(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: Air cavity in the digestive tract affects the dose distribution of radiation therapy plans when locates in the beam path. The number, size, and location of air cavity changes during the treatment course and the overall impact to different treatment approaches needs to be evaluated.
Methods: We retrospectively picked 30 pancreas cancer patients who received radiation therapy. All patients were setup pretreatment based on daily acquired CBCT (OBI on TrueBeam, Varian Medical). For each patient, one VMAT, one fixed angle IMRT, and one traditional 4 fields 3DCRT plan have been generated. The daily CBCT were manually registered to the planning CT. In each CBCT set, the air cavity, PTV, and selected OARs were contoured. To remove the impact caused by the uncertainty of CBCT HU calibration, the patient body was assigned with mass density of 1g/cc except the air cavity, which was assigned to 0g/cc. All three plans were copied to the CBCT and the dose volumetric histogram of the PTV and OARs were computed. For each type of plan, the cumulative dose from all CBCTs were compared against each other and the planning CT based plan.
Results: Preliminary results illustrated that the 3DCRT was the least vulnerable treatment type to the variation of air cavities. For 3DCRT plans the average variations of the max, min, and mean dose of PTV in the CBCT plans and the planning CT were 0.8%, 0.3%, and 0.2%, with standard deviation of 1.3%, 0.7%, and 1.1%; for IMRT the average variations were 1.1%, 0.8%, and 0.7%, with standard deviation of 1.2%, 1.1%, and 1.5%; for VMAT the average variations were 2.3%, 2.4%, and 1.5%, with standard deviations 0.5%, 0.4%, and 0.6%.
Conclusion: The impact of air cavity maybe significant for certain types of treatment approaches and clinical decisions needs to be made accordingly.
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