Effects of head and neck positioning errors and external contour changes on radiation dose in radiotherapy for nasopharyngeal carcinoma
Objective To acquire the external contour changes and positioning errors during fractionated intensity-modulated radiotherapy for nasopharyngeal carcinoma using kilovoltage cone-beam computed tomography(CBCT),and to analyze their effects on the dose distributions in target areas and organs-at-risk.Methods Twenty-one patients with nasopharyngeal carcinoma were enrolled in the study,and the positioning errors in the left-right,superior-inferior and anterior-posterior directions were obtained by matching 137 CBCT images with the positioning CT images.The transverse width of the external contour was measured at 3 different layers of the neck,and its effects on positioning errors were analyzed using Pearson's coefficient and independent sample t-tests.Additionally,simulation plans based on CBCT images were created to analyze the effects of positioning errors and external contour changes on radiotherapy dose,with Mann-Whitney U tests applied for significance analysis.Results The positioning errors in the left-right,superior-inferior and anterior-posterior directions were(1.04±0.73),(1.13±0.87)and(1.38±0.95)mm,respectively.The rates of external contour changes at the A,B and C layers of the head and neck were 15.36%,14.94%and 14.99%,respectively.Compared with executed plans,simulation plans had lower GTV D98,CTV1 D95 and CTV2 D95(P<0.05),and higher Dmax for the brainstem and spinal cord(P<0.05).Conclusion The simulation plans indicate that the presence of uncertainties such as positioning errors and external contour changes will significantly affect dose distributions in target areas,with the largest decrease observed in GTV D98(11.49%)and the maximum change rates in CTV1 D95 and CTV2 D95 being 12.88%and 21.64%,respectively.Except for the left and right lenses and left parotid gland,significant differences are observed in the doses for the other organs-at-risk and target areas,suggesting that positioning errors and external contour changes in actual radiotherapy will exert significant effects on dose distributions.
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