ESTRO 2022 - Abstract Book

S710

Abstract book

ESTRO 2022

Failing GPR for highly modulated SBRT/SRS plans can be an indication for sub-optimal DLG values in Eclipse. MCS can be used to indicate for which energy and treatment technique the DLG needs adaption. Adaption of the DLG lead to GPR that were clinically acceptable. [1] Vieillevigne, L.; Khamphan, C.; Saez, J.; Hernandez, V. On the Need for Tuning the Dosimetric Leaf Gap for Stereotactic Treatment Plans in the Eclipse Treatment Planning System. Journal of Applied Clinical Medical Physics 2019 , 20 (7), 68–77 DOI: 10.1002/acm2.12656. [2] LoSasso T.; Chui C.S.; Ling C.C. Physical and dosimetrical aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy. Med.Phys. 1998 , 25, 1919-1927. [3] Masi L; Doro R; Favuzza V; Cipressi S; Livi L. Impact of Plan Parameters on the Dosimetric Accuracy of Volumetric Modulated Arc Therapy. Medical Physics 2013 , 40 (7), 071718–071718 DOI: 10.1118/1.4810969.

MO-0792 Altered breathing pattern can jeopardize PBS proton therapy of locally advanced NSCLC

L.B. Hysing 1 , L. Bolstad Hysing 2 , C. Grindeland Boer 1 , K. Fjellanger 1,2 , I.M. Sandvik 1 , M. Ugland 1 , G.M. Engeseth 1,3

1 Haukeland University Hospital, Oncology and Medical physics, Bergen, Norway; 2 University of Bergen, Physics and technology, Bergen, Norway; 3 University of Bergen, Clinical science, Bergen, Norway Purpose or Objective Patients with locally advanced non-small cell lung cancer (LA-NSCLC) have poor prognosis, with treatment intensifications prevented by high toxicity rates from state-of-the-art multimodal therapy. Pencil beam scanning proton therapy (PBS-PT) has potential to spare the lungs and heart compared to IMRT. However, little knowledge exists on how uncertainties occurring between planning (Plan) and start of treatment (Start) influence organ at risk sparing and target coverage. The purpose of this prospective simulation study was to evaluate if the clinical potential of PBS-PT persists from Plan to Start, in order to guide its use in LA-NSCLC. Materials and Methods 4DCT imaging at Plan and Start (fraction 2 or 3) was carried out for 15 patients that received state-of-the-art IMRT with prescribed doses of 60-66 Gy in 2 Gy fractions. Three PBS-PT plans were created per patient: 3D-robust single field uniform dose (SFUD), 3D-robust intensity-modulated proton therapy (IMPT) and 4D-robust IMPT (4DIMPT). Target coverage and dose- volume parameters relevant for toxicity were compared across PBS-PT and IMRT. Robustness towards setup and range , breathing motion and interplay were investigated at Plan, and robustness towards changes in breathing motion and anatomy was investigated at Start (Table).

Results Sparing at Plan with PBS-PT compared to IMRT was significant and largest with IMPT, followed by 4DIMPT, SFUD and IMRT, and persisted at Start (Figure 1). All plans met the preset criteria for target robustness at Plan (Figure 2). At Start, D98% was >95% for 12/15 patients with PBS-PT. Three patients had lack of CTV coverage (worst for IMPT with D98% of 82-93%), mainly in the mediastinal lymph nodes, caused by changes in breathing-motion pattern between Plan and Start (Figure 2).

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