Consensus Guide on Hounsfield Look-up Table Generation – Adjustments for Photon Therapy
Abstract
Purpose
Radiotherapy dose calculation is typically based on a computed tomography (CT) scan and requires a conversion of CT numbers to the relevant physical parameter, mass density (MD) or relative electron density (RED) for photon therapy, and stopping-power ratio (SPR) for particle therapy, so-called Hounsfield look-up tables (HLUTs). Recently, a consensus guide was published for generating SPR HLUTs for particle therapy, but similar considerations can be used for HLUT creation in photon therapy. This study extends the consensus guide to be applicable also in photon therapy.
Methods
As tissue-equivalence of phantom inserts is an important aspect of HLUT generation, it was evaluated specifically for photon-based HLUTs (i.e. MD and RED). The open-source code for SPR HLUT generation was updated to also include MD and RED HLUT generation for photon therapy, as well as for vendor-specific CT reconstructions such as DirectDensity (Siemens Healthineers), a kV-independent RED, MD and SPR prediction. To show the wide applicability of the open-source code, it was evaluated for three different CT scanners from different vendors, several tube potentials, three tissue-equivalent CT phantoms (CIRS EPTN, Gammex AED, QRM Multi-Energy), and both regular and DirectDensity reconstructions.
Results
Limited tissue-equivalence was identified for MD of phantom inserts which were otherwise tissue-equivalent in terms of RED and SPR. The strategy for generating MD HLUTs was therefore slightly modified compared to RED and SPR HLUT generation to account for this behavior. The HLUT generation open-source code worked as intended in all evaluated scenarios. HLUTs generated based on CT scans of the three CT phantoms deviated in median over thirteen tube potentials by less than 1% from each other, demonstrating its robustness.
Conclusion
Based on considerations relevant for HLUTs in photon therapy, the open-source code to generate and evaluate a HLUT following the consensus guide was extended to work for photon therapy.