Progressive resolution optimizer (PRO) predominates over photon optimizer (PO) in sparing of spinal cord for spine SABR VMAT plans

Table 1 Planning constraints of planning target volume, normal organs and planning organ at risk volumes for spine stereotactic ablative radiotherapy plans

Structure	Planning constraints	Priority
PTV	D0% < 18.5 Gy	200
PTV	D100% > 18 Gy	150
Spinal cord (or spinal cord PRV)	D_1.2 cc < 7 Gy	120
	D_0.35 cc < 10 Gy	120
	D_0.035 cc < 14 Gy	150
Cauda equine (or cauda equine PRV)	D_{5 cc} < 14 Gy	120
Cauda equine (or cauda equine PRV)	D_0.035 cc < 16 Gy	150

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