To measure the in-air output ratio of a 15-MV photon beam using locally designed miniphantoms. Materials and methods: Columnar and brass miniphantoms were designed locally to accommodate 0.6 cc and 0.13 cc ionization chambers. The in-air output ratio (SC) was measured for square, rectangular, and wedged fields for 15 MV. The influences of the orientation of the miniphantom, miniphantom material, and chamber volume on SC and the collimator exchange effect were also studied. Results: The SC measurements ranged from 0.944 to 1.0321 for the studied field sizes. The orientation of the miniphantom had no influence on SC for the range of field sizes studied. The collimator exchange effect was found to be within 1.57%. The SC increased with wedged field sizes and wedge angles due to greater attenuation and production of low energy scatters. Conclusion: This study suggests the combination of a polymethylmethacrylate miniphantom for larger field sizes and a brass miniphantom for smaller field sizes when measuring SC in high-energy photon beams. The brass miniphantom with 0.6 cc and 0.13 cc ion chambers gives acceptable SC values for small field sizes. The chamber volume (0.6 cc or 0.13 cc) has no impact when used with a brass miniphantom.

To measure the in-air output ratio of a 15-MV photon beam using locally designed miniphantoms. Materials and methods: Columnar and brass miniphantoms were designed locally to accommodate 0.6 cc and 0.13 cc ionization chambers. The in-air output ratio (SC) was measured for square, rectangular, and wedged fields for 15 MV. The influences of the orientation of the miniphantom, miniphantom material, and chamber volume on SC and the collimator exchange effect were also studied. Results: The SC measurements ranged from 0.944 to 1.0321 for the studied field sizes. The orientation of the miniphantom had no influence on SC for the range of field sizes studied. The collimator exchange effect was found to be within 1.57%. The SC increased with wedged field sizes and wedge angles due to greater attenuation and production of low energy scatters. Conclusion: This study suggests the combination of a polymethylmethacrylate miniphantom for larger field sizes and a brass miniphantom for smaller field sizes when measuring SC in high-energy photon beams. The brass miniphantom with 0.6 cc and 0.13 cc ion chambers gives acceptable SC values for small field sizes. The chamber volume (0.6 cc or 0.13 cc) has no impact when used with a brass miniphantom.