In vivo dosimetry for hadron therapy: Gadolinium as a surrogate for dose measurements

Abstract:
The feasibility of using a Gd-based contrast agent as mean of in vivo dosimetry was explored up until now in the scope of the project. The characteristic x-ray peak of Gd (ca 43 keV) was studied under several scenarios to access the possibility of using it as a surrogate for in vivo and in real time dose measurements. The dose and dose rate dependence of the Gd signal was experimentally verified.

An increase of the Gd x-ray peak height was observed for increasing deposited doses at the target and a decrease in the signal was measured for increasing dose rates. Several other variables, such as medium attenuation, angle of detection and concentration of Gd in the tumor volume were researched in order to understand their impact in the response of the dosimeter.

Finally, the dosimeter was tested in a scenario that aimed to simulate a treatment plan subjected to uncertainties (in this case coming from movements of the target, either by inter-fraction displacement, intra-fraction organ motion or overall change in the patient anatomy). A correlation between the x-ray Gd signal and the amount of irradiated volume was observed, which makes it possible to track spatial changes in the deposition of dose.

For in depth displacements of the target, a dependence with the energy of the incident particles was observed, not being possible to access any correlation between Gd x-ray signal and total dose at the target.

The physics behind the secondary emission of the x-rays of interest was also studied and it was seen that several processes are in the origin of the observed signal, mainly particle induce x-ray emission and neutron capture events.