Extended CT density range: Acuros XB algorithm performance

Abstract

Modern treatment planning systems (TPS) offer photon algorithms capable of enhanced performance in heterogeneous media and high density materials, such as metal implants. Acuros XB was implemented by Varian Medical Systems in their Eclipse TPS. Computed tomography (CT) reconstruction can be performed in two ways: a 16 bits depth for HU or 12 bits. A 12 bit study would have a reduced HU scale, describing normal tissues accurately, but underestimating metal objects. Whenever metal objects can be found in the images, 16 bits are needed to have a realistic estimate of their densities. This work addresses the calculation differences between Acuros XB 13.6 using these two CT configurations. A Philips Brilliant Big Bore CT scanner has been used to scan phantoms with different inserts mimicking different metal implants and both configurations. Head and Neck VMAT and conformal plans (two full arc plans, as per local protocol) were prepared and optimized in Eclipse v13.6. Coverage and dose figures of merit were assessed, and DVH curves compared. Dose to water was used throughout this study. Acuros XB results show an increase in maximum dose for distributions obtained in 16 bits image sets: for instance, from 111.0% (12 bits) to 138.2% for PTV60, in a phantom with Ti dental implants (insert) along the mandible. Differences between Accuros XB results with 12 or 16 bits reconstructions are statistically significant (Student’s T test, p = 0.05). Therefore, the use of extended HU ranges helps address the effect of metal objects inside a patient.