Dosimetry for Small, Irregular and Rectangular Field Size for Electron Beam Therapy

Purpose: This study was to investigate the effect of irregularity in shape of field size of electron beam on the percentage depth dose and the output factor. Study the variety of the Percentage Depth Dose (PDD) and the output factors for rectangular and small field sizes when one of field dimensions less than the Practical Range (Rp) of the electron energy used. Compare the measured output factors for every rectangular and small electron lead cutout with its equivalent square field size. Material and Method: Measure the percentage depth dose PDD for small, irregular and rectangular lead cutout against reference field size (10 X 10) cm using 3D water phantom and parallel plate ion chamber on linear accelerator (Elekta precise) with electron beam energies 6, 9, 12, 15 and 18 MeV. Measure and calculate the output factor for rectangular lead cutout and its equivalent square field size. Results: The PDD parameters for irregular lead cutout varied from that for the reference field size depend on the shape of lead cutout and energy, for 6 MeV the variation ranged from 0.5 to 2%, for 9 MeV from 0.2 to 1.8%, for 12 MeV from 1.6 to 6.4%, for 15 MeV from 2.25 to 9.9% and for 18 MeV from 0.7 to 3.2%. The PDD for the small lead cutout varied from the reference field size when one of the field dimension is smaller than the practical range of the electron energy used. The variation in output for the rectangular field sizes and its equivalent square field size are ranged from 5 to 5.5% for (2 X 9) and (3 X 3), ranged from 2.3 to 4.5% for (3 X 9) and (4.5 X 4.5) and for (6 X 9) and (7 X 7) ranged from 0.1 to 0.7% at all electron energies. Conclusion: The PDD parameters and output factors for irregular, small and rectangular varied from the reference field size (10 X 10) cm2 depended on the field shape and the electron energy used.