Effects of depth, energy and shield size on Block Transmission Factor in radiotherapy

INTRODUCTION: High density blocks are usually used to shield normal structures in radiotherapy. The recommended block transmission factor (BTF) is below 0.05. In this research the dependence of BTF under blocked area with depth and block size has been investigated . METHODS AND MATERIALS: Depth doses were measured with and without block in a solid phantom for 10x10 cm2 field, at 100 cm SSD, at different depths ranged from 0 to 22 cm for 6 and 18 MV photon energies. Eight cm thick lead blocks having 2 and 3.3 cm width were used as the shields and put at the center of fields, separately. MCNP4C Monte Carlo code was used to simulate the experimental design. CorePlan 3D treatment planning system was also used to calculate BTFs for the same experimental conditions . RESULTS: The depth doses and beam profiles indicated good agreement between the MCNP and experimental results. For the 2 cm block, the measured as well as calculated BTFs for the 6 and 18 MV energies changed from 0.05 to 0.20 and 0.10 to 0.18 at different depths, respectively. But, for the 3.3 cm block, relevant BTFs for the above energies ranged from 0.03 to 0.14 and 0.05 to 0.10. The maximum difference observed between the BTF values obtained from the measurement and the TPS was that of the 2 cm block at 18 MV photon energy . DISCUSSION: As noted the BTF under the thin block was not less than 0.05. This can be mostly due to the phantom scattering. Hence, it can be concluded that the under block dose should be considered according to the depth of the structure which is going to be shielded. Furthermore, required modification should be made in TPS algorithms for dose calculation to consider these situations .

کلیدواژه ها:

block factor ، MCNP ، radiotherapy treatment planning ، shielding ، dosimetry