Practical Dosimetry Considerations for Small MLC-Shaped Electron Fields at 60 cm SSD

Abstract

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 (R90), dose fall-off range R80-20 (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. R90 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 R90, shorter DFR but larger P80-20. The DFR increases and R90 decreases for small fields at higher beam energies and more distal tissue will receive doses > 20%.

Keywords

• electrons
• multi-leaf collimator
• energy
• monte carlo method
• radiometry