PROton Stopping power estimation in Ion Therapy (PROSIT)

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Charged particle therapy (CPT) is an effective procedure for irradiating tumors with lethal doses while sparing healthy tissue and neighboring organs at risk. The dose profile of single beams gives CPT its strength: a pronounced maximum, the Bragg peak, followed by a steep drop. This feature also makes CPT very sensitive to small mismatches between the actual stopping power (SP) and the computed SP used for therapy planning. Safety margins are thus introduced to prevent the effects of possible planning errors and changes in patient’s morphology. However, monitoring and verification of the actual dose deposition (or at least the particle range) are necessary to take full advantage of the beam physical properties. At present no monitoring method has reached clinical routine, but many techniques are under investigation. Among them, prompt-gamma timing (PGT) shows great potential. PGT relies on the time profiles of detected prompt-gamma (PG) rays which originate from de-excitations along the beam path. In our joint work with INFN - Turin, we have shown using in-silico data of a pencil proton beam that not only the particle range but also the spatiotemporal distribution of the PG emission vertices can be reconstructed from PGT profiles from various detectors. Our method, SER-PGT, reconstructs estimates of the spatiotemporal PG emission distribution through maximization of the Poisson likelihood and the modeling of the measurement process. Within PROSIT we will first improve and extend our models and algorithms for a better reconstruction, including compensation for background signals and data truncation, as well as parameter optimization. Supported by preliminary works with homogeneous targets and in-silico data, we plan to go beyond and obtain the stopping power from the reconstructed spatiotemporal PG distribution, as the latter contains information about the particle motion within the target. To this aim, we analytically invert the equation proposed by Bortfeld to model the particle kinematics using Geiger’s range rule. Then, the parameters of the equation are fitted to the post-processed output of SER-PGT. We will extend this procedure to non-uniform targets, also taking into account that in pencil-beam scanning several beams of different energies and directions target the tumor volume. Finally, we envision developing a procedure able to skip the intermediate SER-PGT reconstruction to directly estimate the SP from measured PGT data using models of the PG emission. Our long-term goal is to allow for in-vivo treatment verification and the personalized readjustment of the treatment plan based on the comparison of the theoretical and the actual stopping power, the latter estimated through a multi-detector PGT system in combination with our models and procedures.