Treatment Planning FAQ: Acuros XB Dose Reporting Mode

If you’ve ever run a dose calculation in Varian’s Eclipse treatment planning system using the Acuros XB (AXB) algorithm, at some point you may have had some uncertainty around the dose calculation reporting options available to you. As we explained in our FAQ feature on AAA vs Acuros, AXB is an algorithm that analytically solves the Linear Boltzman Transport Equation (LTBE), providing similar results to more time-consuming Monte Carlo calculation models. For other conventional treatment planning system models like AAA, we’re familiar with dose reported to water with variable electron density. But which dose reporting method for AXB (and other MC-based algorithms) is best for clinical use: dose-to-water (D _w) or dose-to-medium (D _m)?

In calculating the final dose to each voxel, AXB is similar to Monte Carlo algorithms in taking into account the physical properties of the materials based on the Hounsfield Units (HUs) present in the CT images. A physical density is assigned to voxels based on CT values, and the LTBE solver calculates the energy-dependent electron fluence through voxels. That means the default calculation reporting mode for AXB is dose-to-medium. If dose-to-water is chosen, the original calculated dose is converted in a post-processing step using different dose response functions dependent on mass collision stopping power ratios.

 

To assess what impact the two reporting modalities might have in comparison to plans previously calculated with AAA, Zifodya, et al. selected ten different treatment plans with mixed sites to recalculate dose using both D _w and D _m reporting methods. For largely homogeneous plans, the differences among plans for the two AXB reporting modes were relatively minor (<1.5%). However, in high density tissues such as bone, larger disparities (>4%) emerged. In their evaluation of the dosimetric differences between D _w and D _m, Chen et al., found differences in the two reporting modes to be potentially clinically significant, as high as 10% near high density materials.

Nearly a decade after the release of AXB, there is still healthy debate on whether to use D _m or D _w for the reported final dose, with compelling arguments on both sides.

 

• Dosimetric commissioning measurements and dose calibration are done in water.
• Our historical clinical experience for tumor response has been using dose-to-water calculation algorithms.
• There are inherent issues involved in using material conversion tables to convert HU values in determining tissue composition for D_m calculations.

• The dose-to-medium mode best represents the true dose based on tissue composition.
• Reporting as Dw requires a stopping power ratio conversion, and there is uncertainty in this conversion.
• Differences in the two reporting modes are not significant for most tissues.

As recently as 2017, regarding clinical implementation of Acuros, Yan, et al. admitted that “further investigation and clarification is needed about which dose reporting mode (dose-to-water or dose-to-medium) should be used in clinics.” In reviewing the literature, there appears to be a slight tendency toward using D _m for patient calculations, and C-M Ma and Jisheng Li agree that D _m is recommended for dose prescription and outcome analysis. With that said, Chen et al. suggest using D _w when calculating to quality assurance phantoms (unless the phantom is a water phantom) due to the CT-to-biological materials conversion potentially not performing well for non-biological materials such as plastics.

With the debate being far from settled, which reporting method do you use? A Twitter poll of clinical users revealed that 53.8% prefer D_m while 30.8% use D_w in the clinic for patient calculations (and 15.4% aren’t sure). Two other votes were provided via email, both in favor of dose-to-medium reporting. What does your clinic use?