Electron collision stopping power is a fundamental physical quantity describing energy loss process in matter. The ICRU has been evaluated for the energy loss and range of electrons and positrons for various materials. However, it is not enough to include the material which we often used for the dosimetry in radiation therapy. To make the calculation of mass collision stopping power for electron beams, we carried out the correction for density effect using appropriate approximation formulas in addition to Sternheimer and Peierls correction. Estimated electron mass collision stopping power agrees within 0.5% for the most materials of Ivalues less than 180 eV mentioned in ICRU report 37.

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In the radiation therepy of cervical cancer using a remote after loding system(RALS), it is important to decrease the occurrence of complication in colon and bladder. To decrease the irradiation dose for these critical organs, we considered dose distributions depending on shape and material of a shielding object such as lead and tungsten included in the applicator. From the results of simulations and experiments, we showed the impotance of computer simulations in the design of the applicator which yields an appropriate dose distribution.

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