Design and development of an add-on automated multi-leaf collimator for telecobalt therapy machine and study of its characteristics

Purpose: The purpose of the paper is to present the design, development and dosimetric characteristics of an automated Multi-leaf collimator (MLC) integrated along with an inexpensive Treatment Planning System (ROPS), developed for existing telecobalt units as an add-on without making any modifications to the treatment machine. Method: The prototype MLC design consists of 28 tungsten alloy leaves (14 leaf pairs) having a mass density of 18 g cm -3 . Each of the leaves projects 10 mm width at the isocenter, which is at 80 cm from the source. The automation has been achieved with a dedicated linear actuator for each leaf. Radiochromic films and IC PROFILER™ (Sun Nuclear Corporation, Melbourne, FL) were used for the measurement of beam profiles and the profiles were analyzed to arrive at radiation field width, beam flatness, symmetry and beam penumbra. Results: The prototype MLC can define radiation fields of up to 14 × 14 cm² within the prescribed tolerance values of 2 mm. The flatness and symmetry were found to be within the prescribed tolerance value of 3%. The penumbra for a 10 × 10 cm² field size is 9.5 mm which is less than the generally acceptable value of 12 mm for a telecobalt machine. The maximum leakage through the leaf ends in closed condition was observed to be 7.3% which is less than the values reported for other MLCs designed for medical linear accelerators. Conclusion: It is concluded that dosimetric parameters and the leakage radiation of the prototype MLC are well below their recommended tolerance values. The MLC can be used for carrying out conformal radiotherapy with existing telecobalt machines.