S1816
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Mini-Oral 1077 Technical advancements in intracranial stereotactic radiosurgery and their impact on predicted risk of oedema and radionecrosis Caisa Kjellström 1,2 , Klara Stefansson 2 , Tobias Pommer 2 , André Haraldsson 2,1 , Sofie Ceberg 1 , Peter Siesjö 3 , Per Munck af Rosenschöld 1,2 1 Medical Radiation Physics, Lund University, Lund, Sweden. 2 Radiation Physics, Department of Hematology, Oncology, and Radiation Physics, Skåne University Hospital, Lund, Sweden. 3 Department of Neurosurgery and Section of Neurosurgery, Department of Clinical Sciences, Lund University, Lund, Sweden Purpose/Objective: Intracranial stereotactic radiosurgery (SRS) is a common and successful treatment for brain metastases but carries a risk of developing oedema and radiation-induced necrosis (RN) in surrounding brain tissue [1]. Over time, technical improvements have been introduced, including advanced treatment delivery techniques and treatment monitoring systems [2]. This study aims to compare dosimetric data from a large historical cohort of consequently treated SRS patients with respect to treatment technique and patient characteristics, particularly the number of treated targets. Additionally, a published normal tissue complication probability (NTCP) model [1] was applied to estimate the risk of oedema and RN, and to evaluate whether this risk varies between subgroups. Material/Methods: Intracranial SRS treatment plans for brain metastases treated with 30 Gy with 3 fractions between 2012 and 2025 were retrospectively collected from the clinical treatment planning system. Dosimetric parameters, including mean dose and irradiated brain volume V12Gy, V18Gy, V20Gy (both including and excluding the gross or clinical tumour volume, GTV/CTV), were acquired. The NTCP model by Milano et al. [1] was applied after conversion of doses to single-fraction equivalent doses using the linear-quadratic formalism with an α / β =2. Statistical analyses included Mann- Whitney U tests ( α =0.05) and Spearman rank correlation. Power-function regression fits were used for visualisation. Results: A total of 473 SRS treatments for 770 targets were analysed. Treatment techniques included intensity modulated radiotherapy (IMRT), helical arc, volumetric modulated arc therapy (VMAT) and automated SRS- optimised VMAT (HyperArc). The mean patient age was 67 years (range 25-89). Multiple linear regression showed that NTCP correlated with number of targets, total GTV volume, and field geometry (R2=0.51, p<0.001). The number of targets and total GTV/CTV
was not delineated and could not be evaluated for compliance. However, V40Gy and V30Gy for bowel bag, as well as bladder and rectum, were collected for 236 cervical cancer patients treated between 2020 and 2025 (202 before the implementation – group I, and 34 after – group II).The Wilcoxon ranksum test was used to compare the organ of interest (OI) dose in group II against group I.The prescription was 45 Gy in 25 fractions to the pelvic region, with or without a simultaneous integrated boost (SIB) (of 50-57.5 Gy) to lymph nodes, and/or SIB to cervix (to 50 Gy). The treatment technique was 6 MV VMAT with a PTV margin of 9 mm for the pelvic region and 6 mm for the lymph-node SIB. A coverage of D98%> 95% was applied for all PTVs. Results: The compliance with the constraints for groups I and II are listed in the table for each OI, respectively, and the distribution of parameter values is shown in the figure. The greatest impact was seen on V30Gy to the rectum, where the constraint was fulfilled in 47% of the patients in group II, compared to 27% in group I. There was a small increase in dose for the bowel bag, likely due to the competition introduced by the constraints for rectum and bladder.Differences between our clinical practice and the EMBRACE II protocol could explain some of the lack of compliance with the soft constraints. In a preliminary investigation, the lower PTV coverage constraint (D95% >95%) in the protocol, as well as the use of an inhomogeneous dose distribution of the SIB volumes, seemed to play an important role.
Conclusion: The implementation of EMBRACE II dose constraints reduced the rectal- and bladder dose; however the compliance with the soft constraints was still low. The compliance appears to be strongly dependent on using similar PTV planning aims, which could be reviewed in the future. Keywords: Organs at risk, gynaecology, cervical cancer
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