Design and Dosimetry Consideration for the Fabrication Of Manual Multileaf Collimator For A Cirus Cobalt-60 Teletherapy Machine

Abstract

A manual eight multi-leaf collimator (MLC) has been designed and fabricated for the Cirus cobalt-60 machine at the Komfo Anokye Teaching Hospital. The fabricated MLC is made up of a high carbon steel leaves. The leaf in each bank has a standard width of 7.5 cm projected at the isocentre at a thickness of 4.6 cm. The leaf transmission factor was determined to be 4.8%, which is within the acceptable standard of below 5%. The cobalt-60 beam characteristics such as beam profiles, output factors, MLC effect were determined. Inter-leaf leakage was determined with the 2D dose profile showing a linear increase between field size and leakage at the ends of the leaves. Output factors measurements were done for open and closed fields using an ion chamber and a 30 x 30 cm2 water phantom. The MLC output factor was measured for square fields from 4 x 4 to 32 x 32 cm2 and varying the depths from 0.5 to 15 cm in steps of 0.5cm. Output factors for open MLC fields ranged from 0.952 to 1.250. The MLC effects were measured for square and rectangular fields and depth dependent within the phantom. MLC effect measurement for 7.5 and 15 cm square fields had output factor ranges of 0.979 to 1.218 and 1.143 to 1.218 respectively. The fabricated MLC needs further refinement in terms of the number of leaves, design of the ends of the leaves and limiting the interleaf leakage radiation.