Table 1.

Selected proton beam parameters of the CSI fields.

Parameters	Cranium	Cranium	Upper spine	Lower spine
Proton energy at nozzle entrance (MeV)	180	180	160	160
Range in patient (cm H2O)	16	15.7	10.9	10.7
SOBP width (cm H2O)	16	16	5	6
Gantry angle (degree)	255	105	180	180
Air gap (cm)	10.8	11.3	11.5	11.8
Aperture thickness (cm)	6	6	4	4
Proximal margin around CTV (cm)	0.3	0.3	0.3	0.4
Distal margin around CTV (cm)	0.8	0.8	−0.2	0
Ratio of field size to brass aperture size	0.6	0.6	0.34	0.38

SOBP denotes spread-out break peak; CTV denotes clinical target volume.