Abstract
Background and objectives: Decreasing the ability of conventional external radiation therapy with increasing of tumor size is one of the problems in the treatment of malignant tumors, because of restriction of tissue tolerance. Grid therapy is a technique used to deliver a high dose of radiation (15-20 Gy) in a single fraction to many small volumes within a large treatment field. The grid is a sheet consists of many small holes that shields some parts of treatment field.
Dose distribution from megavoltage grid therapy differs significantly from that of conventional radiation therapy (i.e., many large dose gradients exist within the tumor volume).
In this study we have designed and fabricated the Grid for use with the linear accelerator (Neptun 10 PC) of Imam Reza hospital of Mashhad that produces 9MV x-ray photons.
Material and methods: First, we designed a grid block in a 7-cm thick piece of lead with solid work soft ware. The grid has a 16×square array of holes. When the button surface of the Grid is at the distance of 70 cm to radiation source, the holes diameter and center to center distance of beyond holes is 1.3 and 1.8 cm at the level of Isocenter, respectively. For fabrication of the Grid a custom block of lead alloy (75% Pb, 15% Sb, 10% Sn) that has adequate resistance against pressure and drilling thud, was selected. Then the Grid drilled with VMC (vertical machining center) curler. At the end, the Grid fixed on the linear accelerator of Imam Reza hospital of Mashhad and dosimetric properties of the grid (using 9 MV photon beams) has been measured.
Results: The fabricated grid with this method has economic gain and is inexpensive than other methods. The hole directions has scrupulosity, and dosimetric characteristic of the grid is our evidence. The depth dose curves for the grid fields lie between those for an open portal and a narrow beam. The ratio of the dose delivered to the center of the shielded regions to that under the center of the holes, expressed as Valley-to-peak ratios, ranges from 17% to 28% at Dmax. The output factor for 9 MV photon beam is 78%.
Conclusion: Characterization of the dosimetric properties has allowed clinical implementation of the grid. Grid therapy has allowed palliative treatment of large, deeply seated tumors and massive tumor bulk would most likely not benefit from a conventional course of radiation therapy.