A columnar mini phantom is designed as recommended by ESTRO to measure the Head Scatter Factor (S C ) for 6 MV beam of two linear accelerators. The measurement of S C at different orientations of the chamber, parallel and perpendicular at 1.5 cm depth predicts the deviation of 2.05% and 1.9% for Elekta and Siemens linear accelerators respectively. The measurement of S C at 1.5 cm is higher compared to 10 cm depth for both the linear accelerators suggesting the electron contamination at 1.5 cm depth. The effect of wedges on S C yields a significant contribution of 3.5% and 5% for Siemens and Elekta linear accelerators respectively. The collimator exchange effect reveals the opening of upper jaw increases the S C irrespective of the linear accelerator. The result emphasizes the need of S C measurement at 10 cm. The presence of wedge influences the S C value and the SSD has no influence on S C . The measured S C values are in good agreement with the published data.

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The 4 MV photon beam offers equal build-up region behavior like Co-60 beam and it plays a major role in head and neck and pediatric radiotherapy. In this study an attempt is made to study the head scatter factor (S C ) for 4 MV photon beam using locally designed PMMA and Brass miniphantoms. The S C is measured in combination of PMMA miniphantom with 0.6 cc chamber and Brass miniphantom with 0.6 cc and 0.13 cc chambers. The measured S C is compared with the literature data and it agrees within 61.98%. The study reveals that either 0.13 cc or 0.6 cc chamber with PMMA or Brass phantom materials can be used for S C measurements in a 4 MV photon beam. The variation of SSD does not alter the head scatter factor. The collimator exchange effect is found to be within 1, and it is less than that of other linear accelerators. It is also found that the presence of internal wedge has significant contribution to head scatter factor. The Phantom scatter factor is also calculated and it agrees within 61% with published data.

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