after 3D and 6D corrections, respectively. A significant difference was noted in the volume coverage for 56 Gy between the two correction methods. Additionally, the average maximum dose percentages after 3D and 6D corrections were 5.9% ± 8.0% and 3.3% ± 6.2% for the spinal cord, and 1.8% ± 8.5% and 2.7% ± 6.0% for the brain stem, respectively; however, no significant difference was observed between the two correction methods. CONCLUSION: We conclude that 6D correction for CBCT images achieved better dose coverage of the target tumour volume than 3D correction in patients with nasopharyngeal carcinoma.