In radiotherapy, the radiation beam is sometimes shaped so as to deliver different doses to different organs or give a homogeneous dose to structures of different densities. This objective is achieved by the use of attenuating materials introduced into the radiation beam. These attenuators alter the primary as well as the scattered radiation components of the beam and there is at present no accurate method of dose calculation for these situations. Most calculations are performed considering only the effect of the attenuators on the primary radiation beam and can produce large errors in dosimetry. In this study, the broad-beam attenuation is investigated in homogeneous phantoms for various radiation field sizes, photon beam energies and depths in phantom. A mathematical method taking account of primary as well as first scattered radiation is developed. This method predicts reasonably well the transmission through lead attenuators for various experimental conditions.

We have observed a number of blazars at wavebands ranging from radio to γ-ray. We find bright γ-ray emission to be associated with strong synchrotron flares observed at lower frequencies. The X-ray flux and entire radio spectrum of 4C 39.25 have each increased in strength by 30% over a 2-year period, in agreement with the prediction of the bent relativistic jet model.