Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy

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‐

Independent Monte-Carlo dose calculation for MLC based CyberKnife radiotherapy

“…Differences of up to 5.6% were found in metal fiducial markers in comparison to the TPS dose calculation partly explained by the IDC framework calculating dose to medium. In contrast to material conversion based on hydrogen content alone (Demol et al 2015), we use CT conversion to 14 different biological materials.…”

Dosimetric characterization of MRI‐only treatment planning for brain tumors in atlas‐based pseudo‐CT images generated from standard T1‐weighted MR images

“…pCTs were registered within the true CT frame using the patient MR image to patient CT image registration, and dose calculation were performed using the  Monte Carlo platform. 24 No pCT-to-true-CT registration was used to avoid introducing a bias in the results. Statistical uncertainties inside the dose voxels were below 1% (2σ) in the Monte Carlo simulations.…”

Integration of the M6 Cyberknife in the Moderato Monte Carlo platform and prediction of beam parameters using machine learning

“…The positions of the nodes, target coordinates and MUs of all beams from a patient plan are retrieved from an xml file (as well as the collimator rotation for the MLC), whereas CT images, structures and doses are imported in Dicom format using the Moderato interface. The CT Hounsfield units (HU) are converted into densities and material composition using CTcreate (BEAMnrc) and an image value to density table (IVDT) obtained with a stoichiometric calibration [16,17]. Absolute calibration was obtained from the Monte Carlo dose for the 60 mm reference field (Gy/primary history), to provide a calibration factor to be multiplied with the number of MUs in the plan.…”