ESTRO 2020 Abstract book

S432 ESTRO 2020

outcome. In this study, we investigated potential correlations between the plan robustness (to both range and set-up uncertainty) and tumor recurrence for skull base Chordoma and Chondrosarcoma patients treated at PSI with pencil beam scanned (PBS) proton therapy between 2003-2017. Material and Methods Between 2003 and 2017, 222 patients with skull-base chondrosarcomas and Chordomas were treated with PBS PT at PSI, to a median total dose of 70CGE and 74CGE respectively. All plans were optimized on a 5mm isotropical expansion of the CTV using the in-house developed treatment planning system. Follow-up MRIs have been systematically acquired and local failures (LFs) identified and contoured in 49 (22%) out of 222 patients. For all cases, treatment robustness to both set-up (1.76 mm for bite-block and 2.25 mm for mask considering Confidence Interval of 85%) and range (3%) have been retrospectively computed, and error bar distributions generated using the 'worst case scenario' approach (1,2). Finally, dosimetric and robustness parameters calculated from Dose Volume Histograms and Error Volume Histograms (EVH) were computed for both the full CTV, as well as the union of the recurrence volume and CTV, for the full delivered treatment plan. Results A minor correlation has been found between target under- dosage and recurrence (Figure 1) with the mean dose (Dmean) to the GTV and CTV being lower (95% compared to 98%) for those patients who suffered a local failure, but this dos-difference was not significant. However, no correlations were found between LF and robustness for either the whole CTV or for the overlapping regions of recurrence (Figure 2). Indeed, dose robustness has been found to be generally better (EVH shifted to the bottom left) in the recurrence region compared to the whole CTV (figure 2).

Conclusion Since protons are more sensitive to density variations than photons, the translation of override methods from VMAT to IMPT planning requires some caution. The override of regions to higher densities in the proton beam path can create hotspots. There was no benefit of the diaphragm override approach in terms of inter-fractional robustness. We therefore do not recommend this approach as a default option in IMPT robust optimisation. OC-0702 Is there a correlation between robustness and tumor control for skull base proton PBS treatments? C. De Angelis 1 , F. Albertini 1 , D.C. Weber 1 , M. Walser 1 , A. Lomax 1 , A. Bolsi 1 1 PSI, Protontherapy, Villigen, Switzerland Purpose or Objective Chordomas and Chondrosarcomas are Skull base bone tumors typically treated with proton therapy. Due to their proximity to critical organs, the surgical approach for a total resection, as well as the delivery of high dose radiation with photons, is often not possible. Due to the well-defined range of protons, highly conformal dose distribution to the target can be delivered while sparing neighboring critical structures. However, there are fears that plans might be sensitive (unrobust) to range and set- up uncertainties, potentially affecting patient’s clinical