Abstract
The purpose of this study was to demonstrate that Monte Carlo treatment planning systems require tissue characterization (density and composition) as a function of CT number. A discrete set of tissue classes with a specific composition is introduced. In the current work we demonstrate that, for megavoltage photon radiotherapy, only the hydrogen content of the different tissues is of interest. This conclusion might have an impact on MRI‐based dose calculations and on MVCT calibration using tissue substitutes. A stoichiometric calibration was performed, grouping tissues with similar atomic composition into 15 dosimetrically equivalent subsets. To demonstrate the importance of hydrogen, a new scheme was derived, with correct hydrogen content, complemented by oxygen (all elements differing from hydrogen are replaced by oxygen). Mass attenuation coefficients and mass stopping powers for this scheme were calculated and compared to the original scheme. Twenty‐five CyberKnife treatment plans were recalculated by an in‐house developed Monte Carlo system using tissue density and hydrogen content derived from the CT images. The results were compared to Monte Carlo simulations using the original stoichiometric calibration. Between 300 keV and 3 MeV, the relative difference of mass attenuation coefficients is under 1% within all subsets. Between 10 keV and 20 MeV, the relative difference of mass stopping powers goes up to 5% in hard bone and remains below 2% for all other tissue subsets. Dose‐volume histograms (DVHs) of the treatment plans present no visual difference between the two schemes. Relative differences of dose indexes D98,D95,D50,D05,D02, and Dmean were analyzed and a distribution centered around zero and of standard deviation below 2% (3σ) was established. On the other hand, once the hydrogen content is slightly modified, important dose differences are obtained. Monte Carlo dose planning in the field of megavoltage photon radiotherapy is fully achievable using only hydrogen content of tissues, a conclusion that might impact MRI dose calculation, but can also help selecting the optimal tissue substitutes when calibrating MVCT devices.PACS numbers: 87.55.D‐, 87.55.dk, 87.55.K‐, 87.57.Q‐
Highlights
118 Demol et al.: Hydrogen content for Monte Carlo calculation scale is always divided into a finite number of subsets to link computed tomography (CT) number to chemical elemental composition.[1,2] Usually, six or fewer media of average composition are defined
Mass attenuation coefficients for a mixture can be determined by the weighted sum of the constituent elements.[9] for the mass stopping powers of compounds, this method can be used as a close approximation.[10]. As these two quantities govern dose deposition in a Monte Carlo algorithm, it is reasonable to assume that only the hydrogen content could be sufficient for dose calculation in human tissues, and it is considered as the main hypothesis of this study
Discrepancies between the CT numbers calculated from the stoichiometric model and the measured values are presented in Table 2, allowing a qualitative appreciation of the coherence between the model and the measures
Summary
118 Demol et al.: Hydrogen content for Monte Carlo calculation scale is always divided into a finite number of subsets to link CT number to chemical elemental composition.[1,2] Usually, six or fewer media of average composition are defined (e.g., air, lung, fat, water, muscle and bone). In this way, some media are averaged on a large range of CT numbers (tissue compositions) (e.g., between soft bone and high density cortical bone the composition changes dramatically).(3) It has been shown by Verhaegen and Devic[4] that large dose errors (up to 10%) for MV photon beams can occur from inaccurate assignment of media. As these two quantities govern dose deposition in a Monte Carlo algorithm, it is reasonable to assume that only the hydrogen content could be sufficient for dose calculation in human tissues, and it is considered as the main hypothesis of this study
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