Feasibility of treatment planning on contrast enhanced CTs for H&N cancer
PO-1372
Abstract
Feasibility of treatment planning on contrast enhanced CTs for H&N cancer
Authors: Gober|, Manuela(1)*[manuela.gober@wienerneustadt.lknoe.at];Riegler|, Andreas(1);Zagler|, Norbert(1);Cires|, Dragos-Nicolae(1);Beck|, Florian(1);Kirschner|, Monika(1);Steiner|, Elisabeth(1);Metz|, Martina(1);
(1)Landesklinikum Wiener Neustadt, Institute of Radiatiotherapy and Radiooncology, Wiener Neustadt, Austria;
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Purpose or Objective
For head and neck (H&N) treatment planning, contrast-enhanced CTs (CECT) are helpful for telling the difference between lymph nodes/vessels and blood vessels. For this purpose, a native CT scan and a subsequent contrast agent (CA) enriched CT scan are registered rigidly. Uncertainties in the registration, additional imaging dose for the patient and additional steps in the workflow for radio-oncologists are disadvantages of this method. This study is looking into the feasibility of treatment planning on the CECT itself, as intravenous CA not only increases the contrast but also the Hounsfield units (HU) of the tissue with CA uptake. The aim of this work is to evaluate the influence of CA on dose calculations in H&N treatment planning with focus on VMAT.
Material and Methods
A treatment-planning study was performed which involved the generation of treatment plans on unenhanced CTs and a subsequent transfer of those plans onto the registered CECT. The study included 43 VMAT plans of 19 H&N patients who were treated from August 2018 to January 2019. The dose calculation was performed in Varian Eclipse using the algorithms AAA 11031 and AcurosXB 11031 with dose to medium.
The derived dose distributions were compared and analysed by evaluating the near-minimum D98(%), the near-maximum D2(%) and the median D50(%) absorbed dose values for the PTV. Furthermore, the dose volume histograms (DVHs) were compared visually
The analysis of the dose distributions of the native CT and the dose distribution of the CECT indicated discrepancies in patient positions (deglutition, respiration, etc.) between the two CT data sets. In order to avoid the uncertainty introduced by the setup error, the tissue with CA uptake was contoured and altered by assigning new CT values of 40-50 HU. By using the CA corrected CTs, an evaluation of the effect of CA on dose calculation without the uncertainty arising from setup errors could be performed.
Results
The differences in dose distributions between the native CT and the CECT were 0.13% ± 0.19% for D50(%), -0.36% ± 1.19% for D98(%) and -0.04% ± 0.19% for AAA. For AcurosXB dose differences of 0.15% ± 0.40% for D50(%), -0.20% ± 1.16% for D98(%) and 0.06% ± 0.20% were found.
The comparison of the dose distributions between the CECT and the CA corrected CT showed dose differences of -0.03% ± 0.08% for D50(%), -0.03% ± 0.08% for D98(%) and -0.04% ± 0.07% for AAA and -0.06% ± 0.09% for D50(%), -0.05% ± 0.12% for D98(%) and -0.03% ± 0.08% for AcurosXB.
Conclusion
The setup error between the native CT and the CECT showed to have a larger influence on the dose calculation than the CA itself. The dose difference in treatment planning caused by the presence of the CA is small and no clinical effect is expected. The use of the CECT for VMAT treatment planning is admissible for H&N cancer.