Dose difference between anisotropic analytical algorithm (AAA) and Acuros XB (AXB) caused by target’s air content for volumetric modulated arc therapy of head and neck cancer

BACKGROUND: We clarified the dose difference between the anisotropic analytical algorithm (AAA) and Acuros XB (AXB) with increasing target’s air content using a virtual phantom and clinical cases. MATERIALS AND METHODS: Whole neck volumetric modulated arc therapy (VMAT) plan was transferred into a virtual phantom with a cylindrical air structure at the center. The diameter of the air structure was changed from 0 to 6 cm, and the target’s air content defined as the air/planning target volume (PTV) in percent (air/PTV) was varied. VMAT plans were recalculated by AAA and AXB with the same monitor unit (MU) and multi-leaf collimator (MLC) motions. The dose at each air/PTV (5%–30%) was compared between each algorithm with D(98%), D(95%), D(50%) and D(2%) for the PTV. In addition, MUs were also compared with the same MLC motions between the D(95%) prescription with AAA (AAA_D(95%)), AXB_D(95%), and the prescription to 100% minus air/PTV (AXB_D(100%-air/PTV)) in clinical cases of head and neck (HNC). RESULTS: When air/PTV increased (5–30%), the dose differences between AAA and AXB for D(98%), D(95%), D(50%) and D(2%) were 3.08–15.72%, 2.35–13.92%, 0.63–4.59%, and 0.14–6.44%, respectively. At clinical cases with air/PTV of 5.61% and 28.19%, compared to AAA_D(95%), the MUs differences were, respectively, 2.03% and 6.74% for AXB_D(95%) and 1.80% and 0.50% for AXB_D(100%-air/PTV). CONCLUSION: The dose difference between AAA and AXB increased as the target’s air content increased, and AXB_D(95%) resulted in a dose escalation over AAA_D(95%) when the target’s air content was ≥ 5%. The D(100%-air/PTV) of PTV using AXB was comparable to the D(95%) of PTV using AAA.