Time-resolved Imaging of Secondary Gamma Ray Emissions for in vivo Monitoring of Proton Therapy

Abstract

Particle therapy (PT), including proton therapy, has important advantages compared to external beam photon therapy (section 1.1). This is because most of the therapeutic effect of a proton beam is localized at the endpoint, where most of its energy is imparted to the medium (Bragg peak), with nearly no dose deposited beyond that point. However, the highly localized dose deposition makes proton therapy more sensitive to (1) patient morphological alterations, including tumor progression / regression, (2) organ motion, (3) patient setup errors, (4) tissue lateral heterogeneities that render the results obtained with non-Monte-Carlo-based treatment planning algorithms unreliable to some degree, (5) beam characteristics utilized for treatment planning, and (6) the conversion of Hounsfield units (computed tomography data), to tissue density and stoichiometry. In addition, uncertainties in the mean excitation potential I, necessary to calculate the stopping power of the penetrating ions, further contribute to potential beam range inaccuracies. Given the aforementioned sources of treatment error, an imaging technique capable of providing feedback proportional to the quality of the treatment being delivered is highly desired and a very active field of research in proton therapy (section 1.2). Specifically, it is of utmost importance to develop an imaging technique capable of providing feedback with respect to the in vivo beam range, especially when highly-heterogeneous beam paths are crossed by a pencil beam. Such a imaging technique can make use of secondary gamma (γ) radiation emitted by the patient, as a result of nuclear interactions between the projectiles and the nuclei of the irradiated medium. These techniques are mainly divided into two categories, according to the type of secondary γ rays probed: (1) positron emission tomography (PET), which makes use of delayed emission, namely pairs of 511 keV annihilation photons, resulting from β+-decay; and (2) prompt gamma (PG) imaging, which makes use of the emission of single photons typically on a sub-nanosecond timescale…