ESTRO 2023 - Abstract Book

S1796

Digital Posters

ESTRO 2023

PO-2022 Feasibility of MR-guided (MRgRT) isotoxic re-irradiation in moving targets

D. Murray 1 , B. George 1 , M. Robinson 2 , P. Camilleri 3 , K. Owczarczyk 4 , A. Gaya 4 , J. Good 3 , D. Yucel 5 , J. Drabble 1 , H. Harford- Wright 5 , E. Chinherende 1 , P. Maungwe 1 , A. Huynh 1 1 GenesisCare UK, MR-Linac Physics and Dosimetry, Oxford, United Kingdom; 2 Oxford University Hospitals NHS Foundation Trust, Radiotherapy Physics, Oxford, United Kingdom; 3 GenesisCare UK, Radiotherapy, Oxford, United Kingdom; 4 GenesisCare UK, Radiotherapy, London, United Kingdom; 5 GenesisCare UK, MR-Linac Physics and Dosimetry, London, United Kingdom Purpose or Objective Reirradiation doses are often conservative due to accumulation uncertainties from previous radiotherapy, the extent of normal tissue recovery and the limitations of onboard imaging. MRgRT has advantages over CT-based techniques; exquisite soft-tissue contrast, daily on-table plan adaption, real-time cine imaging, tracking and gating. MRgRT allows opportunities to explore the feasibility of isotoxic dose escalation that may improve tumour control with acceptable toxicity (Kerkmeijer, 2021). We conducted a retrospective dosimetric review of SABR reirradiation plans to identify patient suitability for isotoxic dose escalation using MRgRT. Materials and Methods We considered patients who met the following criteria: 1. MRgRT SABR re-irradiation to either primary tumour or oligorecurrent/oligometastatic disease; 2. >90% of the planning target volume (PTV) received the original prescription dose (PTV: V(100%)>90%); 3. Dose prescriptions 30Gy/3 or 5#. Patient-specific OAR tolerances were calculated using UK SABR Consortium reirradiation guidelines (SABR Consortium, 2019). Dose prescriptions for all plans were increased, with the same fraction number, until one of the stopping criteria was met: 1. OAR tolerance reached; 2. PTV coverage V(100%)<70%; 3. Plan quality (prescription dose spillage (PDS)) not within mandatory guidance levels; 4. Ablative threshold of 80Gy BED (a/b= 10Gy) was not reached. Results 5 patients were included: Lung (n=2), Mediastinum, Pancreas and Renal. Original re-irradiation prescriptions; 30Gy/5# (n=4); 30Gy/3# (n=1). All plans achieved V(100%) >70% (Fig 1) and a minimum 10Gy increase in prescription, whilst achieving OAR constraints. A mean increase of 16Gy was observed, with 20Gy increase from the original prescription in n=2 (Fig 1). Minimum dose to GTV & PTV (Fig 2) (GTV, PTV: V(98%)) improved from 30Gy to 40Gy prescriptions (p<0.05); no significant improvement was shown for 40Gy to >42.5Gy prescriptions (p>0.05). N=4 plans <1.20 PDS guidance tolerance, but all plans inside mandatory guidance (PDS<1.30). Conclusion Dose escalation was successful, highlighting the feasibility of isotoxic MRgRT SABR re-irradiation in moving targets. OAR proximity to the target, and re-irradiation constraints, determined the level of dose escalation. Given the small sample, investigation involving more patients, dose prescriptions and treatment sites is required. We envisage this approach could be tested prospectively in pelvic recurrence, rectal organ preservation and definitive radiation boosts to medically inoperable pelvic disease. 1. Dorr, W,, (2018). The Principles and Practice of Re-irradiation in Clinical Oncology: An Overview. Clinical Oncology, 67-72 2. Kerkmeijer, L. G., (2021). Focal Boost to the Intraprostatic Tumor in External Beam Radiotherapy for Patients With Localized Prostate Cancer. Journal of Clinical Oncology, 787-796. 3. UK SABR Consortium. (2019). Stereotactic Ablative Body. The Faculty of Clinical Oncology of The Royal College of Radiologists.

PO-2023 Dosimetric Evaluation of an MR Linac System compared to multiple Stereotactic Radiotherapy systems

R. Tolakanahalli 1 , R. Kotecha 2 , D.J. Wieczorek 1 , Y. Lee 1 , K. Mittauer 1 , N. Bassiri 1 , M. Hall 1 , M. Chuong 1 , M. Mehta 1 , M. McDermott 1 , A. Gutierrez 1

1 Miami Cancer Institute, Radiation Oncology, Miami, USA; 2 Miami Cancer Institute, Radiation Oncologys, Miami, USA

Purpose or Objective A new brain treatment package consisting of a dedicated head coil with an integrated stereotactic brain immobilization system was recently released for the ViewRay® MRIdianTM system. This, coupled with high-resolution volumetric MR imaging, enables treatment of intracranial lesions using a stereotactic technique. This work aims to compare the dosimetric performance of MR-guided Linac (MRL) treatment plans against established stereotactic delivery platforms, specifically Gamma Knife® (GK), CyberKnife® (CK), and Volumetric Modulated Arc Therapy (VMAT), for brain metastases resection cavities Materials and Methods Ten consecutive patients previously treated on GK to resection cavities were re-planned using CK, VMAT, and MRL intensity- modulated radiation therapy (IMRT) to 30 Gy in 5 fractions. Plans were created for CK, VMAT, and MRL modalities with a setup margin of 1 mm added to the clinical target volume (CTV) defined as 2 mm margin around the rim enhancement of the cavity. Plans across all modalities were optimized to not exceed organs-at-risk (OARs) dose constraints and achieve a minimum target coverage of 99.5% while maximizing Paddick Conformity Index (PCI) and minimizing Gradient Index (GI)and beam-on time (BOT) in this specified order of importance. Plan quality metrics and delivery parameters between GK, CK, VMAT, and MRL were compared. Wilcoxon signed rank test (p<0.05) was used to evaluate statistical significance.