Measurement of the in-air output ratio for high-energy photon beams used in radiotherapy

Aim: 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 (S-C) 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 S-C and the collimator exchange effect were also studied. Results: The S-C measurements ranged from 0.944 to 1.0321 for the studied field sizes. The orientation of the miniphantom had no influence on S-C for the range of field sizes studied. The collimator exchange effect was found to be within 1.57%. The S-C 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 S-C in high-energy photon beams. The brass miniphantom with 0.6 cc and 0.13 cc ion chambers gives acceptable S-C values for small field sizes. The chamber volume (0.6 cc or 0.13 cc) has no impact when used with a brass miniphantom.