A head immobilization system for immobilizing a patient's head in a supine position of the patient includes a support rail structure adapted to be coupled to a patient rest. The system can further include a mask frame adapted to be coupled to at least one deformable upper mask sheet. 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. Each one of the pin-receptions receives one of the pins. A catch-mechanism for each pin-reception and each corresponding pin allows the pin to be pushed further into the pin-reception in the first direction, but interlocks in case of an attempted withdrawal of the pin from the pin-reception in a second, opposite direction.