ESTRO 36 Abstract Book

S232 ESTRO 36 2017 _______________________________________________________________________________________________

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.

Conclusion Robust CTV-based VMAT optimization in head and neck patients resulted in improved estimated actual given dose distributions with lower normal tissue dose and equal target coverage compared to non-robustly optimized plans. This is the first study to compare robustly optimized photon plans to non-robustly optimized photon plans in terms of dose accumulation using daily CBCT images. The differences in dose are deemed clinically relevant and are expected to lead to an improved method of patient selection for proton therapy.