Design and Fabrication of a Turnable System for Dosimetry Investigation on a Cobalt-60 Tomotherapy Treatment System

ABSTRACT

The objective of this research is to ascertain if it is feasible to replicate tomotherapy treatment with telecobalt machine based on phantom studies. To replicate gantry motion around a patient during tomotherapy treatment, the phantom was placed on a fabricated turn-table capable of translational and rotational motions while the gantry of the telecobalt machine was kept constant. The treatment simulations were performed with Cirus cobalt 60 teletherapy machine at the Komfo Anokye Teaching Hospital. The fabricated turn-table was made of a metal steel connected to wiper motors of 22, 35 and 50 rpm to control the translational and rotational motion of water phantom. The Co-60 beam dosimetric characteristics such as beam flatness beam, symmetry beam profile, tissue air ratio, output factors and percentage depth dose were determined with tomotherapy procedure. Measurements were done at source -to -surface distance of 80cm for tomotherapy beam using a Farmer type ionization chamber connected to a UNIDOS electrometer and 30 x 30cm2 water phantom. Measurements were carried out using various field sizes ranged from 4x4 to 30x 30 cm2 at depths of 0.5cm, 5cm, 15cm and 20cm. The percentage depth dose increased from the surface to a depth of maximum dose which is 0.5cm for cobalt and decreased as the depth increased It was found that as field size increased the output factor and tissues air ratio increased due to an increased in scatter which increased the dose and decreased as depth increased. The beam flatness and beam symmetry were found to be 0.60 %, and 1.19% (at depth 10cm and width of 3cm from the central axis; and 0.74% at depth 5cm and width of 2 cm from the central axis) which agreed with the International Electrotechnical commission range of 3% and 2% respectively. The fabricated turn table needs refinement in term of high rpm rotational motor to rotate fully-filled 30x30cm2 water phantom and potentiometer to regulate the speed of the rotational motor.