Effects of metal implants and a metal artifact reduction tool on calculation accuracy of AAA and Acuros XB algorithms in small fields

PubMedID: 31508819Purpose: In this study, the dosimetric accuracy of analytical anisotropic algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms (Varian Medical Systems, Palo Alto, CA) was investigated for small radiation fields incident on phantoms of various metals that include stainless steel grade 316L (SS316L) and titanium alloy grade 5 (Ti5) implants. In addition, the effects of using metal artifact reduction for orthopedic implants (O-MAR, Philips Healthcare, Cleveland, OH) were evaluated. Methods: The evaluations of AAA and AXB were performed by comparing the crossline profiles calculated by AAA and AXB with GafChromicTM EBT3 film measurements at the phantom–implant interfaces and in close vicinity of implant materials for small field sizes (1 × 1 cm2, 2 × 2 cm2, 3 × 3 cm2, and 4 × 4 cm2) of a 6 MV flattening filter free photon beam. O-MAR corrected and uncorrected (UC) computed tomography (CT) images were used for dose calculations. The values of average and standard deviations (SD) of Hounsfield unit (HU) for selected regions of each case were evaluated. The differences in average dose percentages in defined regions were calculated to quantify the relative dosimetric changes between doses calculated on UC and O-MAR corrected CT images. Results: Compared to UC images, the values of SD were reduced, and the average HU became closer to its reference value in the O-MAR images. There was some discrepancy in average dose percentage differences between calculations using UC and O-MAR images at 1 cm above the SS316L implant (average dose percentage differences were AXB/UC = 5.9% and AXB/O-MAR = -1.2%; AAA/UC = 2.2%, and AAA/O-MAR = -0.8%). Neither AAA nor AXB algorithms predict increase in dose at upper phantom–implant interface (4.9%, 9.9%. 13.5%, and 13.8% for the fields from 1 × 1 cm2 to 4 × 4 cm2, respectively). At the side of the SS316L implant (where dark streak artifacts exist), dose difference averages were estimated as - 1.1% and 22.3% when AXB/O-MAR and AXB/UC calculations are compared with EBT3 measurements, respectively. Dose predictions at 1 cm below the SS316L implant were underestimated by AXB/O-MAR (average -0.5%) and AXB/UC (average 2.0%). Conclusions: The O-MAR tool was shown to have a favorable dosimetric effect or no effect on the calculations in the upper proximity of the implant materials. The dose differences between EBT3 film measurements and calculations at upper phantom–implant interfaces were smaller when they were calculated using O-MAR images. However, the dose differences increased when O-MAR corrected images were used for AAA calculations at lower phantom–implant interfaces. Use of O-MAR enabled closer agreement for the AXB algorithm, especially in the dark streak artifact regions. The O-MAR algorithm should be used when the dose is calculated with the AXB algorithm in cases of patients with the metal implants. The estimations using AAA and AXB algorithms, in phantom setups, with Ti5 implant material were found to be closer to the EBT3 film measurements, when compared with the same estimations using SS316L implant material. © 2019 American Association of Physicists in MedicineFBA-2018-8701 Philips Oral HealthcareThe authors received no financial support from Philips Healthcare for conducting this research. The dosimetric data acquisition of this study was performed in the Acibadem Hospital, Adana, Turkey. CT images used in this study were scanned at the Adana City Training and Research Hospital, Turkey. The authors thank medical physicist Gökay Ceyran for his invaluable support and help. This project was supported by Cukurova University Research Project Development and Coordination Unit under the project number FBA-2018-8701. We thank the Cukurova University