Analyzing the Feasibility of the Margin From CTV to PTV for Esophageal Carcinoma with Retraction Method
c gao, C Han*, Z Chi, D Liu, The Fourth Hospital of HeBei Medical University, Shijiazhuang, Hebei ProvinceSU-E-T-712 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Discuss the feasibility of the retraction method by comparing with the result of calculating methed about the margin from CTV to PTV based on the influence of target dosimetry of setup errors during esophageal carcinoma patients treatment.
Methods:Forty two patients with esophageal cancer were treated by Elekta. The setup errors were measured six times during their treatment for once a week by EPID. The values of setup errors were simulated into the treatment planning system by moving the isocenter and then observing the dose distribution . The margin from CTV to PTV will be concluded both by the method of retracting (Fixed the PTV of the original plan, and retract PTV a certain distance defined as simulative organization CTVnx. The distance from PTV to CTVnx which get specified doses, namely guarantee at least 99% CTV volume can receive the dose of 95%, is the margin CTV to PTV we found) and the former formula method.
Results:(1) The systematic setup errors of the 42 patients were -2.31mm left to right, -0.55mm anterior to posterior and -0.16mm superior to inferior, and the random errors were 4.42mm, 4.35mm and 4.48mm, respectively. (2)The setup errors resulted of a dose reduction of GTVD95 with 32cGy and CTVD95 with 88cGy. (3) The CTV to PTV margin by the retraction method based on our setup errors for esophageal cancer were 6mm left to right, 6mm anterior to posterior and 5mm superior to inferior, and 7.71mm left to right, 4.15mm anterior to posterior and 3.46mm superior to inferior by the formula method.
Conclusion:The setup errors reduced the dose of GTV and CTV, the retraction method is more reliable than the formula method for calculating the margin from the CTV to the PTV, of which increasing a new method in this field.