ESTRO 2024 - Abstract Book

S1592

Clinical - Lung

ESTRO 2024

We evaluated our patients treated with lung stereotactic ablative body radiotherapy (SABR) to define whether low dose bath could be reduced by magnetic-resonance image guided (ViewRay MRIdian, ViewRay Inc., Oakwood Village, OH, USA) linear accelerator based intensity modulated radiation therapy (MRI-Linac IMRT) without compromising plan quality for same PTV margin on integrated GTV (iGTV) following 4-dimensional computed tomography (4D-CT) simulation, in comparison to volumetric modulated arc therapy (VMAT).

Material/Methods:

Our retrospective cohort included 31 patients who received 50 Gy in four fractions treated with conventional VMAT technique having four partial coplanar arcs using Agility 160 multileaf collimator of beam former with a 6 MV flattening ‐ filter free (FFF) beam on Elekta Versa HD linear accelerator (Elekta Oncology Systems, Crawley, UK). Each case was replanned with MRI-Linac IMRT using the same set of CT images including the same organs at risk (OAR) and the planning target volume (PTV) from original VMAT plans on Elekta Linac. Tumor tracking or breath hold treatment were not considered for Mr guidance, and an isotropic margin of 5mm on iGTV on 4D-CT dataset formed the PTV on both plans. All plans of the MRI-Linac system made with a 6 MV FFF photon beam using the RayZR MLC, a double-stacked and double-focused 138-leaf MLC with a smaller penumbra, tongue-and-groove effect than conventional linac. The dose calculation algorithm is also based on the Monte Carlo simulation of the MRI-Linac Treatment Planning System for IMRT steep and shoot, with a calculation grid size of 0.2 cm3. The VMAT and MRI Linac IMRT plans were compared for conformity index (CI), gradient index (GI), beam-on time (BOT, min), monitor unit (MU), and volume of receiving doses on body (V40Gy, V30Gy, V20Gy, V10Gy, V5Gy).

Results:

MR-Linac IMRT had statistically significant higher BOTs than VMAT (BOTMR-IMRT 9.80 vs BOTVMAT 3.81; p<0.05) as MUMR-IMRT were non-significantly lower (MUMR-IMRT 4860 vs MUVMAT 4910; p=0.429). Dose bath parameters of V40, 30, 20, 10, 5Gy were statistically lower with MR-Linac IMRT (V40Gy; 26.60 vs 13.57 cm3, p < 0.01, V30Gy; 46.27 vs 24.54 cm3, p<0.01, V20Gy; 91.02 vs 59.83 cm3, p < 0.01, V10Gy; 361.07 vs 285.61 cm3, p < 0.01, V5Gy; 905.30 vs 712.18 cm3, p < 0.01). MR-Linac IMRT revealed significantly lower GI values but no difference in CI values (GIMR IMRT 3.75 vs GIVMAT 4.44, p<0.01, CIMR-IMRT 0.95 vs CIVMAT 0.97, p=0.310).

Conclusion:

The ViewRay MRI-Linac IMRT with its Multi Leaf Collimator (MLC) design and unique assembly features seems to reduce dose bath in our cohort treated with lung SABR in comparison to conventional VMAT for various tumor locations, without considering tumor tracking or breath hold which theoretically would further decrease the PTV and related dose bath.

Keywords: MRIdian-linac, MR-guidance, IMRT, VMAT, SABR

References:

1. Aznar MC, Warren S, Hoogeman M, Josipovic M. The impact of technology on the changing practice of lung SBRT. Physica Medica 2018; 47: 129-138.