An Evaluation Of Setup Errors And Their Effects On Dose Distribution Of IMRT For Nasopharyngeal Carcinoma Using On-board KV-CBCT

Objectiv: Using Kilovoltage X-ray cone-beam CT To study the setup errors for nasopharyngeal carcinoma treated by intensity modulated radioation therapy(IMRT), and to analyze the effect of setup errors against the physical dosimeter on target coverrage and normal tissues.Methods Thirty two patients diagnosed with nasopharyngeal carcinoma and treated with IMRT were selected. Patients were immobilized by head and neck thermoplastic mask and were posed by original position. Kilovoltage cone-beam scan was performed weekly before treatment. The setup errors on translational directions(X,Y,Z)were obtained by matching the CBCT images and the planning CT images.Change in head and neck contours during treatment at the 15 th,20th and 25 th fractions were measured from the treatment planning system. Analyzed the effects of weight loss and change of head an neck contours on setup errors. And defined the margin for the Clinic TargetVolume(CTV) to the Planning Target Volume(PTV).The effects of setup error on the dose distribution was simulated by shifting the patient isocenter along the three Cartesian axes.Results There were Thirty two patients received CBCT scans and a total of 217 CBCT scans were obtained. The mean values of setup errors were(1.50±0.87)mm,(1.59±0.87)mm and(1.95±0.99)mm in the left-right, head-feet and anterior-posterior direction respectively. The PTV margins were 2.9mm in the left-right direction, 4.5mm in the head-feet direction and 3.1mm in the anterior-posterior direction. There was no correlation between the weight loss and setup errors. The mean ratios of change of head and neck contours at A, B,C level were 5.57%,6.13% and 4.46% respectively. The ratio of change of head and neck contours was significantly associated with the displacement in head-feet direction(r=0.68、P=0.01 at level A, r=0.56、P=0.04 at level B, r=0.67、P=0.01 at level C). The setup errors in the patient with ratio>8.0% at A,B,C level are more apparent than those with ratio<8%. The dose variations for GTV D98, CTV D95, left parotid, right parotid, left lens,right lens, braistem and spinal cord was(-1.68-0.76) %,(-0.95-1.37)%,(-12.25-13.15)%,(-1.37-12.52)%,(-15.06-12.39)%,(-12.15-19.28)%,(-17.85-13.25)%,(-4.28-15.75)%. When the center of actual setup position was shifted 2mm to the right direction, it resulted in 5.94% dose meanly increasing of right parotid D50. With the center ofactual setup position shifting 3mm to the cranial direction, the max dose of optic chiasm meanly increased with 6.29%.With the center of actual setup position shifting 2mm to the dorsal direction, the max dose of spinal cord meanly increased with 8.12%. With the center of actual setup position shifting 3mm to the dorsal direction, the max dose of brain stem meanly increased with 8.12%.Conlusion: 1. The 5mm of CTV-PTV margins for nasopharyngeal carcinoma were feasible in all three directon, it could be appropriately reduced in the left-right and anterior-posterior direction. 2. The contours of head and neck decrease gradually in the course of radiotherapy. The effect of changes on the setup errors is not obvious. Replanning should be considered for those who have a large reduction ratio in contour over the three level during the treament. 3. The setup errors had obviou influence on the dose of organs at risk. And we should pay attention to the parotid because it is more sensitive. 4. The setup errors of three directions should be controled within 2mm so as to reduce the damage of important tissue and organs.