Abstract Book

S153

ESTRO 37

Purpose or Objective To investigate the feasibility of hypoxia-PET based dose escalation (DE) in head and neck cancer (HN C) patients with a high-field MR-linac and to compare treatment plan quality of DE applied with the MR-linac

and 4DMRI. Entire liver and lung volumes below T8 vertebral level were contoured on inspiratory and expiratory phases of 4DMRI. Change in lung volumes were calculated between maximal inspiratory and expiratory bins. In addition, Euclidian distance was used to measure liver motion. Results To date, five patients and three volunteers have been scanned. Results of patient tumour movement are shown in Table 1. The mean amplitude of movement seen on 4DCT and 4DMRI was 1.36cm (range 0.28-2.59cm) and 1.36cm (range 0.53-2.19cm) respectively. Tumour delineation appeared equivalent or superior on the 4DMRI compared to 4DCT (see Figure 1). All 4DMRI sequences were <7minutes in duration and were well tolerated. In three healthy volunteers, AC reduced amplitude of liver movement and lung volumes as seen on 4DMRI.

and a standard linac. Material and Methods Ten patients with locally advanced HNC who

received dynamic FMISO PET/CT scans and a treatment planning CT prior to definitive radio-chemotherapy in a phase II dose escalation trial were retrospectively included in this study. Two treatment plans were created for each patient with Monaco Research 5.19.03 (Elekta AB, Stockholm Sweden) using the same patient model and fractionation scheme with 30×2 Gy and 30×1.8 Gy in the PTV 60 and PTV 50 , respectively and a sequential boost of 5×2 Gy in the PTV 70 . Dose was escalated to 2.2 Gy for all fractions in hypoxic sub-volumes (HV) of the PTV 70 identified by dynamic FMISO PET. Delineations of HVs were available from the original treatment plan. For the standard linac, VMAT plans with two full arcs were created. For the MR-linac (Elekta AB, Stockholm Sweden), Step-and-Shoot (SNS) plans with nine beams were created with automatically optimized beam angles based on the angular MU distributions in the corresponding VMAT plan. Specifications of both treatment units are given in Tab. 1. Treatment plans were optimized to show similar mean doses in the escalation volume and PTV 70 . Typical dose parameters were compared between the two plans using a Wilcoxon signed rank test in R 3.2.3.

Results For both techniques, it was generally possible to create plans adhering to our internal guidelines, though compromises were slightly more frequent for MR-linac plans, e.g. with respect to D max in the mandible. Hypoxia dose escalation as per study protocol was possible for MR- linac plans, with a median D mean of 75.6 Gy in the escalation volume. MR- and standard linac plans showed similar coverage of the escalation volume with a median difference of 0.24 Gy in D mean (p=0.32). For PTV 70 a small, but significant, dose increase (0.22 Gy, p=0.02) was found in MR-linac plans. This trend persists in PTV 60 (0.85 Gy, p=0.01) and PTV 54 (0.66 Gy, p=0.01). The reason is a lower conformity of the MR-linac dose distributions. Only for one major organ at risk parameter a significant difference was found, an increased D mean in the right parotids in MR-linac plans (Fig. 1 shows positive differences for an increased dose in MR-linac plans). Further differences were observed in OARs with lower priority, e.g. a median increase in mandible D max of 0.96 Gy (p=0.04).

Conclusion Further evaluation of motion, artefact and image quality is under way in a larger cohort of patients. Superior soft tissue delineation and reduced radiation exposure may mean 4DMRI is a suitable replacement for 4DCT. AC combined with 4DMRI is a feasible technique and may benefit patients having liver SBRT. OC-0296 High-field MR-linac treatment plans for hypoxia dose escalation in head and neck cancer D. Mönnich 1 , M. Nachbar 1 , E. Neuhaus 1 , C. Gani 2 , S. Boeke 1,2 , S. Welz 2 , D. Zips 2,3 , D. Thorwarth 1,3 1 University Hospital Tübingen, Section for Biomedical Physics- Department of Radiation Oncology, Tübingen, Germany 2 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany 3 German Cancer Research Center DKFZ, German Cancer Consortium DKTK partner site Tübingen, Heidelberg, Germany