In Vitro sub-lethal and non-targeted effects on normal human cells along the Bragg curve for different ion beams.

Linear Energy Transfer (LET) is the main physical parameter to compare charged particles to photons and predict their higher effectiveness. Damage complexity governs cellular radioresponse and reflects how energy is deposited by ions as described by the Bragg curve. It is likely to change along penetration depth and with track structure. Hence, ion beams of different Z but similar LET may differ radiobiologically. Construction of “biological” Bragg curves may improve modeling of ion action. There are also uncertainties on non-cancer effects of relevance in hadrontherapy. In fact, most experimental data are almost exclusively on tumour lethality and the Spread-Out Bragg Peak (SOBP), overlooking sub-lethal damage on normal cells at various positions along the Bragg curve. Further, little is known on non-targeted effects (NTE) by ion-irradiated normal cells expressing long-term sub-lethal damage. We studied Stress-Induced Premature Senescence (SIPS) by -galactosidase assay and chromosome aberrations (CA) by WCP-FISH and mFISH along the Bragg curve for several ion beams on 3 cell lines (AG01522 fibroblasts, MCF-10A breast epithelial cells, endothelial HUVEC cells). NTE occurrence was studied by medium-transfer from irradiated prematurely senescing cells at time points after irradiation on MCF10A and breast cancer epithelial MCF7 cells. To explore changing biological effectiveness along the Bragg curve we used up to 20 MeV 12C and 16O beams at a 3-MV Tandem accelerator (Department of Physics, Naples); and 60 MeV/u 16O and 20Ne beams at INFN-LNS cyclotron, Catania. Data show SIPS being effectively induced by ions, with a clear dependence on ion type and Bragg curve position, persisting for 2 months post exposure. CA data indicate a similar dependence, and the elevated frequency of complex-type CA agrees with these aberrations being a cytogenetic signature of high-LET ions. However, incidence of CA and SIPS point to a significantly greater efficiency of ion beams compared to x-rays even at very high LETs, contrary to the notion of a close-to-unity RBE above 200 keV/m. Onset of SIPS at ion beam entrance may have important implications for hadrontherapy patients. We also observed a significant bystander effect by senescing cells manifesting itself with an increase in tumour cell proliferation, in accord with reports of a Senescence-Associated Secretory Phenotype. Geant4 Monte-Carlo modeling is under way to correlate ion-track structure with such results.