ESTRO 2023 - Abstract Book

S895

Digital Posters

ESTRO 2023

Department of Radiation Oncology and Radiotherapy, Berlin , Germany; 5 Charité - Universitätsmedizin Berlin, Department of Radiation Oncology and Radiotherapy, Berlin, Germany; 6 University of Toronto, Department of Radiation Oncology, Toronto, Canada; 7 Alma Mater Studiorum - Università di Bologna, Department of Biomedical and Neuromotor Sciences, Bologna, Italy; 8 Charité - Universitätsmedizin Berlin, Department of Radiation Oncology and Radiotherapy, Belrin, Germany; 9 German Cancer Consortium (DKTK), German Cancer Research Center (DKFZ), Heidelberg, Germany; 10 Charité - Universitätsmedizin Berlin, Department of Neurosurgery, Berlin, Germany; 11 Charité - Universitätsmedizin Berlin , Radiation Oncology and Radiotherapy, Berlin, Germany; 12 Berlin Institute of Health, Clinician Scientist Program, Berlin, Germany Purpose or Objective Neoadjuvant stereotactic radiosurgery (Na-SRS) of brain metastases has gained importance, but it is not routinely performed. While awaiting the results of prospective studies, we aimed to analyze the changes in the volume of brain metastases irradiated pre- and postoperatively and the resulting dosimetric effects on normal-brain-tissue (NBT). Materials and Methods We identified patients treated with SRS at our institution to compare hypothetical preoperative gross tumor and planning target-volumes (preGTV and prePTV) with original postoperative resection cavity volumes (postGTV and postPTV) as well as with a standardized-hypothetical PTV with 2.0 mm margin (Wilcoxon test). We used Pearson-correlation to assess the association between the GTV and PTV changes with the preGTV. A multiple linear regression analysis was established to predict the GTV-change. The hypothetical planning for the selected cases to assess the volume effect showed NBT exposure with a median of only 67.6% (range: 33.2%-84.5%) of postPTV receiving the fractionation-specific evaluated dose. We performed a literature review on Na-SRS and searched for ongoing prospective trials. Results We included 30 patients in the analysis. The pre/postGTV and pre/postPTV did not differ significantly. We observed a negative correlation between preGTV and GTV-change, which was also a predictor of volume change in the regression analysis, in terms of a larger volume change for a smaller pre-GTV. In total, 62.5% of cases with an enlargement of >5.0 cm3 were smaller tumors (preGTV<15.0 cm3), whereas larger tumors (>25.0 cm3) showed only a decrease in postGTV. The hypothetical planning for the selected cases to assess the volume effect showed NBT exposure with a median of only about 67.6% (range: 33.2%-84.5%) of postPTV receiving the fractionation-specific evaluated dose. Nine published studies and twenty ongoing studies are listed as an overview. Conclusion Patients with smaller brain metastases may have a higher risk of volume increase when irradiated postoperatively. Target volume delineation is of great importance because the PTV directly affects the exposure of NBT, but it is a challenge when contouring resection cavities. Further studies should identify patients at risk of relevant volume increase to be preferably treated with Na-SRS in routine practice. Ongoing clinical trials will evaluate additional benefits of Na-SRS. 1 Federico II University School of Medicine, Department of Advanced Biomedical Sciences, Naples, Italy; 2 Federico II University School of Medicine, Department of Advanced Biomedical Sciences, Naples, Italy, Italy; 3 Federico II School of Medicine, University Hospital, Naples, Italy; 4 Federico II School of Medicine, Department of Advanced Biomedical Sciences, Naples, Italy; 5 Institute of Biostructures and Bioimaging, National Research Council of Italy (CNR), Naples, Italy; 6 University of Naples Federico II, Department of Advanced Biomedical Sciences, Naples, Italy; 7 University of Naples Federico II, Department of Advanced Biomedical Sciences,, Naples, Italy Purpose or Objective SRT is a proven effective treatment for brain metastases (BM), but radiation necrosis (RN) is a possible late effect affecting patient's QoL. We report the outcome and the incidence of RN in patients treated at a single institution. Materials and Methods 44 patients treated for treated for 116 BM with SRT from 2016 to 2021 with clinical and MRI follow up ≥ 6 months were selected. Survival outcomes were estimated with the Kaplan-Meier method. RN was scored according to the CTCAE 5.0. Clinical (KPS, histology, extracranial disease status, smoking status, chemo-, immuno- or target-therapy, steroids use during RT, previous brain treatments), anatomical (PTV volume, location) and dosimetric (BED2 corrected brain minus GTV Dmax and Vx across 2 Gy increments of dose) factors were collected. To evaluate factors possibly associated with RN, univariate and multivariate logistic regression analyses. ROC analysis was also performed. Results At a median follow up of 16 (6-42) months the crude local control is 90%. Estimated median OS, extracranial-PFS, brain PFS are 25, 9 and 11 months, respectively. Estimated LFFS rates at 1-, 2- and 3-years result 89.7%, 85.3% and 85.3%, respectively; extracranial-PFS 36.7%, 23.8%, and 15.9%; brain-PFS 37.9%, 21.4%, 10.7%. OS is significantly better when KPS is ≥ 70 (p=.004) (Figure 1). G1-2 RN occurred in 11 patients (25%) and 16 lesions (14%), with a median time to onset of 6 (3 46) months. G2 patients with neurological deficits received steroids, and 40% improved or stabilized with treatment, although for all of them a phase of 5-16 months of steroids dependence has been faced (Figure 2); for others, the event is still ongoing (20%), or not evaluable (40%). No one required surgical decompression (G3). At univariate analysis, previous surgery (p=.02), PTV (p=.006) and GTV (p=.003) size resulted significantly related to RN. Dosimetric parameters resulted not significantly related with RN. Median PTV size was 1.21(0.02-25.2) cc for the G0 RN, and 2.72(0.75-89.38) cc for the G1-2, respectively. A cut-off value of 1.48 cc for the PTV size was found by ROC analysis. G1-2 events occurred in the 26% PO-1120 Radiation necrosis in patients treated with brain SRT: evaluation of clinical and dosimetric factors A. Barillaro 1 , M. Caroprese 2 , C. Oliviero 3 , S. Clemente 3 , C.A. Goodyear 4 , A. Russo 4 , L. Cella 5 , R. Pacelli 6 , M. Conson 7