No abstract

Purpose/Objective(s): Historically, radiation dose prescription and constraints to organs at risk (OAR) for head and neck cancers were validated using type A/B calculation algorithms. Switching to more advanced calculation algorithms results in significant dosimetric differences particularly for high density structures. This work aims to evaluate the dosimetric consequences of replacing the Anisotropic Analytical Algorithm (AAA) by the latest version of Acuros XB (AXB), dose-to-water (Dw) or dose-to-medium (Dm), for RapidArc plans of nasopharyngeal carcinomas (NPC), with special focus on the mandible. Materials/Methods: Seventeen locally advanced NPC plans generated with AAA (v.15.6) were recalculated with AXB (v.15.6) Dw and Dm. The dose-volume histogram (DVH) parameters for the planning target volumes (PTV) and relevant OARs were compared between AXB and AAA. The high dose PTV was divided into bone, air and tissue components and the DVH comparison was performed for each of them. Several DVH parameters correlated with an increased risk of osteoradionecrosis were considered for the mandible. 2D Gamma analysis was performed to compare AAA dose distributions to each of the AXB dose reporting modes in axial planes at the mandible level. Results: The DVH calculation using AXB Dw revealed no significant differences to the PTVs compared to AAA. It showed significantly lower absolute values to spinal cord, brainstem, oral cavity and parotids (up to 2.3% median difference), and higher values to cochleas (up to 5.4% difference) and mandible (up to 6.7% difference). AXB Dm predicted lower DVH values than AAA for all PTVs (up to 2% difference) and OARs (up to 6.1% difference). For the PTV bone subvolume, AXB Dw and Dm predicted respectively higher (up to 2.4% difference) and lower (up to 3.4% difference) DVH values. No significant differences were noted for the air component. Both AXB Dw and Dm predicted lower absolute values than AAA for soft tissues. The largest differences were observed for the mandible V 60Gy parameter, with a median difference of 6.7% higher for AXB Dw and 6.0% lower for AXB Dm, when compared to AAA. Gamma analysis showed the largest dose discrepancies over the mandible for both comparisons. The median Gamma agreement, with a 2%/2 mm acceptability criteria, was 97.3% and 96.7% for AXB Dw to AAA and AXB Dm to AAA comparisons, respectively. Conclusion: Significant dose differences were noted when switching from AAA to AXB Dm or Dw in NPC cases. The dose prescription as well as the tolerance limits for some OARs, particularly those of high density, may need to be adjusted depending on the selected dose calculation algorithm and reporting mode. Meanwhile, recalculating an AXB optimized plan with AAA might be considered.