Purpose. To compare the dosimetric differences between volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT)\nin treating early T-stage nasopharyngeal carcinoma (NPC). Method. Ten patients with early T-stageNPCwho received tomotherapy\nusing simultaneously integrated boost (SIB) strategies were replanned with VMAT (RapidArc of Varian, dual-arc). Dosimetric\ncomparisons between the RapidArc plan and the HT plan included the following: (1) D98, homogeneity, and conformity of PTVs;\n(2) sparing of organs at risk (OARs); (3) delivery time andmonitor units (MUs). Results. (1) Compared with RapidArc,HT achieved\nbetter dose conformity (CI of PGTVnx + nd: 0.861 versus 0.818, P = 0.004). (2) In terms of OAR protection, RapidArc exhibited\nsignificant superiority in sparing ipsilateral optic nerve (Dmax: 27.5Gy versus 49.1Gy, P < 0.001;D2: 23.5Gy versus 48.2Gy, P < 0.001),\ncontralateral optic nerve (Dmax: 30.4Gy versus 49.2Gy, P < 0.001; D2: 26.2Gy versus 48.1Gy, P < 0.001), and optic chiasm (Dmax:\n32.8Gy versus 48.3Gy, P < 0.001; D2: 30Gy versus 47.6Gy, P < 0.001). HT demonstrated a superior ability to protect the brain stem\n(D1cc: 43.0Gy versus 45.2Gy, P = 0.012), ipsilateral temporal lobe (Dmax 64.5Gy versus 66.4 Gy, P = 0.015), contralateral temporal\nlobe (Dmax: 62.8Gy versus 65.1Gy, P = 0.001), ipsilateral lens (Dmax: 4.27Gy versus 5.24Gy, P = 0.009; D2: 4.00Gy versus 5.05Gy,\nP = 0.002; Dmean: 2.99Gy versus 4.31Gy, P < 0.001), contralateral lens (Dmax: 4.25Gy versus 5.09Gy, P = 0.047; D2: 3.91Gy versus\n4.92Gy, P = 0.005; Dmean: 2.91Gy versus 4.18Gy, P < 0.001), ipsilateral parotid (Dmean: 36.4Gy versus 41.1Gy, P = 0.002; V30Gy:\n54.8% versus 70.4%, P = 0.009), and contralateral parotid (Dmean: 33.4Gy versus 39.1Gy, P < 0.001; V30Gy: 48.2% versus 67.3%, P\n= 0.005). There were no statistically significant differences in spinal cord or pituitary protection between the RapidArc plan and the\nHT plan. (3) RapidArc achieved a much shorter delivery time (3.8 min versus 7.5 min, P < 0.001) and a lower MU (618MUs versus\n5646MUs, P < 0.001). Conclusion. Our results showthat RapidArc andHT are comparable inD98, dose homogeneity, and protection\nof the spinal cord and pituitary gland. RapidArc performs better in shortening delivery time, lowering MUs, and sparing the optic\nnerve and optic chiasm. HT is superior in dose conformity and protection of the brain stem, temporal lobe, lens, and parotid.
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