ESTRO 2022 - Abstract Book

S610

Abstract book

ESTRO 2022

dose outside the GTV. To avoid LR, dose up to 1.6cm outside the GTV should be increased for patients with high peritumour density by larger margins or constraints on PTV heterogeneity to reduce sharpness of dose fall-off.

PD-0674 Verification protocol of Fractionated Thoracic Radiation Treated with Deep Inspiratory Breath Hold

S. appel 1 , Z. Symon 1 , Y. Lawrence 1

1 Sheba Medical Center, Radiation Oncology, Ramat Gan, Israel

Purpose or Objective Deep inspiration breath-hold (DIBH) may reduce lung exposure to radiation and reduce tumor motion, thus improve local control and minimize pneumonitis risk. However, department specific confirmation of the robustness and reproducibility of the technique in lung cancer patients with VMAT delivery is a prerequisite for safe practice. Materials and Methods We retrospectively studied cases treated with DIBH for locally advanced lung and thymic malignancies starting 1/2019- 8/2021. Indications for DIBH use were cases expected suffer radiation pneumonitis. Simulation, CBCT and radiation delivery were synchronized automatically using the RPM system (Varian) with audio guidance; predefined threshold was used for automated gating. Treatment verification was daily online CBCT with soft tissue registration. Margins used from GTV to CTV: 5 mm; CTV to PTV: 5-8 mm. For each case we retrospectively reviewed the respiratory waveform and CBCT s acquired prior to each fraction. For every case at least six CBCT and waveforms were analyzed (2 in each third) Poor compliance with DIBH was defined if treatment was prolonged beyond 6 DIBH cycles or inability to maintain a stable breath hold of at least 20 sec. DIBH reproducibility (lung inflation) was determined by comparing lung and diaphragm silhouette between planning CT and CBCT. If this matched the planning CT (< 1 cm difference) it was "adequate". Minor changes were "satisfactory" and significant changes on several fractions were "inadequate". Set up accuracy was determined from CBCT: perfect- tumor within the CTV; adequate- tumor within PTV; inadequate- tumor outside the PTV. Results 106 pts were treated: Histology: NSCLC 80/106 (75%), SCLC 21/106 (20%), thymic malignancies 5/106 (5%). Definitive chemoradiation in 101/106 (95%), or adjuvant radiation in 5/106 (5%) to mean radiation dose 59.9 Gray delivered in 30 fractions with VMAT. 13 cases treated concurrent with SBRT to primary lung lesion that was peripheral to the mediastinal nodal spread. Indications for DIBH were COPD (11%), large PTV (44%), small lung volume (10%) prior immunotherapy (IT) (5%), re-radiation (3%), or prior lobectomy (13%). Compliance with DIBH was optimal 93% (99/106) completed treatment. 4 patients were unable to comply with DIBH in the planning CT; 3 cases have begun treatment with DIBH but could not finish (total 7/106, 6.6%, all female). DIBH reproducibility (lung inflation) was reproduced throughout fractions adequately in 91/102 (89%), satisfactory in 8/102 (8%) but inadequate in 3/102 (3%). Set up accuracy was perfectly registered to the CTV in 86/102 (84%), adequate (inside PTV) in 13/102 (13%) but inadequate in 3/102 (3%).