Baroni, G.

Durante, M.

Fattori, G.

Krämer, M.

Orecchia, R.

Riboldi, M.

Scifoni, E.

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Retrospective analysis of treatment plans robustness in image-guided carbon ion treatment of skull-base tumours. G. Fattori1, M. Riboldi1, E. Scifoni2, M. Krämer2, M. Durante2,3,R. Orecchia4,5 and G. Baroni1,4 1Politecnico di Milano, Milano, Italy; 2GSI, Darmstadt, Germany; 3Technische Universität, Darmstadt, Germany; 4CNAO foundation, Pavia, Italy; 5IEO, Milano, Italy;  Clinical procedures for patient positioning are designed to minimize systematic errors in the treatment geometry. However, despite the use of advanced technologies, ana-tomical variations and practical issues limit the accuracy of daily setup. In addition, the estimation of stopping power for different tissues from CT data has intrinsic un-certainties. In order to quantify the degradation of dose distribution due to such residual uncertainties, we have retrospectively tested the robustness of intensity modulated carbon ion plans for the treatment of clivus chordoma. Materials and Methods Ten patients treated at the Centro Nazionale di Adroter-apia Oncologica (IT) were selected for this study. The treatment consisted of 16 fractions, 4.4 Gy (RBE) each. In our study we considered the enlarged PTV applied in the first 10 fractions before boosting the treatment to a small-er volume. The PTV was obtained by applying 2 mm ex-pansion from the CTV. The brainstem was included as organ at risk, with a maximum 30% of the fraction dose.  We derived the setup errors from the clinical practice at the institution [1] featuring six degrees of freedom couch and 2D-3D image registration software. We have identi-fied the ±1.0 mm and ±1.0° ranges to cover the 95% of observed residual setup errors. Orthogonal sampling was applied on the error space, thus defining 64 trials to ex-plore the effects of setup errors in a statistically equiva-lent dataset. Accordingly, the patients’ planning CTs were rigidly transformed and considered for dose recalculation. Treatment planning was based on TRiP98 and LEM-I (α/β = 2) using 6 mm beam FWHM with 2 mm by 3 mm raster grid in lateral and depth dimensions.  We have evaluated DVH bands at 95% (D95CTV) and 105% (D105CTV) dose for CTV and 5% (D05OAR) for the brainstem. In addition, inhomogeneity (IC) and conformi-ty (CI) indexes were calculated for the CTV.  Setup error cases resulting in larger variation on the target DVH were considered for the analysis of stopping power uncertainties. Only the worst-case scenario report-ed by Rietzel et al. [2] for head&neck tissues was consid-ered, thus applying ±2.6% deviations from nominal val-ues.  Results  On 7 patients out of 10, the D95CTV error was always below 10% in presence of setup errors. Instead, larger discrepancies were observed in combination with range uncertainties, whereas their effect on the organ at risk is related to the specific patient anatomy (Figure 1).  The CICTV exhibited reduced sensitivity to setup errors with respect to the ICCTV. Conversely, range uncertainties affected significantly the CICTV, but minor variations were reported for ICCTV. Higher values for ICCTV in nominal conditions were reported to be prone to large deviation when uncertainties occur. Finally, a strong correlation (ρ = 0.91) was found between the nominal treatment plan ICCTV and its score in presence of setup errors. Figure 1: axial dose cut in nominal geometry (left panel), in presence of setup errors (top-right panel) and combination of setup and HU-WE uncertainties (bottom-right panel). Conclusions and outlook We have recalculated the patient dose in presence of setup and range errors to test the treatment plan robust-ness. Even though conformity is generally preserved, the combination of setup and range errors has an impact on the treatment quality. On this basis, we have further in-vestigated the role of fractionation to mitigate the patient dose degradation [3].  References [1] Desplanques M et al. A comparative study between the imaging system and the optical tracking system in proton therapy at CNAO. J Radiat Res 2013;54:i129–35.  [2] Rietzel E et al. Range accuracy in carbon ion treat-ment planning based on CT-calibration with real tis-sue samples. Radiat Oncol 2007;2:14.  [3] Fattori G et al. Dosimetric effects of residual uncer-tainties in carbon ion treatment of head chordoma. Radiother Oncol 2014; 133 (1):66-71. GSI SCIENTIFIC REPORT 2014 APPA-HEALTH-25DOI:10.15120/GR-2015-1-APPA-HEALTH-25 317

Retrospective analysis of treatment plan robustness in image-guided carbon ion treatment of skull base tumors.

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Retrospective analysis of treatment plan robustness in image-guided carbon ion treatment of skull base tumors.

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