A proton or ion particle beam irradiation system comprising: a dose-monitor system, a beam shape and beam positionmonitoring system; one or more processors; and one or more computer-readable non-transitory storage media coupled to one or more of the processors and comprising instructions operable when executed by one or more of the processors to cause the system to: receive treatment information input into the system and relating to a treatment plan for proton- or ion-beam therapy intended to irradiate a target tissue; receive machine-limitation information input into the system and relating to one or more limitations of one or more machines involved in the proton- or ion-beam therapy; determine a time-optimized beam current for a proton or ion beam based on the treatment information and the machine-limitation information, wherein the time- optimized beam current minimizes the time required to deliver a required quantity of monitor units to one of a plurality of spots, wherein each of the plurality of spots is a particular area of the target tissue; and deliver the time-optimized beam current to the particular area; characterised in that determining of the time-optimized beam current comprises, for each energy layer of a target volume with respect to a particular field: determining a smallest value of prescribed monitor units for the each energy layer from all prescribed spots in the each energy layer; determining a minimum time of exposure required to deliver the prescribed monitor units; calculating a ratio of the smallest value of prescribed monitor units per energy layer to the determined minimum time of exposure; and if the calculated ratio is higher than a maximum beam current specified by the machine- limitation information for the dose-monitor system and the beam- position and beam-shape monitoring system involved in the proton- or ion- beam therapy, setting the time-optimized beam current for the each energy layer to the maximum beam current for the dose-monitor system and the beam position and beam shape- monitoring system. a dose-monitor system, a beam shape and beam positionmonitoring system; one or more processors; and one or more computer-readable non-transitory storage media coupled to one or more of the processors and comprising instructions operable when executed by one or more of the processors to cause the system to: receive treatment information input into the system and relating to a treatment plan for proton- or ion-beam therapy intended to irradiate a target tissue; receive machine-limitation information input into the system and relating to one or more limitations of one or more machines involved in the proton- or ion-beam therapy; determine a time-optimized beam current for a proton or ion beam based on the treatment information and the machine-limitation information, wherein the time- optimized beam current minimizes the time required to deliver a required quantity of monitor units to one of a plurality of spots, wherein each of the plurality of spots is a particular area of the target tissue; and deliver the time-optimized beam current to the particular area; receive treatment information input into the system and relating to a treatment plan for proton- or ion-beam therapy intended to irradiate a target tissue; receive machine-limitation information input into the system and relating to one or more limitations of one or more machines involved in the proton- or ion-beam therapy; determine a time-optimized beam current for a proton or ion beam based on the treatment information and the machine-limitation information, wherein the time- optimized beam current minimizes the time required to deliver a required quantity of monitor units to one of a plurality of spots, wherein each of the plurality of spots is a particular area of the target tissue; and deliver the time-optimized beam current to the particular area; characterised in that determining of the time-optimized beam current comprises, for each energy layer of a target volume with respect to a particular field: determining a smallest value of prescribed monitor units for the each energy layer from all prescribed spots in the each energy layer; determining a minimum time of exposure required to deliver the prescribed monitor units; calculating a ratio of the smallest value of prescribed monitor units per energy layer to the determined minimum time of exposure; and if the calculated ratio is higher than a maximum beam current specified by the machine- limitation information for the dose-monitor system and the beam- position and beam-shape monitoring system involved in the proton- or ion- beam therapy, setting the time-optimized beam current for the each energy layer to the maximum beam current for the dose-monitor system and the beam position and beam shape- monitoring system. determining a smallest value of prescribed monitor units for the each energy layer from all prescribed spots in the each energy layer; determining a minimum time of exposure required to deliver the prescribed monitor units; calculating a ratio of the smallest value of prescribed monitor units per energy layer to the determined minimum time of exposure; and if the calculated ratio is higher than a maximum beam current specified by the machine- limitation information for the dose-monitor system and the beam- position and beam-shape monitoring system involved in the proton- or ion- beam therapy, setting the time-optimized beam current for the each energy layer to the maximum beam current for the dose-monitor system and the beam position and beam shape- monitoring system.