ESTRO 37 Abstract book

S1069

ESTRO 37

EP-1963 Is accurate contouring necessary for salivary and swallowing structure-sparing radiotherapy? A. Delaney 1 , M. Dahele 1 , B. Slotman 1 , W. Verbakel 1 1 VUMC, Cancer Center Amsterdam- Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Current approaches to organ-at-risk (OAR) contouring emphasize accurate, anatomically faithful structures. Not only is this prone to inter-observer variation, but it is also time consuming and results in many individual structures which can make treatment plan optimization more challenging. This can create barriers to the implementation of advanced radiotherapy techniques, especially when resources are restricted. We therefore investigated if a simplified approach to salivary and swallowing structure contouring combined with auto- mated plan optimization could lead to OAR sparing comparable to that of clinical locally-advanced head and neck cancer (HNC) plans, in which OARs were accurately and individually contoured. Material and Methods For 15 HNC patients, the oral cavity, parotid and submandibular glands and the swallowing structures were replaced by simple tube-shaped structures. Simplified contouring was based on interpolating between the cranial and caudal extent of an OAR, without considering the original clinical accurate contours (red contours in Figure). Individual clinical OAR contours in a RapidPlan TM model were combined to create composite salivary and swallowing structures. The simplified contours of each patient were assigned to the composite structures and the oral cavity of the model and used to create “simplified-contouring” knowledge-based plans (SC- plans). SC-plans were compared with their respective clinical plans, which were also automatically optimized, using accurate contours. Metrics for comparison were the boost planning target volume (PTV B )/elective PTV (PTV E ) receiving 95% and 107% of the prescription dose, mean dose to individual OARs using the accurately delineated clinical structures for dose reporting, and mean dose to the composite salivary/swallowing structures (comp sal /comp swal ).

Conclusion Simplified contours for locally advanced HNC treatment planning could be created in minutes and resulted in clinically acceptable automated HNC treatment plans. This shows that it is possible to actively spare clinically relevant salivary and swallowing OARs without the need for numerous high-accuracy, time consuming delineations. Such strategies may help to reduce the resources needed to provide advanced HNC radiotherapy and aid clinical implementation, especially when combined with automated planning. EP-1964 Use of 4DCBCT Symmetry Elekta to evaluate organ motion and set-up error in lung SBRT. A pilot study. C. Di Carlo 1 , M. Nuzzo 1 , A. Allajbej 1 , R. Jimenez De Lorenzo 2 , M. Taraborrelli 1 , L. Caravatta 1 , M. Trignani 1 , M.D. Falco 2 , D. Genovesi 1 1 "SS. Annunziata" Hospital- "G. D'annunzio" University, Radiotherapy, Chieti, Italy 2 "SS. Annunziata" Hospital- "G. D'annunzio" University, Medical Physics, Chieti, Italy Purpose or Objective To evaluate organ motion and set up errors in patients treated with stereotactic radiotherapy for lung cancer or mediastinal lymphadenopathy, using 4D-Cone Beam CT (CBCT) (Symmetry Elekta X-Ray volume imaging system) both in treatment planning and delivery. Material and Methods From September 2016 to April 2017, 6 patients were selected to receive stereotactic radiotherapy to thoracic lesions. Patients characteristics were: mean age 75 years (range 68-85); M/F: 6/0; pulmonary nodules: 4 (66.7%), mediastinal lymphadenopathy: 2 (33.3%). Patients under- went a 3D simulation computed tomography (CT) without contrast, with 3 mm slice thickness and acquired from humeral heads to include the 12 th rib. A 4 fixation points

Results Delineation of simplified-contours required an average of 8 minutes. On average, simplified contours included 95%/75%/93% of the individual salivary glands/oral cavity/swallowing structures. SC-plans were comparable to the clinical plans: composite salivary structures/oral cavity improved by an average of 1.1Gy/1.0Gy in the SC- plans, while dose to composite swallowing structures was an average 0.6Gy higher, largely attributable to higher doses in small-volume swallowing structures (Table). For instance upper esophageal sphincter/cricopharyngeal (smallest swallowing structures, on average) mean dose increased by 9.5Gy/7.5Gy for SC-plans over clinical plans.