| Objective：Through the application of kilovolt cone beam CT (KV-CBCT) to measure the setup errors in patients receiving radiotherapy withsuperior thoracic esophageal carcinoma. The effect of setup errors on thedosage of targets and peripheral organs at risk (OAR) were analyzed bysimulating the actual isocenter in radiotherapy treatment plans. Dosimetricmethods were used to estimate the effective distance of the margin in clinicaltarget volume-planning target volume (CTV-PTV) for patients with superiorthoracic esophageal carcinoma.Methods：13patients with superior thoracic esophageal carcinoma wereanalyzed for this study. These patients scanned by KV-CBCT and followed byintensity modulated radiation therapy (IMRT). The CBCT scan images werematched with simulated CT scan images from treatment plans to obtain thepatients’ setup errors. The CTV was first expanded outwardly2mm from theleft to right (represented by the X-axis),3mm from superior to inferior(represented by the Y-axis), and2mm from anterior to posterior (representedby the Z-axis) to form the internal target volume (ITV). The ITV wasprogressively expanded outward by1mm10times to form novel PTVs withalternative margins. Re-plans were developed based on the10novel PTVs andthey were used to simulate setup errors. The original treatment plan isocentercoordinates were moved to the practical position, while all other treatmentconditions remained stable (e.g. ray radian, beam number, beam shape,prescription dose, multi-leaf collimator, machine hops, etc.) withoutreoptimization of the treatment plan. The number of CBCT scans could decidethe frequency of shifting the isocenter. Moved the treatment plan isocenter tothe actual isocenter, then multiplied by the number that the CBCT represented,and then merge to get the simulated actual exposure curve. That was to find a suitable PTV even with presence of setup errors still can assure95%ITVvolume to prescription dose, and also can guarantee CTV volume toprescription dose, CTV-ITV and ITV-PTV, which was the sum of the distancebetween the CTV-PTV expanding distance.Results：(1)128group record of setup errors were obtained with CBCTin13patients superior thoracic esophageal carcinoma. The setup errors in theX direction was the direction of left and right range about (2.86±2.06) mm, inthe Y direction was the direction of superior and inferior range about(3.42±2.19) mm, in the Z direction was the direction of anterior and posteriorrange about (3.00±2.17) mm. The setup errors mean were maximum in Ydirection, moderate in Z direction and minimum in X direction. In the X, Y, Zdirection system error on the right, superior and posterior were1.21mm,1.19mm,2.46mm, and the system errors were maximum in Z direction. Andrandom error in the X, Y, Z direction were3.33mm,3.89mm,2.77mm. Therandom error Y direction was the most.(2) Through the method of PTVexpanding obtained the confines of ITV-PTV was5mm for the upper thoracicesophageal cancer patients in my center. The expanding confines of CTV-PTV in upper thoracic esophageal cancer patients in the left and right, superiorand inferior, anterior and posterior were7mm,8mm,7mm. This couldguarantee95%ITV volume in90%of patients can still achieve prescriptiondose in the presence of setup errors.(3) According to the method of PTVexpanding obtained the CTV-PTV distance to make radiotherapy plan(planPTV1). Moved the isocenter to the practical position, under the condition inthe presence of setup errors (planPTV2): double lung V5(%), spinal cord D1cc(Gy), increased by about0.87%,4.95Gy, P values were0.006and0.001, thedifference was significant. Heart V40(%), D95PTV (Gy), V100PTV (%),D95ITV (Gy), V100ITV (%) are reduced to about0.62%,4.95Gy,8.38%,1.84Gy,1.87%, P values were0.008,0.007,0.000,0.001,0.001, thedifference was significant.(4) The plans of conventional margin of CTV-PTV5mm in the presence of setup errors (plan5mm2) showed that D95ITV was(57.00±4.94) Gy, V100ITV(90.36±7.12)%. And only23.08%of patients95% ITV volume could reach the prescription dose. If setup errors existed, theconventional margin of CTV-PTV5mm could not ensure95%ITV volumeachieve prescription dose.Conclusions：1Through CBCT scan we found the setup errors of upper thoracicesophageal cancer patients were maximum in Y axis direction of superior andinferior, followed by Z axis which was the direction of anterior and posterior,the last was X axis in the direction of left and right. The Z direction was thehighest in system errors. The maximum random error belonged to Y direction.2Through the PTV expanding method we obtained the confines of ITV-PTV was5mm for the upper thoracic esophageal cancer patients in my center.The expanding range of CTV-PTV of upper thoracic esophageal cancerpatients, in the left and right, superior and inferior, anterior and posterior were7mm,8mm,7mm, respectively. It could ensure95%ITV volume of90%ofpatients still achieved prescription dose in the presence of setup errors.3With the presence of setup errors, the conventional5mm expandingrange could not ensure prescription dose cover95%ITV volume of90%ofpatients.4When the CTV-PTV expanding range of upper thoracic esophagealcancer patients were7mm,8mm,7mm in the left and right, superior andinferior, anterior and posterior, comparing with the origin scheme that therewas an increase in the double lung V5and the D1ccof spinal cord under thecondition of the existence of setup errors, but they were all within thetolerance range. |
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