SU‐FF‐J‐161: Feasibility Analysis On Converting Conventional Orthovoltage Biological Irradiator to a Micro‐Beam Array for Small Animal/cell Irradiation

Purpose: To evaluate the feasibility and restrictions in converting an X‐ray orthovoltage biological irradiator, PXI‐X‐RAD‐320, to a micro‐beam array using Geant4 Monte Carlo Simulation. Two important aspects of the beam, the peak‐to‐valley ratio (PVR) and beam flux, were evaluated quantitatively. Method and Materials: X‐ray tube used in the irradiator (MXR‐321) was simulated to generate the angular dependent X‐ray spectrum. With this spectrum, X‐ray beams impinged onto a plate with micro‐grids (micro‐beam plate) or a control plate (normal‐beam plate), with photons collected by detectors underneath at spatial resolution of 0.02mm. The impact of such factors as source‐to‐plate distance, plate‐to‐detector distance, with/without blocking, and the diameter of blocking on the beam profiles and flux was evaluated. Kodak EDR2 films were analyzed by VIDAR‐Dosimetrypro‐Advantage scanner. Results: PVR increased and beam flux decreased with decreasing plate‐to‐detector distance. For instance, keeping the source‐to‐plate distance at 345mm from source, a decrease of the plate‐to‐detector distance from 5cm to 1mm led to PVR increase from 1.3 to 687.3 and beam flux increase from 1.16% to 1.27%. The relative low resolution of scanner caused micro‐beam doses being averaged within vicinity area. It was showed that a PVR of 1.916 got from film was consistent with 1.843 from average of the simulation data for 1mm plate‐to‐detector distance. As the source‐to‐plate distance became larger at constant plate‐to‐detector distance, PVR increased whereas beam flux decreased. In situations where a larger plate‐to‐target distance is desired, a blocking with small orifice must be used. While providing an outstanding PVR at orifice diameter of 0.65mm, this method resulted in an extremely low beam flux. Conclusion: A conventional x‐ray irradiator can be converted to a micro‐beam array but with limitations. The primary restriction is caused by machine's large irradiation angular coverage. Advanced method should be developed to evaluate micro‐beam dose from film.