A new model of the telecobalt unit, Bhabhatron-I, was designed and developed by Bhabha Atornic 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 repairlservicing 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 telecobait units. Source head of Bhabhatron-l 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. lsocentre 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 crn2 field is 11.3 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 1 Ox1 0 crn2 field relative to maximum depth dose is 20.6%. Head leakage in source OFF and ON conditions as well as collimator trsnsmission are well within the specified limits. Mechanical and dosimetry parameters of this unit are also comparable to commercial telecobalt units. Bhabhatron-l can therafore be used safely for tho clinical application in beam therapy.

Total skin electron treatment(TSET) using electrons of energy about 2 - 4 MeV are widely practiced in the treatment of cutaneous T cell lymphomas. In high energy electron accelerators, the treatment technique needs to be designed using 6 MeV to 9 MeV electron beams. The physical parameters of these low energy electron beams should be characterized after high energy electron beam is degraded. At the new linac facility installed at our center we have standardized the above technique. A 10 mm plexiglass beam spoiler has been locally designed which provides 4.25 M focusto- skin distance (FSD) at the Clinac-2300 CD linear accelerator. Measurements were performed with high dose rate mode 2500 MU/min. The beam energy and absorbed dose measurements were carried out with plane parallel chamber (PPC 40) and solid water phantom. Beam flatness measurements were performed using 0.6 cc Farmer chamber. Using this flatness profile for single beam, the beam tilt was calculated so as to provide homogeneity at the junction. The mean energy of the direct electron beam is 2.28 MeV, most probable energy 3.4 MeV, and practical range 15 mm. The measured bremsstrahlung background is less than 0.5% for the horizontal single beam. Two wide angle electron beams with 12.20 tilts against horizontal direction provided large field size of 260 cm x 75 cm at FSD, with dose uniformity within + 8%. The measured absorbed dose rate is 0.109 cGy/MU at FSD. The treatment room dimensions provided large treatment distance. To achieve 130 cm distance between field centers, required tilt of the beam was 12.20. Assuming that one primary electron produces another scattered electron, doubling of the calculated dose by inverse square law, showed good agreement with measured dose/MU within 5%. Additional phantom measurements are required to apply the horizontal beam measurements for executing Stanford technique of TSET.

Clinical radiotherapy applications of low energy electrons in the treatment of total skin involvement is a complex and rarely practiced modality world over. The physics and technical aspects of execution of the techniques involve clear understanding of the use of relevant parameters. After finalizing the treatment geometry and characterizing the parameters of total skin electron treatment (TSET) beam in a linear accelerator, the treatment has to be delivered using 3 pairs of fields in the Stanford technique. There is need to find out the integrated effect of 6 fields by further dosimetric measurements to derive monitor units for single field. Measurements were performed using humanoid phantom in simulated treatment position on a rotating stool. The factor for 6 pairs' of field related from single field calibrated dose per MU is found out using a 0.6 cc Farmer chamber at two selected positions on the humanoid phantom. The overdose ratio at the junction is also quantified. Monitor units are calculated using the single field output and the 6 pairs dosimetric factor. A factor of 4.26 is derived for the 6 pairs of wide angle electron fields of the Stanford technique. The overdose ratio at the junction of 12.20 tilted upper and lower halfrbody fields is found to be 1.09. For the treatment delivery of planned dose of 1 Gy, the required mohitor units for the 6 MeV beams were 428 MU for 3 pairs of fields on alternate days. Ameasured value of 0.1063 cGy/MU on humanoid phantom'with 0.6 cc is in agreement with measured 0.1092 cGy/MU with parallel plate chamber with cubic phantom. The value of dose for 6 pairs of fields to 1 pair of fields D6p/lD1p=2.79 which agrees with the factor between 2.5 to 3.0 reported by AAPM. Our methodology in implementation of TSET will be helpful for other centers rendering this rare radiotherapy practice.