While there is a growing clinical interest and emerging technological developments for FLASHRT with different ionizing particle beams, the majority of relevant research has been done with the use of electron, photon or proton beams. Investigations of achievable FLASH effect with heavy ions are therefore needed for understanding the clinical potential of particle beams for FLASH-RT. Methods: The study investigates the dose threshold effect of FLASH-RT with proton and heavy ion beams, by modelling the achievable FLASH effect through the production yields of solvated electrons and spatial differences of production yields due to LET dependency. Solvated electrons have been chosen as surrogate for indication of FLASH effect based on their reactivity with cellular oxygen, therefore standing as possible mediators of oxygen dependency of FLASH-RT. Pristine pencil beam Bragg peak 3D dose distributions of various ion energies were simulated with GEANT4 and biological spread-out Bragg peaks (SOBP’s) were calculated from these sets for different field sizes and treatment depths. Various heavy particle types were investigated - protons and ions of 3He, 4He, 6Li, 7Li, 9Be, 10B, 12C, 14N, 16O and 20Ne. Mixed beam linear energy transfer (LET) 3D distributions were calculated and were used for estimation of solvated electron production yields. Results: FLASH-RT dose threshold levels for different heavy ion beams are estimated, indicating impact of SOBP geometry and field size on this threshold. Dose threshold effect spatial difference maps are indicated due to solvated electron production yield dependency on LET.