Purpose: The application of biocompatible metal nanoparticles (NPs) in radiotherapy to potentially improve tumor control is promising due to their radiosensitizing effect. There is still no clarification of the underlying physical, chemical and biological mechanisms of the NPs' radiosensitivity when interacting with proton radiation. This work presents various experimental studies to investigate the energy deposition of protons in matter loaded with platinum nanoparticles (PtNPs).
Methods: Tissue-like samples were created with homogeneously dispersed laser ablated surfactant-free PtNPs (37 nm) in gelatin cuboids. A concentration of 300 µg/ml was used, leading to a strong radiosensitizing effect during proton irradiation based on supplementary studies of reactive oxygen species. Cuboids without PtNPs were created as control samples. Clinical X-ray imaging was utilized to verify the samples’ properties. Various detectors were used to determine the proton stopping power downstream of the samples with and without PtNPs at a pencil beam scanning beam line as well as a passive beam line with proton energies from about 56 up to 200 MeV.
Results: The samples’ water equivalent ratios and the protons’ mean deposited energy in the detectors downstream of the sample with PtNPs compared to the one without PtNPs were consistent within the experimental uncertainties of about 2%. Since both quantities are a measure of proton stopping power, the latter is not increased in the presence of PtNPs during proton irradiation.
Conclusion: This study presents the experimental evidence that the radiosensitizing effect of PtNPs in proton irradiations is not caused by an enhanced proton energy deposition. The results more likely indicate a catalytic effect of the PtNPs. Therefore, this work contributes to the highly discussed radiobiological topic of the proton therapy efficiency with metal NPs and offers first indications that the proton dose calculation in treatment planning with PtNPs is unproblematic.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the MERCUR-Stiftung graduate school 'Praezisionsprotonentherapie - Praxisbezogene Physik und Chemie an der Schnittstelle zur Medizin' (grant number St-2019-0007) and the project 'Synergistische Effekte von Gold-Nanopartikeln und Protonenbestrahlung bei der Behandlung von Hirntumoren im Kindesalter' (grant number EFRE-0801289) by the European fund for regional development (EFRE).
Protons, Radiobiology, Radiosensitivity
TH- Radiobiology(RBio)/Biology(Bio): RBio- Particle therapy- Protons