Fluence modulated proton computed tomography: a new approach for low-dose image guidance in particle therapy

• Applicants: Dr. Georgios Dedes; Professor Dr. Guillaume Landry
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388731804

Fractionated proton therapy offers theoretically advantageous radiation dose distributions when compared to conventional photon therapy due to the physical nature of proton interactions with tissue. The sharp dose gradient at the end of a proton beam can be used to deposit a high dose in the tumour volume while optimally sparing healthy tissue. However, the increased conformity of proton therapy also requires increased precision in dose delivery. Volumetric image guidance is making its entry in proton therapy, with several cone beam com-puted tomography (CBCT) scanners installed recently. While image guidance may improve the precision of proton therapy by ensuring correct target alignment, the dependence of the dose distribution to the radiological thickness traversed to reach the target means that geometric alignment alone is not sufficient to ensure precise dose delivery. For this reason calculation of the so-called water equivalent thickness (WET) to the target on daily volumetric images is key to ensure high precision. Ideally diagnostic quality CT imaging would be employed for this calcula-tion due to their superior integrity when compared to CBCT images, which suffer from severe non-homogeneities. Diagnostic CT imaging in treatment position is unfortunately not readily pos-sible in a proton therapy treatment room due to space considerations, and CT image conversion to the necessary stopping power ratio (RSP) suffers from significant uncertainties (as does CBCT). Proton computed tomography (pCT) offers a potential clinical solution to the issues raised above by using the treatment beam to directly measure the RSP at the treatment position. By using single particle tracking and an energy detector, high accuracy RSP images have been obtained from prototype pCT scanners. Furthermore the physical nature of proton interactions suggests that images of CT-equivalent noise levels can be obtained at low imaging radiation doses, which is critical to enable frequent imaging.Up to now, pCT has been performed with broad proton beams. Our proposal is to develop pencil beam scanning pCT, in collaboration with the international collaboration operating the pCT pro-totype (of which we are members). This would open the door to the main idea behind our re-search proposal: fluence modulated pCT (FMpCT) where pencil beam intensities are adjusted to yield the lowest possible imaging dose while maintaining sufficient image quality in all areas where proton beams interact, thus allowing high accuracy, daily up-to-date WET estimates.To evaluate the performance of our novel image guidance strategy, an extensive comparison to state-of-the-art CBCT imaging will be performed both in a simulation environment and experi-mentally. We will thus aim at establishing the value of FMpCT for proton therapy image guidance.

• DFG Programme: Research Grants
• Co-Investigator: Professorin Dr. Katia Parodi