spot sizes in an exemplary embodiment the spot size was selected to be u03c3 0 32 mm 3 29 u03c3 98 1 mm by defining a value of the betatron function of u03b2 1 m at the patient a scanning system follows the directional triplet bending magnets 55 a first scanning magnet 57 has a maximum dipole magnetic field of b sc1 u00b1 0 73 t the field is required for the protons with maximum kinetic energy of 250 mev and to provide for the maximum transverse distances of u00b1 10 cm u2212 bending angle of 0 0903 rad a second large aperture scanning magnet 58 brings the beam to a normal angle of incidence to the patient a gantry total length which is defined as t for the projection to the horizontal axis is equal to p 6 367 meters or can range from 6 meters to 8 meters a vertical distance of delivery w which is between the patient and the second scanning magnet 42 is equal to w 0 73 m and roughly can be extended between t 0 5 m to 1 5 m a distance between the last magnet of the transport part of the gantry and the beam spot triplet magnets is between 9 cm 12 cm the dimensions described in connection with fig5 are only provided to illustrate a range length when physically constructed referring now to fig6 shown is a path of an ion beam 90 through the gantry the figure is not necessarily drawn to scale and the position and dimensions of the ion beam 90 may be exaggerated for illustration purposes three transverse positions are shown a negative beam offset 59 a zero beam offset 60 and a positive beam offset 61 the zero.