Abstract Book

ESTRO 37

S496

Delivered BED 99% >100 Gy were correlated to excellent local control rates, regardless of dose algorithm, although PB should not be used, since small field dosimetry in low density tissues must be properly handled for lung SBRT. PO-0922 Implementation of swallowing organs in national radiotherapy guidelines for H&N cancer E. Samsøe 1 , C. Grau 2 , J. Johansen 3 , E. Andersen 4 , J.B.B. Petersen 5 , K. Jensen 2 , B. Smulders 6 , J. Friborg 7 , H.M.B. Sand 8 , M. Andersen 8 , A. Logadóttir 9 , Z. Ujmajuridze 10 , C.R. Hansen 11,12 1 University Hospital Herlev, Department of Oncology- Radiotherapy Research Unit, Herlev, Denmark 2 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 3 Odense University Hospital, Department of Oncology, Odense, Denmark 4 University Hospital Herlev, Department of Oncology, Herlev, Denmark 5 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 6 Rigshospitalet, Department of Oncology- Section of Radiotherapy, Copenhagen, Denmark 7 Rigshospitalet, Department of Oncology, Copenhagen, Denmark 8 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark 9 Zealand University Hospital Næstved, Department of Oncology- Radiotherapy, Næstved, Denmark 10 Zealand University Hospital Næstved, Department of Oncology, Næstved, Denmark 11 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 12 University of Southern Denmark, Institute of Clinical Research, Odense, Denmark Purpose or Objective Our national guidelines for head and neck cancer radiotherapy (RT) have been available since 1990, were revised in 2013 and are currently under further revision (2017). One of the current major revisions is the inclusion of additional swallowing organs at risk (OARs) in the dose volume constraints table. The purpose of this study was to perform the first national dummy run including swallowing organs in the optimization procedure and thus, to establish a common national understanding and awareness of the possibilities to increase the quality of dose plans to limit patients’ side effects after RT. Material and Methods All six centers treating head and neck cancer with RT in our country participated in the dummy run. A CT-scan from a single patient, with pre-arranged delineations of targets and OARs, was sent to the centers. The patient had UICC7 stage III oropharyngeal cancer, and the prescribed dose was 66 Gy to the high-dose PTV, 60 Gy to the high-risk elective volume, and 50 Gy to the low-risk elective volume, all in 33 fractions. Each center prepared a dose plan according to the 2017 version of the national RT guidelines and the dose plans were compared and evaluated at a national workshop with special attention to the following organs involved in the swallowing function: Pharyngeal constrictors, larynx, extended oral cavity, buccal mucosa, esophagus, parotids, and submandibular glands. The planning techniques varied over centers; 7-field IMRT (AAA), Rapidarc with 2-3 arcs (AAA/Acuros), RapidPlan with 3 arcs (AAA) (all Varian) and Autoplan with 1 arc (CC) (Pinnacle). The centers’ experience in optimizing the dose according to the new >85 Gy and BED 50%

were found for the smaller lesions or isolated in the middle of lung tissue.

Within the group of metastases treated with PB, 5 lesions relapsed (n=30; 83.3% crude local control rate) after a median of 9 m post-treatment, while for the MC group, no local recurrence was observed (n=17, 100% local control rate), after a median follow-up of 23 m [3-31 m]. The BED 99% and BED 50% delivered to the PTV were those calculated with MC. The temporal evolution of the averaged BED shows a statistically significant gap ( p <0.0001) when MC algorithm was implemented (Fig. 1).

The BED 99% were strongly correlated with observed local control rates: 100% and 61.5% for BED 99% >85 Gy and <85 Gy ( p <0.0001), respectively and 100% and 58.3% for BED 50% >100 Gy and <100 Gy ( p <0.0001), respectively. For the TCP curves, the adjustment parameters were, D 50 : 50.91 Gy ± 1.51 Gy and 58.08 Gy ± 2.68 Gy for TCP 99% and TCP 50% respectively, whereas the slopes, γ 50 : 0.60 ± 0.08 and 0.53 ± 0.10, respectively. Actuarial local control rates for BED 99% <85 Gy were 72.7% at 9 m, 62.3% at 18 m and 49.9% at 35 m ( p <0.002), respectively, whereas for BED 50% <100 Gy were 70.0% at 9 m, 58.3% at 18 m and 43.7% at 35 m ( p <0.001), respectively. Conclusion and BED 50%