ESTRO 2020 Abstract Book

S534 ESTRO 2020

with good tolerance and no interruptions. Acute toxicity were reported in 22% of pts with only 1 case of G3 toxicity (dyspnea) according to CTCAE scale v4.0 and no case of G4 or more side effects. About late toxicities, 23% of pts developed a side effect: only one pt had dyspnea G3 and 3 had G3 pneumonitis, no G4 or more were reported. At time of analysis 5 pts were lost at follow up; 41 pts were still alive, 56% did not have evidence of disease and 44% with loco-regional or metastatic disease; 6 pts died after progression and 5 pts for other or unknown causes . During regular follow up 13 pts developed loco-regional progression, while 13 developed contralateral or extra thoracic metastases (six pts had both). Conclusion In selected pts affected by centrally located pulmonary lesions SBRT seems to be feasible and effective representing an efficient treatment. SBRT seems to be well tolerated and associated with relatively low rates of serious treatment related toxicity. More robust clinical data are awaited from on going international clinical trials. PO-1001 Shortened radiation time promotes recovery from lymphopenia in early-stage NSCLC who received SBRT Q. Zhao 1 , Z. Zeng 1 1 Zhongshan Hospital- Fudan University, Department of Radiation Oncology, Shanghai, China Purpose or Objective To evaluate the potential impact of radiation time on radiation-induced lymphopenia (RIL) and subsequent recovery after stereotactic body radiation therapy (SBRT) and to examine the association between these parameters and patient outcomes in early-stage non-small cell lung cancer (NSCLC). Material and Methods Clinical and laboratory records of 115 patients who had received SBRT for early-stage NSCLC were reviewed to assess changes in total lymphocyte counts (TLCs) following SBRT. Post-SBRT TLC values <1,000 cells/µL indicated lymphopenia. Patients that exhibited post-SBRT TLC levels similar to their pre-SBRT levels at least twice during the first 6 months of follow-up were considered to have recovered from RIL and were classified as the lymphocyte recovery (LR) group. Associations of TLC kinetics with clinical and treatment features and outcomes were analyzed. Results Most patients (100/115, 86.96%) experienced significantly decreased median TLCs following SBRT (1,700 vs. 1,100 cells/µL; P <0.001), and 52 patients (45.21%) met the criteria for lymphopenia (Figure 1A). Six months after SBRT, 44 patients (38.26%) had recovered. A negative correlation between TLC reduction and radiation time was observed (r=-0.381, P <0.001) (Figure 1B). According to the receiver-operating characteristic (ROC) curve analysis, the optimal cut-off value for radiation time to predict LR following RIL was 3,950 sec ( P <0.001) (Figure 1C). Multivariate analyses demonstrated that radiation time was significantly associated with LR (odds ratio [OR], 0.113; 95% confidence interval [CI], 0.029–0.432; P =0.001) but not TLC reduction ( P =0.575). LR within 6 months after SBRT was associated with improved progression-free survival (PFS) in patients without lymphopenia ( P =0.034) but had little effect in patients with lymphopenia ( P =0.405) (Figure 2). Conclusion A longer radiation time was associated with a lower rate of LR within 6 months after SBRT in patients with early- stage NSCLC. Given the association of severe and persistent RIL with survival in NSCLC, further study of the effect of radiation time on immune status is warranted. PO-1002 Early outcomes of stereotactic MR-guided adaptive radiation therapy in 54 high-risk lung tumors

T. Finazzi 1 , C. Haasbeek 1 , M. Palacios 1 , F. Spoelstra 1 , M. Admiraal 1 , A. Bruynzeel 1 , B. Slotman 1 , F. Lagerwaard 1 , S. Senan 1 1 Amsterdam University Medical Centers VUmc location, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Magnetic resonance (MR-)guided stereotactic ablative radiotherapy (SABR) was performed for lung tumor patients in whom treatment delivery was challenging due to pulmonary comorbidity, tumor location or motion. Given the complexity of the stereotactic MR-guided adaptive radiation therapy (SMART) approach, we evaluated early clinical outcomes in this high-risk patient cohort. Material and Methods Fifty consecutive patients (54 lung tumors) underwent SMART between 2016 - 2018 for either a primary lung cancer (n = 29 patients) or for lung metastases (n = 21). Risk-adapted dose fractionation was used to deliver 60 Gy in 8 fractions (n = 28), 55 Gy in 5 fractions (n = 23), 54 Gy in 3 fractions (n = 2), and 60 Gy in 12 fractions (n = 1). All patients had ≥1 factors predisposing to toxicity, including a central tumor location (n = 30 patients), previous thoracic radiotherapy (n = 17), synchronous multiple lung tumors (n = 9), or interstitial lung disease (n = 7). After acquisition of a daily 17-second breath-hold MR scan in treatment position, on-table plan adaptation was performed using the anatomy-of-the-day. Gated SABR delivery was performed during repeated breath-holds under continuous MR-guidance. Local control, overall (OS) and disease-free survival (DFS) were estimated using the Kaplan-Meier method. Results All but one patient completed the planned SMART schedule. With use of daily plan adaptation, a biologically equivalent dose (BED 10Gy ) ≥100Gy to 95% of the planning target volume was delivered in 50 tumors (93%), with prioritized organ at risk sparing in the other cases. Median patient follow-up was 20.9 months (95%CI, 17.2-27.0). Local control, OS and DFS at 12 months were 95.4% (95%CI, 89.4-100.0), 88.0% (95%CI, 79.4-97.5) and 63.5% (95%CI, 51.4-78.5), respectively (Fig. 1). Local failures developed in 2 patients who were re-irradiated for a post-SABR local recurrence of squamous cell carcinoma, and in another 2 patients treated for colorectal metastases. Overall rates of any grade ≥2 and ≥3 toxicity were 32% and 8%, respectively. Commonest toxicities were grade ≥2 radiation pneumonitis (12%) and chest wall pain (8%). No treatment-related deaths were noted.

Conclusion Use of the SMART approach in a cohort of high-risk lung tumors resulted in adequate early local control and low