The effect of material assignment in nasal cavity on dose calculation for nasopharyngeal carcinoma (NPC) using Acuros XB

PURPOSE: To evaluate the effect of material assignment in nasal cavity on dose calculation for the volumetric modulated arc therapy (VMAT) of nasopharyngeal carcinoma (NPC) using Acuros XB (AXB) algorithm. METHODS: The VMAT plans of 30 patients with NPC were calculated using AXB with material auto‐assignment of nasal cavity to lung and reassignment to air respectively. The doses to the planning target volumes (PTVs) overlapping with nasal cavity with material auto‐assignment of lung (AXB_Lung) were compared to the values obtained when nasal cavity was reassigned to air (AXB_Air) under the dose‐to‐medium (D (m)) reporting mode of AXB. RESULTS: For dose calculated under AXB_Lung, the D (98%), D (2%), and D (mean) of the PTV(69.96)_Air Cavity (PTV of prescription dose 69.96 Gy overlapping with nasal cavity) were on average 16.1%, 1.6%, and 8.6% larger than that calculated under AXB_Air, respectively. Up to 19.5% difference in D (98%), 3% difference in D (2%), and 11.2% difference in D (mean) were observed in the worst cases for PTV(69.96). Similar trend was observed for the PTV(5940)_Air Cavity, in which the D (98%), D (2%), and D (mean) calculated under AXB_Lung were on average 14.7%, 2.5%, and 10.2% larger than that calculated under AXB_Air, respectively. In the worst cases, the difference observed in D (98%), D (2%), and D (mean) could be up to 17.7%, 4.5%, and 12.7%, respectively. CONCLUSIONS: Significant dose difference calculated by AXB between the material assignment of lung and air in nasal cavity for NPC cases might imply the possibility of underdosage to the PTVs that overlap with inhomogeneity. Therefore, attention should be put to ensure that accurate material assignment for dose calculation under AXB such that optimal dosage was given for tumor control.