Study of Head Scatter Factor in 4MV Photon Beam Used in Radiotherapy

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 +/- 1.98%. 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 +/- 1% with published data.