COMPARISON OF ELECTRON BEAM TRANSMISSION OF DIFFERENT ENERGIES WITH TWO DIFFERENT BLOCK MATERIALS AT DIFFERENT PLACEMENT POSITIONS WITHIN THE APPLICATOR

INTRODUCTION: Superficial tumors are treated with electron beams. Shielding blocks are used to conform to the shape of the tumor. These shielding blocks are usually kept at lower level of the applicator which is near the skin surface. The scattering property of electron may increase the surface dose which will increase with increasing electron energies. The purpose of this study is to compare electron beam transmission of different energies with two different block materials at different placement positions within the applicator. MATERIAL AND METHODS: Cerrobend alloy (50%bismuth, 26.7%lead, 13.3%tin and 10%cadmium) and 1mm thick lead sheets (94%lead, 6%alloy) in Varian Clinac2300C/D linear accelerator with electron energies 6,9,12,16 and 20MeVs using 10x10 applicator at 3 different holding levels was used. Measurements with RW3 Slab phantom (Water equivalent), PPC05 Parallel Plane Chamber, Dose 1 electrometer was done. The slab phantom 30x30x10 cm3 aligned with PPC05 Parallel Plane Chamber (at R85 of respective energies). Readings measured for open and block fields, for different thickness of shielding material, at different placement positions within the applicator. The percentage transmission calculated manually. RESULTS: Using electron energies 6,9,12,16, and 20MeVs respectively the transmission% were: with lead sheet 1mm thickness-2.48%,8.69%,16.05%, 28.03% and 39.50% at lower placement position, 1.19%,3.76%,7.75%,15% and 23.99% at centre placement and 0.96%,3.02%,6.15% and 20.27% for upper placement; with 2mm thickness-0.89%,1.62%,3.66%, 8.95% and 16.35% at lower level, 0.60%,1.28%,2.54%,5.74% and 10.72% at centre level and 0.57%,0.94%, 2.12%,4.85% and 9.22% at upper level; with 3mm thickness-0.80%,1.53%,2.88%,5.29% and 9.42% at lower position, 0.52%,1.25%,2.06%,4.03% and 7.36% at centre position and 0.51%, 0.90%,1.78%,3.66% and 6.43% at upper position; with 4mm thickness- 0.75%,1.40%, 2.71%,4.81% and 7.76% at lower level, 0.50%,1.18%,1.95%,3.68% and 6.31% at center level and 0.51%,0.80%, 1.70%,3.34% and 5.65% at upper level; with 5mm thickness-0.73%, 1.30%,2.57%,4.56% and 7.20% at lower level, 0.45%,1.06%,1.81%,3.48% and 5.68% at center level and 0.47%,0.79%,1.61%,3.13% and 5.24% at upper level. For Cerrobend material 5mm thickness, the transmission at lower level are 0.79%,1.50%,2.98%,5.58% and 10.39%, at center level are 0.52%,0.99%,2.09%,4.12% and 7.67% and at upper level are 0.49%,0.91%, 1.82%,3.75% and 6.90% for the energies 6,9,12,16 and 20 MeV’s respectively. CONCLUSION: There is not much difference in the transmission values at centre and upper levels so as to keep nearer the skin, the centre position in electron applicator may be optimum. Lead sheets can be used since easy to prepare especially for rectangular or square shapes.