ESTRO 2024 - Abstract Book

S1867

Clinical - Mixed sites, palliation

ESTRO 2024

788

Mini-Oral

Limited Toxicity with Ultra-Hypofractionated MR-guided Radiotherapy for Oligometastatic Disease

Robert Herrera, Michael D. Chuong, Kathryn E. Mittauer, Adeel Kaiser, Nema Bassiri-Gharb, Matthew D. Hall, Diane Alvarez, Alonso La Rosa, Tugce Kutuk, Muni Rubens, Oreoluwa Olorunlogbon, Amy E. Rzepczynski, Nicole C. McAllister, Ranjini Tolakanahalli, Minesh P. Mehta, Alonso N. Gutierrez, Rupesh Kotecha

Miami Cancer Institute, Radiation Oncology, Miami, USA

Purpose/Objective:

Magnetic resonance (MR)-guided radiation therapy (MRgRT) delivered on an MR-linac offers superior soft tissue visualization compared to CT-guided platforms, automatic beam gating due to anatomical motion, and online adaptive planning, thereby facilitating hypofractionation and biological dose escalation. MRgRT has achieved low toxicity and high local control rates even amongst patients with tumors in close proximity to organs-at-risk (OARs), such as the duodenum/bowel. As patients with oligometastatic disease are increasingly evaluated for stereotactic body radiotherapy (SBRT) to distant metastatic sites, there is a pressing need to methods to deliver high biologically effective doses with reduced toxicity risks. Therefore, the objective of this study was to report treatment-related toxicity outcomes in oligometastatic patients treated with ablative MRgRT at a high-volume center.

Material/Methods:

Consecutive patients treated with ultra-hypofractionation (UHFx) defined as 1-6 fractions on a MR-Linac from May 2018 to May 2023 for oligometastatic, oligoprogressive, or oligoresidual disease were evaluated. Biologically effective dose (α/β=10, BED 10 ) was calculated for each course, and the dose delivered to at least 95%, 90%, and 80% (D 95 , D 90 , and D 80 ) of the gross tumor volumes (GTVs) and planning target volumes (PTVs) were recorded. Treatment related toxicities were assessed during follow-up visits by treating physicians using the CTCAE version 5.0 criteria. Acute toxicities occurred within 90 days of MRgRT completion while those occurring thereafter were considered late.

Results:

262 patients (145 [55.3%] male, median [range] age, 68 [15-94] years) were evaluated, with 90.5% having an ECOG 0 1 performance status. The most commonly treated sites included the adrenal gland (59, 22.5%), abdominopelvic lymph nodes (56, 21.4%), thorax (53, 20.2%), liver (39, 14.9%), pancreas (19, 7.3%), and other (36, 13.7%). The median (range) prescribed RT schedule was 49 (16-60) Gy in 5 (1-6) fractions. Respiratory motion management was breath hold in 222 (84.7%) patients. The median BED 10 was 100 Gy 10 with 137 (52.3%) patients receiving an ablative dose of ≥100 Gy 10 and an additional 82 (31.3%) were considered to receive a definitive dose ≥72 Gy 10 . The median (range) of GTVs, clinical target volumes (CTVs), and PTVs were 12.9 (0.6-808.0) cm 3 , 23.7 (1.6-439.0) cm 3 , and 36.3 (2.4-901.3) cm 3 , respectively. Of the 1195 fractions delivered, 717 (60.0%) required online plan adaptation — most commonly for tumors involving the adrenal gland (53, 20.2%) and abdominopelvic lymph nodes (50, 19.1%). Median (interquartile range [IQR]) GTV D 95 , D 90 , and D 80 were 47.8 Gy (IQR 39.5-54.8), 50.5 Gy (IQR 40.9-56.5), and 52.3 Gy (IQR 44.4-57.9),