S1990
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
radionecrosis for 3-fractions SRS, it has not been reproduced in an independent cohort. In this study, we aimed to perform an external validation, and examine the prognostic impact of novel factors. Material/Methods: data were retrospectively collected and statistical analisys was done calculating the relative risk of G3 RN by volume, further Kaplan-Meyer survival analysis was done to evaluate the prognostic impact of the neutrophil/lymphocyte ratio (NLR) at the time of SFRT as a surrogate of systemic inflammation. All patients were given linac-based 3-fractions SFRT with non- coplanar volumetric modulated arc-therapy (VMAT) with 6 FFF MV. Radiation dose was prescribed to the PTV. RN was assessed with perfusion magnetic resonance imaging and/or biopsy confirmation during scheduled follow-up, and scored with Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Results: from 2022 to 2024, 22 patients were evaluated. Median age was 66 years (range 49-83 years). Median ECOG performance status was 1 (range 0-3). Median number of brain metastases per patient was 1 (range 1-4). Median total dose was 27 Gy (range 24-30 Gy). Median follow up was 8 months (range 2-25 months). Median overall survival was 6 months (range 2-25 months). The incidence of any RN was 18.2% (4 patients), and 4.5% (1 patient) for G3 RN (confirmed by biopsy)(see figure below). V23 > 20cc of total irradiated volume correlated with the risk of G3 RN (p=0.07, 95% CI: 0.7355-305.9156). V20 > 20 cc was not associated with an increased risk of G3 RN (p=0.48, 95% CI: 0.1353-66.5294). Median NLR was 4. Lower NLR was associated with longer overall survival (p=0.02): median survival was 16.5 months if NLR<4 and 4.4 months if NLR>4.
(v16.1). The high-risk clinical target volume (CTV-HR: gross tumor + cervix) was expanded by 5 mm to create the PTV Boost.Dose–volume parameters were converted to EQD2 using α / β = 10 for tumor and 3 for OARs. The tumor biologically effective dose (BED ₁₀ ) was also calculated.The SBRT boost was considered feasible if the composite dose (EBRT + boost) achieved D90 CTV-HR ≥ 85–90 Gy EQD2(10) while maintaining D2cc rectum ≤ 65–70 Gy and bladder ≤ 80–90 Gy EQD2(3). Results: The mean composite CTV-HR D90 was 84.6 ± 1.3 Gy EQD2(10) [82.5–86.9 Gy], with 60% of patients achieving D90 ≥ 85 Gy.The mean tumor biologically effective dose (BED ₁₀ ) was 99.9 ± 1.8 Gy [94.3–101.7 Gy].The mean conformity index (CI) was 1.08 [range, 0.90–1.64], and the mean homogeneity index (HI) was 1.51 [1.23–2.03].For organs at risk, the mean composite D2cc values were 79.5 EQD2(3) for the rectum [63.7–86.7 Gy] and 87 EQD2(3) for the bladder [73.7–99.1 Gy].The maximum dose within the PTV reached 130% ± 2 of the prescribed dose. Conclusion: SBRT boost provides adequate target coverage and acceptable OAR doses in most patients, with tumor BED and D90 comparable to conventional brachytherapy objectives. This study demonstrates the dosimetric feasibility of a stereotactic boost in settings where brachytherapy is unavailable. However, these results could potentially be improved if a repeat planning scan were performed at the time of the boost. Keywords: cervical cancer, stereotactic boost dosimetric predictors of radionecrosis in 3-fraction linac-based stereotactic radiotherapy for brain metastases in a homogeneously treated cohort Carlo Furlan 1 , Enrico Raggi 2 , Margherita Crespi 3 , Vittorio Baggio 1 , Alessandro Magli 2 1 UOC Radioterapia, AULSS1, Belluno, Italy. 2 SC Radioterapia, ASUGI, Trieste, Italy. 3 UOC Fisica Sanitaria, AULSS1, Belluno, Italy Purpose/Objective: linac-based stereotactic fractionated radiotherapy (SFRT) for brain metastases is increasingly used and since brain radionecrosis (RN) is a concerning limiting factor in SFRT, it is mandatory to reduce the risk of RN to a minimum. Upadhyay et al.1 identified V23 and V20 to the total irradiated volume (normal brain plus GTVs) as the most robust dosimetric parameters associated with the risk of RN for 3-fractions stereotactic radiosurgery (SRS). Although this represents the largest dataset evaluating Digital Poster 4289
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