Background gamma-ray lines are emitted in passive materials close to the detectors and in the detector material itself. Primary and secondary cosmic ray particles (protons and neutrons) induce excited nuclei in nuclear reactions with nuclei of the passive material. The prompt or delayed (radioactive) de-excitation of these nuclei leads to gamma-ray lines which can be detected by the germanium detectors. Calculations show that lines originating in the mask should not pose a problem for SPI.