ESTRO 37 Abstract book

ESTRO 37

S489

work is needed to evaluate other aspects of IMPT for the treatment of oropharynx cancer.

PO-0912 A comparison of swallowing sparing IMPT and VMAT for head and neck cancer T. Furre 1 , B. Rekstad 1 , C. Ramberg 1 , T. Hellebust 1 , E. Malinen 2 , E. Dale 3 1 Oslo University Hospital, Department of Medical Physics, Oslo, Norway 2 University of Oslo, Department of Medical Physics, Oslo, Norway 3 Oslo University Hospital, Department of Oncology, Oslo, Norway Purpose or Objective Dysphagia is a common side effect after radiotherapy of head and neck cancer. The purpose of this study is to investigate the potential benefit of robust intensity modulated proton therapy (IMPT) compared to volumetric modulated photon arc radiotherapy (VMAT) for head and neck cancer (HNC) patients focusing on sparing swallowing structures in addition to parotids and spinal cord. Material and Methods Ten previously treated patients with stage III-IVa oro- hypopharyngeal cancer, all with bilateral elective nodes, were included in this treatment planning study. Simultaneously integrated boost (SIB) plans were made with 54/64/68 Gy to elective nodes, high risk volume and GTV, respectively. Four plans were constructed for each patient: standard (std) VMAT and robust IMPT (± 3 mm in x-, y- and z-direction, CTV D 98% ≥ 95%), aiming at sparing parotids and spinal cord, and swallowing sparing (sw) VMAT and robust IMPT also focusing on reducing dose to superior pharyngeal constrictor muscle (SPCM), esophageal inlet muscle (EIM) and supraglottic larynx (SGL). OAR doses and normal tissue complication probabilities (NTCP) were compared. NTCP of late dysphagia (physician-rated RTOG grade 2-4) was calculated using a model based on the mean dose to SPCM and SGL (Christianen MEMC, 2012). NTCP of xerostomia (75% reduction in salivary flow rates) was calculated using the mean dose model (Houweling AC et al , 2010). Results Dose to swallowing structures was reduced with swVMAT compared to stdVMAT and further reduced with swIMPT, resulting in estimated dysphagia NTCP of 28.9 %, 21.3 %, 21.6 % and 14.8 % for stdVMAT, swVMAT, stdIMPT and swIMPT respectively (Table 1). Comparing swVMAT with swIMPT, mean dose to SPCM was reduced by 2.1 Gy, mean SGL dose by 11.0 Gy and mean EIM dose by 1.9 Gy. Dose to contralateral parotid was reduced with protons for 9/10 patients for both std and sw plans. Dose to ipsilateral parotid was reduced with protons for 8/10 patients with std plans and 9/10 with sw plans. Mean contralateral parotid gland dose was reduced from 17.0 Gy with VMAT to 13.4 Gy with IMPT, translating to a mean ΔNTCP of 4.0 % using the mean dose model. Ipsilateral parotid gland dose was reduced from 22.1 Gy to 15.4 Gy, translating to a mean ΔNTCP of 6.6 %. Spinal cord D1% was reduced from 42.2 Gy with photons to 28.8 Gy with protons. Table 1. Dose (median values and ranges) to organs at risk and NTCP for xerostomia and dysphagia (n=10)

Conclusion Swallowing sparing optimization reduced dose to swallowing organs for both photons and protons with largest sparing for protons, resulting in reduced NTCP of dysphagia. Dose to parotids and spinal cord was not affected. NTCP of xerostomia was reduced with protons compared to photons. PO-0913 Evaluation of PlanIQ Quality Algorithm to improve quality of treatment planning for prostate cancer A. Girardi 1 , T. Gevaert 1 , C. Jaudet 1 , G. Coussement 1 , A. Defauw 1 , M. Boussaer 1 , M. Burghelea 1 , J. Dhont 2 , T. Reynders 1 , B. Engels 1 , M. De Ridder 1 1 Universitair Ziekenhuis Brussel, Department of Radiotherapy- Vrije Universiteit Brussel, Brussels, Belgium 2 Universitair Ziekenhuis Brussel, Department of Radiotherapy- Faculty of Medicine and Pharmacy- Vrije Universiteit Brussel, Brussels, Belgium Purpose or Objective Creating an optimal plan in order to guarantee both tumor coverage and organs at risk (OARs) sparing is often challenging. What is dosimetrically achievable and which ones are the criteria for acceptability of a treatment plan is frequently a function of the experience of the planner/physician. The goal of this study is to use the PlanIQ (Sun Nuclear, Melbourne, FL) software to build an algorithm for plan scoring and to evaluate the possibility to use the software as guide to improve the quality of treatment plans. Material and Methods Starting from June 2016 forty clinical prostate treatment plans designed with Raystation 5.0 (RaySearch Medical Laboratories AB, Stockholm, Sweden) and delivered with the VERO SBRT system (Brainlab AG, Feldkirchen, Germany and Mitsubishi Heavy Industries, Tokyo, Japan) using the dynamic wave arcs (DWA) technique were evaluated using PlanIQ. A Plan Quality Algorithm containing 12 Quality Metric Score functions was built, with particular emphasis on the coverage of target volumes (TVs) and sparing of bladder, rectum and the distal half of the rectum. An Adjusted Plan Quality Metric (APQM) score was produced and registered for each plan, based on the goals achieved and on a 3-dimensional dose cloud calculated by the software and used as a benchmark. Plans which APQM score belonging to the first quartile of the overall APQM distribution were selected and re-planned by the same expert planner. The metrics from the two subsets were compared using paired sample t-tests or Wilcoxon tests if the datasets were not well-modeled by a normal distribution. Results Each re-planned treatment had APQM greater than the initial plans and the mean of the APQM of the re-planned cases (79.4±8) were significantly higher (p<0.001) than