Design and characterisation of a minibeam collimator utilising Monte Carlo simulation and a clinical linear accelerator

Spatially fractionated radiotherapy is showing promise as a treatment modality. Research has primarily focused on applications in proton therapy, however, there is some evidence that minibeam effects may also be present in photon therapy. This study presents a 3D printed tungsten minibeam collimator intended to produce peak-to-valley dose ratios (PVDR) of between seven and ten with a 1 MeV, bremsstrahlung generated, photon beam. The design of the collimator is motivated by a Monte Carlo study estimating the peak to valley dose ratio for different collimator designs at different energies. This collimator was characterised on a clinical linear accelerator (Elekta VersaHD). It was found that 1 MeV photon sources may generate PVDRs in the region of seven. The performance of the fabricated collimator was measured on a 250 keV superficial treatment unit and an Elekta VersaHD running in unflattened mode with a 6 MV beam. On the superficial unit the measured PVDR was greater than 10. On the Elekta VersaHD units the PVDR was measured to be between approximately 1.5 and 2.0 at 3 cm deep.