SU-E-J-156: Dosimetric Evaluation of Proper Width of Respiratory Gating Window According to Dose Distribution of EBT2 Film.

Abstract

We analyzed dose distribution depending on the width of gating window to determine the proper width of gating window in gated radiation therapy. A three dimensional breathing simulator with house phantom was built to simulate periodic sinusoidal breathing motion of 4 seconds/cycle and 3cm fan shape movement. This was driven synchronized with Real-time Position Management (RPM) system (Varian Medical Systems, Palo Alto, CA, USA), and thereafter 4D-CT images were acquired. Three treatment fields (0°, 120°, 240°) with gating plan using treatment planning system (Eclipse, Varian, USA) were performed aimed to be exposed 200cGy to isocenter. Dose evaluations regarding static, non-gated motion, 60% (phase of 20%-80%), 40% (phase of 30%-70%), 30% (phase of 40%-70%), 20% (phase of 40%-60%) and 15% (phase of 40%-55%) of gated motion were carried out using EBT2 film, and extents of field size, high dose exposed, penumbra were analyzed. In most cases, dose differences compared to static were getting decreased as the width of gating window was decreased. Non-gated motion showed -74.5% dose discrepancies compared to static exposed, whereas 15% respiratory gating window showed within 1% dose difference. Dose differences of 40% gating window was -30.7% whereas 30% gating window showed -6.3% dose discrepancies. Dose difference was rapidly reduced from 30% gating window against non-gated motion to 40% gating window in all cases of field size, high dose area, and penumbra. 15% respiratory gating window was regarded the most ideal width of gating window, but it will significantly increase beam delivery time over a conventional treatment. Therefore, considering beam delivery time and dose distributions of high dose area, field size, and penumbra, 30% respiratory gating window would be recommended. This work was supported by Nuclear Research & Development Program of the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST).