Po‐Thur Eve General‐05: Novel Intermediate Energy Photon Treatment of Small Lesions: A Monte Carlo Simulation and Treatment Planning Study

Stereotactic radiosurgery affords great conformality for small tumour volumes. Our study proposes that the radiological penumbra for an intermediate energy photon beam (IEP, 0.2 – 1 MeV) is greatly reduced compared to a megavoltage beam. From Monte Carlo simulation, an 800kV beam of field size less than 2×2 cm 2 was generated from electrons impinging upon a 0.5mm tungsten target. This beam generated a radiological penumbral width (80%–20%) of less than 300μm for small field sizes at depth=5cm in water. A virtual IEP treatment unit (PDD's and profiles generated from Monte Carlo) was created in a Pinnacle treatment planning system (v6.2). An 11 beam non‐coplanar arrangement was used to cover a target volume situated in the middle of a phantom head and at 1mm from a critical structure. Dose volume histograms generated for both the 800kV and a standard 6MV beam showed that the volume of critical structure receiving 10% of the prescription dose was 27% versus 41%. The maximum dose received by the target was 110% (800kV) and 127% (6MV). The 800kV and 6MV beams were dosed to 92% and 78% isodose lines respectively for comparable target coverage. The reduction of radiological penumbra is linked to reduced photon scattering (using small field sizes) and the reduced secondary electron range (using IEP). An 800 kV beam shows superiority over a standard 6MV beam resulting in greater homogeneity and conformality to the target and better sparing of a critical structure in close contact with the target.