en Medical Physics Medical Physics تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> In Radiation Therapy, the sparing of normal tissues can be performed using either multi-leaf collimators or Cerrobend blocks. The current work focuses on the physical characteristics of Cerrobend blocks including attenuation coefficient, effective penumbra width and isodose curves undulation in penumbral regions. <span style="font-weight: bold">Materials and Methods:</span> All measurements were performed using a dual energy linac and the Cerrobend blocks designed and fabricated using a commercial Cerrobend material. Data were collected using a calibrated ionization chamber as well as EDR2 films. <span style="font-weight: bold">Results:</span> The results showed that the attenuation coefficient was found to be 0.4475 and 0.4276 cm-1 for photon beams 6MV and 15 MV, respectively, and a potential air bubble with a diameter greater than 3 mm affects beam attenuation significantly. The optimum Cerrobend block width was found to be around 16 mm. The isodose curves scalloping achieved for secondary collimator jaws, were also similar. <span style="font-weight: bold">Conclusion: </span>If Cerrobend blocks are used as a basic method to protect normal tissues, its physical characteristics will be recommended to be taken into account comprehensively. Iran. J. Radiat. Res., 2010 8 (2): 93-101 Beam collimation, block-cutter, Cerrobend blocks, shielding. 93 101 http://ijrr.com/browse.php?a_code=A-10-1-356&slc_lang=en&sid=1 A. Taherkhani 790031947532846001823 790031947532846001823 No M. Mohammadi mohammadi@umsha.ac.ir 790031947532846001961 790031947532846001961 Yes M.S. Saboori 790031947532846001962 790031947532846001962 No V. Changizi 790031947532846001963 790031947532846001963 No