1. Therapy system ( 100 ) for irradiating a target volume ( 2 ) within a patient ( 1 ) with a charged particle beam ( 6 ), comprising a charged particle beam generator ( 3 ), a beam transport system ( 4 ) for transporting the charged particle beam ( 6 ), a nozzle ( 5 ) for delivering the charged particle beam ( 6 ) to the target volume ( 2 ), an X-Ray device ( 10 ) comprising an X-Ray source ( 12 ) for emitting X-Rays towards the target volume ( 2 ), the X-Ray device ( 10 ) being movably mounted into the nozzle ( 5 ) between a first position wherein the X-Ray device ( 10 ) is inside of a treatment path envelope ( 23 ) of the charged particle beam ( 6 ) within the nozzle ( 5 ) and a second position wherein the X-Ray device ( 10 ) is outside of said treatment path envelope ( 23 ), an X-Ray receiving device ( 11 ) mounted opposite to the patient ( 1 ) with regard to the X-Ray source ( 12 ), and in optical alignment with the X-Ray source ( 12 ) when the X-Ray device ( 10 ) is in the first position, wherein the therapy system further comprises vacuum means ( 7 ) for creating a vacuum within the nozzle ( 5 ) so that, in operation, an internal air pressure inside the nozzle is lower than an external air pressure outside the nozzle, and the X-Ray device ( 10 ) is rotatably mounted into the nozzle between the first and the second position about a rotation axis ( 20 ).