ESTRO 36 Abstract Book

S234 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective In the Netherlands, a subgroup of patients eligible for proton therapy will be selected using the model-based approach. So far, patients were selected by comparing robustly optimized proton plans to non-robustly optimized photon plans. However, using non-robust optimization for photon plans may not explore the full potential of this technique, possibly leading to an unfair comparison. The main objective of this study was to investigate the benefit of robust optimization for head and neck using photon techniques. Material and Methods A cohort of nine head and neck cancer patients clinically treated with VMAT were included. Their non-robustly optimized (i.e. PTV-based) VMAT plans were copied and their objectives were altered to create robustly optimized (i.e. CTV-based) VMAT plans. Plans were re-optimized with worst case robust objectives for the targets. Hence, the actual given dose was estimated by calculating the dose on 35 daily cone beam CT (CBCT) scans after position correction, deforming the dose distributions to the planning CT and summing doses from all fractions. All estimated actual given dose distributions were inspected and approved by a physician with experience in head and neck radiotherapy. Treatment plan quality was evaluated using dosimetric parameters and normal tissue complication probability (NTCP) values using previously published models for tube feeding dependence, grade 2-4 dysphagia, xerostomia and sticky saliva, 6 months after treatment. Results Robustly optimised plans resulted in a lower actual given dose estimation for all organs at risk (figure 1). The difference in dose distribution and dose volume histograms of a typical case are displayed in figure 2. The average NTCP values were lower for the robustly optimized plans by 0.5% (p = 0.07) for tube feeding dependence, 1.8% (p = 0.04) for dysphagia, 3.0% (p = 0.01) for xerostomia and 1.1% (p = 0.02) for sticky saliva. Moreover, target dose conformity slightly improved using robustly optimized VMAT optimization.

prostate patients by acquiring the conversion algorithms within and outside an automatically contoured bone outline. The quality of the produced sCT images was quantified by HU and dose distribution comparisons against standard CT images. The treatment planning was conducted with VMAT. Results Figure 1 shows examples of the constructed sCTs with the original MR images of each scanner. The mean HU difference for the sCTs was 11 HUs and 90 HUs in the soft and bone tissue volumes, respectively (n=9). The target volume dose differences compared to the CTs were within 0.8% in all cases (0.2±0.5% [average±SD, n=9]). Table 1 presents the HU and dose distribution differe nces between the sCTs and the actual CT images.

Conclusion This study revealed the feasibility of generating high quality sCTs directly from intensity values of standard T2- weighted MR images. The applied sCT generation method is adjustable for images applied by multiple manufacturers’ scanners with different clinical settings. This work can further contribute to wider clinical implementation of MRI-only based radiotherapy treatment planning.

Proffered Papers: Optimatisation algorithms for treatment planning

OC-0443 Robust optimization of VMAT in head and neck patients D. Wagenaar 1 , R.G.J. Kierkels 1 , J. Free 1 , J.A. Langendijk 1 , E.W. Korevaar 1 1 UMCG University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands