Experimental investigation of the characteristics of prompt gammas produced in tissue during proton therapy treatment

Includes bibliographical references.Protons entering matter have a finite range while photons have a gradual, exponential attenuation as they travel through material. Proton radiation therapy takes advantage of this difference, by aligning the cancerous tissue with the end of the proton range where it deposits the maximum amount of energy, resulting in significant tissue sparing compared to traditional photon therapy. These properties of protons also require a more precise delivery of the radiation since a small uncertainty in the positioning of the proton beam could result in the over- or under-dosing of critical structures. This highlights the importance of an in-vivo dose measurement technique. Due to the nature of protons, there are no primary particles exiting the patient to be used for verification purposes. One option is to use secondary radiation for dose verification purposes, like prompt gammas produced by proton-nuclei inelastic collisions. The primary aim of this work was to report on the detection of prompt gammas and their specific characteristics when produced by the important elements of tissue, like oxygen, carbon, hydrogen and nitrogen. A number of different targets containing these elements such as water, Perspex, graphite and liquid nitrogen were irradiated in a passive-scatter proton therapy treatment facility and the gammas were detected by a high resolution 2' x 2" LaBr3 detector. The measurements were carried out at iThernba LABS in Somerset West, South Africa using the proton therapy beamline. In order to determine the shielding required to sufficiently block the secondary neutrons and scattered gamma-rays emitted from the beam line elements, a significant problem in a passive-scatter proton beam, preliminary Monte Carlo simulations were performed. The energy spectra of the prompt gammas produced in the various targets was measured, looking specifically at the discrete elemental prompt gamma peaks at 4.44 MeV from ¹²C and 6.13 MeV from ¹⁶0. Measurements were also performed to investigate prompt gamma emission as a function of depth along the beam path. The depth measurements were carried out for water and Perspex phantoms at several detector positions surrounding the depth location of the Bragg peak in each material. The discrete prompt gamma-ray measurements reveal that in-vivo range verification is feasible for clinical passive-scatter proton irradiations. The secondary aspect of this work was to develop a Monte Carlo model of the entire experimental measurement set-up including the entire iThemba LABS passive-scatter proton beam line. In recent years, the Geant4 Monte Carlo toolkit has played an important role in the development of a device for real time dose range verification purposes using prompt gamma radiation. Unfortunately, in Geant4, the default physics models were not suitable for replicating measured prompt gamma emission. Determining a suitable physics model for low energy proton inelastic interactions will boost the accuracy of prompt gamma simulations