ESTRO 2020 Abstract book

S581 ESTRO 2020

SBRT. This update includes our first report of long-term outcomes and tolerances. Material and Methods We retrospectively analyzed consecutive patients with either primary lung cancer or oligometastatic disease treated with lung SBRT and CPAP between 6/2014- 8/2019. All patients received at least 40 Gray. After obtaining consent, patients underwent simulation and treatment with CPAP pressure set at 15 cmH2O for 15 minutes using either free breathing (FB) or deep inspiratory breath hold (DIBH) technique. Patient medical records were search for outcomes. Lung volume with and without CPAP were compared using Wilcoxon signed-rank test. Local control (LC) and progression free survival (PFS) were determined by CT or PET/CT, performed at 3-6- month intervals. Overall survival (OS) was determined according to national records. LC, PFS, and OS were estimated using Kaplan Meir. Data cutoff was on 10/2019. Results Sixty-one patients with either primary lung cancer 23 (25.8%) or oligometastatic disease 66 (74.6%) received treatment for 89 lung lesions using SBRT and CPAP (table 1). Indications for use of CPAP included disease in lower lobes 47 (52.8%) or presence of multiple lung lesion in 26 (29.2%). Mean radiation dose was 52 Gy (SD 4.39). CPAP was used with FB in 54 (60.7%) and DIBH in 35 (39.3%). CT simulations with and without CPAP were available for comparison of lung volumes in 51/61 patients. Use of CPAP increased mean lung volumes in both FB (3139 ml to 3927 ml, p= 0.0001) and DIBH (4913 ml to 5284 ml, p= 0.002). Most patients tolerated CPAP well. One patient suffered prolonged syncope before the first treatment and 2 patients were unable to tolerate the pressure increase. Median follow-up was 702 days; LC at 18 months: 80.1% (96% CI 67%-88%), and at 24 months was LC 68.5% (96% CI 50.5-80%) median not reached. LC did not differ according to DIBH or FB, tumor origin, tumor location or dose (Figure 1). PFS at 18 months was 45.6% (96% CI 30%-60%), and at 24 months was 35.3% (96% CI 20.5%-50.7%), median 526d. OS at 18 months was 77.7% (96% CI 62.4%-87.3%) and at 24 months 62.3% (96% CI 45.7%-75%), median survival not reached. PFS and OS were better for patients with primary lung cancer than for patients with metastatic disease (p=0.001).

Conclusion CPAP was tolerated well and increased lung volume significantly. LC was independent of tumor type, origin, location, CPAP technique or dose. Patients treated for primary lung cancer had better PFS and OS than patients with metastatic disease Additional follow up studies are warranted. PO-1005 Influence of PTV underdosage on local control in patients with central NSCLC treated with SBRT M. Duijm 1 , N. Van der Voort van Zyp 2 , P. Granton 1 , P. Van de Vaart 2 , M. Mast 2 , E. Oomen - de Hoop 1 , M. Hoogeman 1 , J. Nuyttens 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 2 Haaglanden MC, Department of Radiation Oncology, The Hague, The Netherlands Purpose or Objective Stereotactic Body Radiation Therapy (SBRT) of central lung tumors is characterized by risk-adapted fractionation schedules with accompanying Organs At Risk (OAR) dose- constraints. PTV underdosage is often accepted to respect these dose-constraints and to avoid high rates of toxicity. The purpose of this study was to analyze the effect of PTV underdosage on local tumor control and to determine whether other factors influence local- or disease control. Material and Methods Two hundred and twenty patients with centrally located NSCLC treated with ≤12 fractions of SBRT were included. A central tumor was defined as a tumor <2cm from the esophagus and/or main bronchus or first branches of the bronchial tree. PTV underdosage was allowed in order to meet the OAR constraints. Because of variations in treatment schedules, doses were converted into a biologically equivalent dose using an α/β-value of 10Gy (BED 10 ). The following dosimetric PTV parameters were derived from individual DVHs: maximum and minimum point dose (D max , D min ), mean dose (D mean ) and dose to 2% / 50% / 98% of the PTV (D 2% / D 50% / D 98% ). PTV underdosage was defined as the (percentage of) PTV receiving less than 100Gy BED 10 ; (%)V <100 BED 10 . Potential dosimetric- and patient- prognostic factors for local tumor control (LC) and disease free survival (DFS) were evaluated using univariate (UVA) and multivariate (MVA) Cox Regression analysis. The majority of the patients had stage I (38%) or stage II (52%) NSCLC. The most commonly used fractionation schedules were 5 x 9-12Gy (37%), 8 x 7.5Gy (31%) and 12 x 5Gy (15%). Results The median V <100 BED 10 was 4.2cc in patients with a local recurrence compared to 1.2cc in patients without a local recurrence ( p = 0.593). The median %V <100 BED 10 was the same in patients with and without local recurrence (2%, p = 0.127). When the prescribed dose was <100 Gy BED 10 , patients were more likely to develop a local recurrence (HR 2.24; p = 0.045). None of the PTV parameters derived of the DVHs appeared to be a prognostic factor for LC. The UVA results of the other factors are shown in Table 1 . MVA