Performance characteristics of indigenously developed Bhabhatron-I telecobalt unit

A new model of the telecobalt unit, Bhabhatron-l, was designed and developed by Bhabha Atomic Research Centre. Fully software controlled operation which also keeps record of vital operational history of the unit; availability of field sizes smaller than 5 x 5 cm2; automatic closure of collimator to 0 x 0 cm2 field in case of any functional abnormality; software controlled collimator, couch and gantry movement and calibration facilities; and option of remote repair/servicing are the unique features of Bhabhatron-I. Thorough evaluation of this telecobalt unit was carried out to verify its compliance with national and international standards as well as to compare its performance with commercial telecobalt units. Source head of Bhabhatron-I consists of a stainless steel shell filled with lead and natural uranium and is designed to load a 60Co source of maximum capacity 250 RMM. Source movement is controlled by pneumatic drive system. Field limiting devices contain fixed opening primary as well as variable opening secondary collimators. Secondary collimator of this unit includes two pairs of collimating jaws made up of natural uranium. All safety switches and interlocks as required for a telecobalt unit are provided and are functional. Isocentre of this unit was always within 2 mm dia sphere with respect to collimator, couch and gantry rotation. Field flatness and symmetry are within 3%, while penumbra for 10 x 10 cm2 field is 11.4 mm. Percentage depth dose (PDD) data of this telecobalt unit are comparable to BJR supplement 25 data (BJR, 1996) for telecobalt units. Percentage surface dose for 10x10 cm2 field relative to maximum depth dose is 20.6%. Head leakage in source OFF and ON conditions as well as collimator transmission are well within the specified limits. Mechanical and dosimetry parameters of this unit are also comparable to commercial telecobalt units. Bhabhatron-I can therefore be used safely for the clinical application in beam therapy. (author)