practical dosimetry considerations for small mlc- shaped electron fields at 60 cm ssd

Superficial tumours can be treated with megavoltage electron beams. the underlying tissue can be spared through the steep dose fall-off gradients over a range of a few centimetres.an accurate monte carlo model for an elekta precise was determined and dose distribution was simulated. dosimetric parameters were calculated to set guidelines for tumour irradiation. elekta precise multi-leaf collimators (mlc), which shaped electron fields were investigated using a benchmarked monte carlo model. beamnrc modelled the elekta precise and results were benchmarked against measurements. percentage depth dose and beam profile data were simulated within 2% / 2 mm accuracy of the measured data. the dosxyznrc code simulated the 3-d dose data in water between 4 and 15 mev. the relative (p_80-20) penumbra, percentage depth dose (pdd), range to 90% of dose maximum (r_90), dose fall-off range r_80-20 (dfr), and the percentage bremsstrahlung dose (bsd), were extracted from the simulated data. the relative penumbra ranged from 90% to 10% at 6 mev and 15 mev, respectively. r_90 values ranged between 0.8 cm at 4 mev and 4.5 cm at 15 mev. the dfr ranged between 0.8 cm at 4 mev and 3.5 cm at 15 mev. the bsd was the highest for low beam energies and small fields. developed guidelines indicated that intermediate- sized mlc fields are most suited for therapy since they have lower bsd, longer r_90, shorter dfr but larger p_80-20. the dfr increases and r_90 decreases for small fields at higher beam energies and more distal tissue will receive doses > 20%.