Setup Errors Of IMRT For Nasopharyngeal Carcinoma With On-board KV-CBCT And Their Effects On Dose Distributions

Objective: Using KV-cone beam CT (CBCT) on IGRT to study the setup errors for nasopharyngeal carcinoma treated by fractionated intensity-modulated radiation therapy (IMRT) and to explore the effects on dose distributions in order to reduce and/or eliminate the effects on radiotherapy.Methods: Fifteen patients diagnosed with early nasopharyngeal carcinoma by pathology were selected. They were treated by fractionated IMRT. During the treatment courses, patients were posed by the original positions every week in order to get fractional setup errors with on-board KV-CBCT. We applied setup errors into the TPS to acquire dose distributions when patients posed at actual setup positions without image guided radiotherapy (IGRT). The mean values, standard deviations, 95% confidence intervals (CI) of three dimensional and the horizontal rotation errors were calculated. Analyzed the effects of body mass index and weight loss on setup errors. Explored the impact of the setup errors on the dose distributions for targets and organs at risk. And defined margins for planning target volume definition.Results: The mean values of setup errors on ventral-dorsal, cranial-caudal, medio-lateral directions and the horizontal rotation were -0.1833cm, -0.0322cm, 0.0967cm, -0.8333°and 95% confidence intervals were [-0.2117cm, -0.1549cm], [-0.0768cm, 0.0124cm ], [0.0563cm, 0.1371cm] and [-1.0987°, -0.5687°] respectively. The displacements increased with the treatment progress along the ventral-dorsal direction. Absolute values of the first setup errors, the system errors of the medio-lateral direction and the horizontal rotation in the patients with BMI≥25 are more apparent than those with BMI<25. Weight loss can obviously result in the center of actual setup position shifting to the ventral direction and the horizontal rotation counterclockwise. The center of actual setup position shifting to the dorsal direction 0.2cm is able to meanly increase brain stem D1 and spinal cord D1cc, with 6.58% and 4.70%. The center of actual position shifting to the cranial direction 0.2cm resulted in 8.94% dose meanly increasing of optic chiasm D1. The center of actual setup position shifting to the left direction 0.2cm resulted in left parotid D50 meanly increasing with 4.95%. The variations of CTV2 D95 and GTVnd D98 are relevant to 3-dimensional shift vector and the rotation setup error (P<0.05, |r|>0.4); the variations of CTV1 D95 and GTVnx D98 are not relevant to the rotation setup error (P>0.05); the variation of GTVnx D98 is not relevant to 3-dimensional shift vector (P=0.077, |r|=0.187). Patients with BMI≥25 of MPTV values were significantly increased in all directions.Conclusion: 1. The nasopharyngeal carcinoma patients with BMI≥25 should be treated by adaptive radiation therapy; 2. The treatment planning should be adjusted when the weight lost over than 7.10% during the treatment course; 3. The effective measures should be adopted to control isocenter point deviation of three directions within 0.2cm in nasopharyngeal carcinoma with IMRT; 4. The doses of GTVnd and CTV2 are more sensitive than that of GTVnx and CTV1 in translational and rotational setup error; 5. The patients with BMI≥25 might benefit from KV-CBCT online or offline correcting fractional setup errors to reduce margins for the PTV.