ESTRO 2021 Abstract Book

S1615

ESTRO 2021

Conclusion The relevance of the dose prescription concept is increasing being eroded by modern SABR practice when treating challenging anatomical sites. In this study SABR for abdominal nodes and liver disease shows nominal changes in dose prescription by up to 5Gy is possible in more than half of cases and is also possible in a significant proportion of pelvic node of pancreas cases. This brings into question the merits of reporting patient outcomes, such as local control, against prescription dose. We are actively evaluating the relevance of the dose prescription concept using daily adaptive plans to evaluate delivered BED dose.

PO-1896 Feasibility of mastication-structure-sparing radiation therapy for head and neck cancer C. Olsson 1 , I. Raunert 2 , N. Pauli 3 , L. Tuomi 3 , C. Finizia 3 , N. Pettersson 1

1 Institute of Clinical Sciences, the Sahlgrenska Academy Gothenburg University, Department of Radiation Physics, Gothenburg, Sweden; 2 Sahlgrenska University Hospital, Department of Medical Physics and Biomedical Engineering, Gothenburg, Sweden; 3 Institute of Clinical Sciences, the Sahlgrenska Academy Gothenburg University, Department of Otorhinolaryngology, Head and Neck surgery, Gothenburg, Sweden Purpose or Objective Although radiation-induced side-effects affecting mastication functionality have been studied in head and neck cancer (HNC) radiation therapy (RT), dose constraints for the associated structures are rarely included during treatment plan optimization. Previous research has identified several radiation dose relationships with mean dose thresholds around 30-40 Gy for masseter muscles, 40-50 Gy for pterygoid muscles, and 15-60 Gy for temporomandibular joint (TMJ) relating to a 10% trismus risk post RT [1] 7 [2] 8 [3] 9 [4] 10 . The purpose of this work was to use a multi-criteria optimization (MCO) approach to investigate to what extent doses to these structures can be lowered without violating existing clinical treatment goals in inverse planning of HNC RT.

[1] Hauge C et al. Acta Oncol 2018:57:1038 [2] Rao SD et al. Acta Oncol 2016:55:99 [3] Kamal M et al. RO 2020:149:142 [4] Pauli N et al. Acta Oncol 2016: 55:788 Materials and Methods

This exploratory treatment planning study used data from 22 HNC patients treated to 68 Gy without mastication-structure- sparing intent in 2017-2019 at one institute in Sweden. Original volumetric-modulated arc therapy (VMAT) plans were re- activated in the Eclipse treatment planning system and masseter muscles, pterygoid muscles (medial and lateral), and TMJ were consistently delineated according to a previously reported delineation manual 4 . Re-planning was done using the MCO function of Eclipse with the resulting dose distribution normalized to match the clinical target volume (CTV T) mean dose of the clinical treatment plan. Dose differences between MCO and clinical plans were not allowed to exceed 2 Gy for any