Dosimetric comparison between VMAT plans using the fast-rotating O-ring linac with dual-layer stacked MLC and helical tomotherapy for nasopharyngeal carcinoma

Table 2 Dose constraints for inverse planning

Priority	Structure	Constraints
1	P-GTV	D₉₅ ≥ 67.2 Gy (100%) or D₉₉ ≥ 95% of the prescribed dose (67.2 Gy)
	P-GTV	V_{73.9 (110%)} ≤ 1 cm³
	P-CTV	D₉₅ ≥ 56.0 Gy (100%) or D₉₈ ≥ 95% of the prescribed dose (56.0 Gy (100%))
	P-cord^a	D_max ≤ 45.0 Gy
	Optic nerve and optic chiasm	D_max ≤ 50.0 Gy
2	Brain stem	D_max ≤ 50.0 Gy
	Parotid glands	D_mean ≤ 26.0 Gy
	Oral cavity	D_mean ≤ 30.0 Gy
	Oral cavity	V₃₀ ≤ 30%
3	Lens	D_max ≤ 10.0 Gy
	Eyeball	D_max ≤ 50.0 Gy
	Esophagus and Cochlea	D_mean ≤ 35.0 Gy
4	Submandibular glands	D_mean ≤ 30.0 Gy
4	Brain	D_max ≤ 60.0 Gy

P-GTV, planning gross target volume; P-CTV, planning clinical target volume; P-cord, planning spinal cord; D_V, D dose delivered to V% of organ volume; V_D, absolute or percentage of organ volume receiving D Gy or higher; D_max, maximum dose; D_mean, mean dose
^aP-Cord means the planning volume for the spinal cord which was generated by adding 3–5 mm margin to the actual spinal cord

ISSN: 1748-717X