en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> In conventional radiation therapy, regarding normal tissue tolerance, the treatment of bulk tumors is one of the remaining challenges. Grid Radiation Therapy (GRT) is a technique to deliver high doses, approximately 15 – 20 Gy per fraction, to several small volumes located in a large radiation field. This can be performed using a grid block. The current work has concentrated on the dosimetric characteristics of a designed mega-voltage grid, used for a unique treatment modality. <span style="font-weight: bold">Materials and Methods:</span> All measurements performed using a Neptune linear accelerator (9 MV photon beam). A square 16 × 16 array grid block was designed and fabricated. Several dosimetric characteristics including: depth dose, Valley To Peak (VTP) ratio, and grid out-put factor were evaluated using a calibrated diode dosimeter for a range of radiation fields. <span style="font-weight: bold">Results: </span>The percent depth dose curves, measured in the presence of grid block, lie within those measured for the corresponding open field and a narrow beam. At the Dmax, the VTP ratio was found to be within 17% - 28%, while these ranges between 23% - 35% at a depth of 10 cm. The grid out -put factor found to be 0.78 and it slightly decreases with increasing of radiation field size. <span style="font-weight: bold">Conclusion:</span> The VTP ratio found to be dependent strongly on the grid design and manufacturing properties. However, other parameters such as radiation field size and the depth of measurement should also be addressed as important factors. The measured dosimetric characteristics of grid block indicate that the mega-voltage grid therapy can be applied as a possible clinical modality for palliative cases. Iran. J. Radiat. Res., 2011 9(1): 49-56 Grid therapy, palliation, mega-voltage, radiation dosimetry. 49 56 http://ijrr.com/browse.php?a_code=A-10-1-384&slc_lang=en&sid=1 K. Ghazi-khanlou Sani ghazi@umsha.ac.ir 790031947532846002227 790031947532846002227 Yes M. Mohammadi 790031947532846002228 790031947532846002228 No A. Eskandarlo 790031947532846002229 790031947532846002229 No H. Gholam-hosseinian 790031947532846002230 790031947532846002230 No