Dosimetric feasibility of brain stereotactic radiosurgery with a 0.35 T MRI‐guided linac and comparison vs a C‐arm‐mounted linac

Abstract

Purpose MRI is the gold‐standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35 T MRI‐guided linear accelerator (MRL) equipped with a double‐focused multileaf collimator (MLC). Dosimetric comparisons were made vs a conventional C‐arm‐mounted linac with a high‐definition MLC. Methods The quality of MRL single‐isocenter brain SRS treatment plans was evaluated as a function of target size for a series of spherical targets with diameters from 0.6 cm to 2.5 cm in an anthropomorphic head phantom and six brain metastases (max linear dimension = 0.7‐1.9 cm) previously treated at our clinic on a conventional linac. Each target was prescribed 20 Gy to 99% of the target volume. Step‐and‐shoot IMRT plans were generated for the MRL using 11 static coplanar beams equally spaced over 360° about an isocenter placed at the center of the target. Couch and collimator angles are fixed for the MRL. Two MRL planning strategies (VR1 and VR2) were investigated. VR1 minimized the 12 Gy isodose volume while constraining the maximum point dose to be within ±1 Gy of 25 Gy which corresponded to normalization to an 80% isodose volume. VR2 minimized the 12 Gy isodose volume without the maximum dose constraint. For the conventional linac, the TB1 method followed the same strategy as VR1 while TB2 used five noncoplanar dynamic conformal arcs. Plan quality was evaluated in terms of conformity index (CI), conformity/gradient index (CGI), homogeneity index (HI), and volume of normal brain receiving ≥12 Gy (V12Gy). Quality assurance measurements were performed with Gafchromic EBT‐XD film following an absolute dose calibration protocol. Results For the phantom study, the CI of MRL plans was not significantly different compared to a conventional linac (P > 0.05). The use of dynamic conformal arcs and noncoplanar beams with a conventional linac spared significantly more normal brain (P = 0.027) and maximized the CGI, as expected. The mean CGI was 95.9 ± 4.5 for TB2 vs 86.6 ± 3.7 (VR1), 88.2 ± 4.8 (VR2), and 88.5 ± 5.9 (TB1). Each method satisfied a normal brain V12Gy ≤ 10.0 cm3 planning goal for targets with diameter ≤2.25 cm. The mean V12Gy was 3.1 cm3 for TB2 vs 5.5 cm3, 5.0 cm3 and 4.3 cm3, for VR1, VR2, and TB1, respectively. For a 2.5‐cm diameter target, only TB2 met the V12Gy planning objective. The MRL clinical brain plans were deemed acceptable for patient treatment. The normal brain V12Gy was ≤6.0 cm3 for all clinical targets (maximum target volume = 3.51 cm3). CI and CGI ranged from 1.12–1.65 and 81.2–88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured vs computed absolute dose difference was −0.1%. Conclusions The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤2.25 cm. Large lesions (>2.25 cm) should be treated with a linac capable of delivering noncoplan...

Adoption of Hybrid MRI-Linac Systems for the Treatment of Brain Tumors: A Systematic Review of the Current Literature Regarding Clinical and Technical Features

“…In the above-mentioned experience by Tseng et al 78 including ten high grade glioma patients (4 GBM) that completed chemoradiotherapy on Unity MRL, FLAIR MRI performed at 10, 20 and 30 days after radiotherapy start revealed a volume change of at least 20% in 3, 3 and 4 patients respectively and 3 patients required replanning (between fractions 15 and 19) due to increase of tumor or edema volume.…”

“…A comparison between plans simulated for MRIdian and VMAT plans was performed by Slagowski et al 78 Clinically acceptable plans were generated for single-isocenter brain SRS (step-and-shoot IMRT with 11 coplanar beams, 20 Gy single fraction) for MRIdian on the basis of data from six cases of BM previously treated on a conventional linac for lesions with a diameter ≤2.25 cm, while for larger lesions the normal brain V12Gy planning objective (≤10.0 cm3) could not be respected and was achieved only with conformal arcs and non-coplanar beams on the conventional linac.…”

On the MLC leaves alignment in the direction orthogonal to movement

“…The MLC design on a single-head linac-based system is different from the three-head 60 Co unit. 5 The defining feature is the doublestack double-focused configuration with an individual leaf width of 8.3 mm as projected at the isocenter (Fig. 1).…”

Evaluation of an MRI linac magnetic isocenter walkout with gantry rotation in the presence of angle-specific corrections

“…The likely reason for this magnetic isocenter movement was traced to the variation in B 0 eddy currents with gantry angle, as the distribution of the conductive mass in the field changed with gantry angular position (Curcuru et al 2021). This was suboptimal, considering that MRIdian's intended use is largely precision image-guided procedures, including intracranial stereotactic radiosurgery (Slagowski et al 2020). An immediate remedial action was to average the imaging isocenter location between a number of gantry angles and align that point with the radiation isocenter, thus reducing the maximum deviation by about one-half.…”