Practical Dosimetry Considerations for Small MLC-Shaped Electron Fields at ۶۰ 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 ۲% / ۲ mm accuracy of the measured data. The DOSXYZnrc code simulated the ۳-D dose data in water between ۴ and ۱۵ MeV. The relative (p ۸۰-۲۰) penumbra, percentage depth dose (PDD), range to ۹۰% of dose maximum (R۹۰), dose fall-off range R۸۰-۲۰ (DFR), and the percentage bremsstrahlung dose (BSD), were extracted from the simulated data.The relative penumbra ranged from ۹۰% to ۱۰% at ۶ MeV and ۱۵ MeV, respectively. R۹۰ values ranged between ۰.۸ cm at ۴ MeV and ۴.۵ cm at ۱۵ MeV. The DFR ranged between ۰.۸ cm at ۴ MeV and ۳.۵ cm at ۱۵ 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 R۹۰, shorter DFR but larger P۸۰-۲۰. The DFR increases and R۹۰ decreases for small fields at higher beam energies and more distal tissue will receive doses > ۲۰%.

کلیدواژه ها:

Electrons ، multi-leaf collimator ، energy ، Monte Carlo Method ، Radiometry