SU-E-T-160: Evaluation of Accuracy for Target Margin Size Obtained From CBCT On Lung SBRT Based On Film Dosimetry

Authors


Abstract

Purpose:

To analysis delivered dose on target using gafchromic films for evaluating accuracy of target margin size obtained from cone beam computed tomography (CBCT) during lung stereotactic body radiation therapy (SBRT)

Methods:

The phantom consists of measurement part and driving part. The motor of Quasar motion phantom (Modus Medical Devices Inc, London, ON, Canada) was used for driving part and we developed measurement part which consist of cork cylindrical body and acrylic target with radiochromic film inserted into central and both ends of acrylic target. In this study lung SBRT cases through both four dimensional computed tomography (4DCT) and CBCT were selected. Target contouring including margin based on 4DCT is defined with a 1 cm margin around gross tumor volume (GTV) in all directions except for inferior direction. The moving range in inferior direction was larger than other directions thus, including 2 cm margin. In case of CBCT, the margin means blurring of target on CBCT images. This study was compared margin size determined through 4DCT and that of based on CBCT and we also evaluated dose profile and the length of margin in superior-inferior direction on CBCT compared with 4DCT.

Results:

The length of target including margin was 2.48 cm (based on CBCT) and 2.66 cm (based on 4DCT), respectively in superior-inferior direction. The difference of delivered dose on target between two margins was only within 1%.

Conclusions:

This study has shown the feasibility of determining target margin using CBCT for delivering more accurate prescription dose to lung cancer.

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