The present invention relates to a head immobilization system for immobilizing a patient's head in a supine position of the patient, the system comprising:



a support rail structure adapted to be coupled to a patient rest and extending at least on both lateral sides of the patient's head,
a mask frame adapted to be coupled to at least one deformable upper mask sheet, wherein the mask frame is releasably connected to the support rail structure via a first interface section and a second interface section, with at least two pins protruding from the first interface section in a first direction, and at least two pin-receptions provided at the second interface section, wherein each one of the pin-receptions receives one of the pins, and
a catch-mechanism for each pin-reception and each corresponding pin, which allows the pin to be pushed further into the pin-reception in the first direction, but which interlocks in case of an attempted withdrawal of the pin from the pin-reception in a second, opposite direction.