Precision Dose Calculator Dose to Water Ratios for Radixact Plans
Abstract
Purpose
To determine if the Accuray Precision dose calculator computes dose to water or dose to tissue for Radixact plans.
Methods
Two dose calculation scenarios are considered: 1) Uniform tissues, and 2) Finite volumes of bone in water. Reference uniform tissue to water dose ratios were calculated by hand using standard textbook equations and NIST data. Three sets of evaluation uniform tissue to water dose ratios were calculated by: 1) Hand calculations incorporating Precision dose calculation approximations and NIST data, 2) Precision calculations using a custom Fluence Attenuation Table (FAT) with the same NIST data, and 3) Precision calculations using the product FAT. For the finite bone volume case, all dose calculations were done with Precision. 2g/cc cylinders 0 to 7mm radius were coaxially embedded in a 1g/cc cylinder whose radius was set to ensure a constant radiological depth to the center of the cylinder. Bone to water dose ratios were then calculated.
Results
For the uniform tissue case, reference, hand calculation, custom FAT, and product FAT dose ratios for adipose were [1.0020, 1.0000, 1.0002, 1.0006]. Dose ratios for soft tissue and bone were [0.9908, 0.9954, 0.9954, 0.9973], and [0.9263, 0.9299, 0.9335, 0.9594] respectively. For the finite bone volume case, the bone to water dose ratio approaches 1.0 as the volume radius approaches zero. For a 1mm radius the custom and product FAT dose ratios were [0.9609, 0.9764], respectively. For a 2mm radius the custom and product FAT dose ratios were [0.9426, 0.9597], respectively. As the radius approaches Dmax, the dose ratio approaches the values seen in the uniform tissue case.
Conclusion
Accuray Precision doses for Radixact plans depend on the tissue volume. Small volume doses are near dose to water and converge to values somewhere between dose to water and dose to tissue as the volume radius approaches Dmax.