Dose-free monitoring of radiotherapy treatments with scattered photons: First experimental results at a 6-MV Linac

Radiotherapy (RT) is nowadays, after surgery, among the most commonly used treatment regarding cancer. For predicted clinical outcomes it is of importance assuring that the treatment plan is correctly delivered without dose deviations, namely target underdosage and/or organ-at-risk overdosage. For that, a new multi-pixel imaging technique for real-time dose verification during photon RT has been proposed (RTmon). First scanned, single-pixel experimental results are presented here. The principle of operation of RTmon relies on the detection of scattered photons emitted perpendicularly to the beam direction. The first experimental results were obtained with a collimated γ-ray detector operated under a 6-MV clinical linac beam. A large contaminating background originating from the head of the linac was found. We show that its mitigation is possible by (1) using appropriate shielding, (2) decreasing the scintillator volume pointing towards out-of-field, background-prone regions, and (3) optimizing the collimation by means of detector positioning and differential signal sampling. Simulation and experimental data showed that multiple scattering inside a phantom target constitutes an additional source of background that increases along the beam direction due to momentum conservation in collisions between the incoming photons and target electrons. This second source of background can be reduced effectively by optimizing the distance between the collimated detector and the beam axis, at the expense of decreased collimation sensitivity. Despite the harsh therapeutic irradiation conditions, background and noise suppression by appropriate shielding and differential signal sampling renders the scanned experimental results quite similar to those obtained by means of Geant4 simulations. Such experimental results suggest therefore that this detector system may be useful for real-time dose verification in external X-ray radiotherapy.