Dosimetric characteristics of a newly designed grid block for megavoltage photon radiation and its therapeutic advantage using a linear quadratic model Ali S. Meigooni, a Kai Dou, Navid J. Meigooni, Michael Gnaster, Shahid Awan, Sharifeh Dini, and Ellis L. Johnson University of Kentucky Chandler Medical Center, Department of Radiation Medicine, Lexington, Kentucky 40536-0084 Received 7 March 2006; revised 29 June 2006; accepted for publication 6 July 2006; published 17 August 2006 Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, com- bined with conventional radiation therapy, has been demonstrated using a prototype GRID block Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114–118 1990. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radia- tion Product Design, Inc. Albertive, MN. This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investi- gated for a Varian 21EX linear accelerator Varian Associates, Palo Alto, CA. These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water™ phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with  /  ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented. © 2006 American Association of Physicists in Medicine. DOI: 10.1118/1.2241998 Key words: grid therapy, megavoltage photon radiation, dosimetry, cell survival, therapeutic ad- vantage I. INTRODUCTION Megavoltage GRID radiation therapy is a new paradigm in the management of bulky 8 cm malignant tumors. 1–4 The Department of Radiation Medicine at the University of Ken- tucky has utilized a modified spatially fractionated technique with megavoltage x-ray beam for treatment of advanced tumors. 3,4 Dosimetric characteristics of these GRID radiation fields for both photon and electron beams have been studied by several investigators. 5–7 It is interesting to know that the published clinical results 1–4 were obtained with a prototype GRID block that was constructed by placing several seg- ments of copper tubing between two plastic trays and filling the space between the tubes with cerrobend material. There- fore, the differences in attenuation between the copper tubing and cerrobend material, as well as the nondivergent match problem of the copper tubing to the divergent beam, brought about a challenge to the design of the prototype GRID block. Despite the problems mentioned above in the prototype GRID block design, Mohiuddin et al. 3 have shown that the overall response rate of patients increased from 62% to 91% when they were treated with a single-field GRID irradiation of 15 Gy plus conventional external beam therapy. Recently, a new GRID block has been designed and fab- ricated by Radiation Product Design, Inc. 5218 Barthel In- dustrial Dr., Albertive, MN. This block was constructed by casting the divergent holes in a cerrobend block, using a stereotactic method which is able to provide the optimal con- ditions of: 1 accurate beam divergence and 2 less trans- mission through the blocked area. This block design allows for a greater volume of tissue to receive the therapeutic dose of greater than 85%. The hole size and center-to-center spac- ing can be customized, and also the divergence of the holes can be designed to match one of the commercially available linear accelerators. Characterization of the dosimetric and biological properties of the new GRID block design is nec- essary for clinical implementation. While the evaluation of dosimetric properties of GRID irradiation is relatively straightforward, the biological analy- sis is much more difficult. The radiobiology of GRID irra- diation is not well understood and is currently being studied by several investigators. 8,9 Proposed mechanisms of action include the production of various cytokines in the irradiated tissues. 9 For a nonuniform radiation field, Niemierko 10 intro- duced a radiobiological model, based on the linear quadratic model, to describe the biological effectiveness of the inho- 3165 3165 Med. Phys. 33 „9…, September 2006 0094-2405/2006/33„9…/3165/9/$23.00 © 2006 Am. Assoc. Phys. Med.