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%.