SU-FF-T-201: Effect of Mini-Phantom Material On In-Air Dosimetry Quantities

Abstract

Purpose: Recent studies suggest that miniphantom material and thickness affect the measured values of in‐air output ratio. These results prompt us to investigate the effect of mini‐phantom depth and material on several in‐air quantities. Method and Materials: Beam data for two photon beams (6x and 15x) were compared for several in‐air quantities: off‐axis ratio (OAR); central axis wedge factor (CAX WF); distance factor (DF) dependence and off‐axis wedge factor (OAX WF). Several miniphantom materials (Lucite, copper,lead and graphite) were used with a radiological depth ranging from 2 to 25g/cm2. Results: OAR decreases with increasing miniphantom thickness and off‐ axis distance x, for the same mini‐phantom material. For open field, the maximum difference is 5% and 8% for 6x and 15x, respectively. CAX for WF were measured for 15°, 45° and 60° wedge angles, for both virtual and solid wedges. WF increases up to 11% with mini‐phantom thickness, when solid wedges are used. Virtual wedges have an effect of less than 1% on WF. Maximum errors were found for 450 solid wedge. DF dependence was measured for several SAD's for 6 and 15x, open and 60° virtual and solid wedges. DF agreed within 3% regardless of beam energy, material and SAD. OAX WF were measured at a few off‐axis points along both wedge and non‐wedge gradient directions for 6x. WF varies by up to 2.6% in the toe direction, by up to 1.1% in the heel direction and by up to 3.3% in the non‐wedge direction. Conclusion: Mini‐phantom material has an effect on measured in‐air dosimetric quantities. Ideally, a water‐equivalent miniphantom should be used. When the miniphantom thickness increases (even for water equivalent material), the value of WF, OAR, DF increase by 11%, 8%, 3%, respectively. The error also increases with Z value of mini‐phantom and photon energy.