S383
Clinical - CNS
ESTRO 2026
and slightly higher than 12Gy GK. Additional margins exaggerated the BED differential further and increases toxicity risk by expanding the high dose region outside the VS into surrounding OARs. These results provide critical guidance to clinicians using SRS linac-based systems on how BED can be impacted by their much faster delivery time. References: 1. Tuleasca C, Faouzi M, Maeder P, Maire R, Knisely J, Levivier M. Biologically effective dose correlates with linear tumor volume changes after upfront single- fraction stereotactic radiosurgery for vestibular schwannomas.Neurosurg Rev 20212. Anthes VB, Schwartz M, Cusimano M, Radovanovic I, Kulkarni A, Laperriere N, Payne D, Heaton R, van Prooijen M, Das S, Tsang DS. Effect of cobalt-60 treatment dose rate on arteriovenous malformation obliteration after stereotactic radiosurgery with Gamma Knife. Neurosurg 20242. Jones B, Hopewell JW. Modelling the influence of treatment time on the biological effectiveness of single radiosurgery treatments: derivation of “protective” dose modification factors. Br J Radiol 2018; Keywords: Radiobiology, Radiosurgery, BED Digital Poster 3741 Diffuse 5-Aminolevulinic acid (5-ALA) Residual Fluorescence Predicts Worse Survival in Glioblastoma Multiforme Vishal D Manik 1 , Gopikrishna Shyam 1 , Ayesha Sathali 1 , Dimitrios Kalaitzoglou 2 , Mark MacDonald 3 , Van Sim 1 , Amoolya Mannava 3 , Christos Soumpasis 2 , Yasir Chowdhury 2 , Kazumi Chia 1 , Keyoumars Ashkan 2 , Ranjeev Bhangoo 2 , Richard Gullan 2 , Lucy Brazil 1 , Omar Al-Salihi 1 , Francesco Vergani 2 , Angela Swampillai* 1 , Jose Pedro Lavrador* 2 1 Clinical Oncology, Guys & St Thomas' NHS Foundation Trust, London, United Kingdom. 2 Neurosurgery, Kings College Hospital, London, United Kingdom. 3 Neuroradiology, Guys & St Thomas' NHS Foundation Trust, London, United Kingdom Purpose/Objective: Glioblastoma multiforme (GBM) carries a poor prognosis despite many attempts to develop new therapies. The standard treatment remains maximal safe resection. 5-ALA guidance increases the extent of resection but complete tumour removal is not always achievable. It is uncertain whether the pattern of residual 5-ALA fluorescence predicts outcomes in GBM patients. Material/Methods: Patients with GBM treated at our centre from January 2018 to June 2022 were included if they underwent 5- ALA–guided tumour excision and were planned for
fast and slow repair half-times of 11.4 and 129.6 minutes, respectively. Results: Mean treatment times were 55.5, 44.3, 30.1 and 8.1 minutes for Model B, Perfexion, Cyberknife and Truebeam, respectively. GK mean BED (mBED) for 12Gy/1 physical dose (55Gy2.47) was the primary comparator for Cyberknife and Truebeam mBEDs (Table 1). No margin, single fraction mBEDs increased by 7.3% and 25.7%, at 12Gy for Cyberknife and Truebeam, respectively, while at 18Gy/3, Cyberknife mBEDs decreased 1% and increased 12.4% for Truebeam. At 25Gy/5 and 27.5Gy/5, Truebeam mBEDs increased 42.9% and 67.6%, respectively. GTV-to-PTV margins increased mBEDs by 42% for 12Gy/1 with 0.5mm margin and by 57% and 83.1% for 25Gy/5 and 27.5Gy/5 respectively with 1mm margin. A decrease of the Truebeam dose to 11Gy/1 resulted in a mBED increase of only 8.5% compared to GK, similar to that of Cyberknife (Figure 1)
Conclusion: Faster Truebeam delivery times yielded mBEDs significantly greater than GK or Cyberknife, for the same physical dose. A Truebeam physical dose of 11Gy resulted in mBEDs similar to 12Gy Cyberknife
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