Purpose: Reirradiation of skull base lesions is often challenging due to the high dose gradient required to spare previously-treated nearby structures. This study investigates the feasibility of stereotactic radiotherapy for skull base lesions on Elekta MR-Linac.
Methods: Two patients with locally-recurrent skull base lesions were treated with stereotactic radiotherapy in 2020. IMRT plans(mIMRT) were re-generated in Monaco for MR-Linac and compared to clinical RayStation VMAT plans(rVMAT) for Varian TrueBeam sTx. Simulation MR images were registered with planning CT images, and then manually shifted 2mm and 4mm individually in each of the lateral, anterior-posterior, and superior-inferior(SI) directions to simulate and test the online adapt-to-position(ATP) daily planning workflow. The quality of daily adaptive plans(dIMRT) was evaluated in terms of target coverage and dose to organ-at-risks(OARs).
Results: Patient-1 had a smaller CTV1(1.4cm3) and a more irregular CTV2(17.4cm3) compared with Patient-2 larger CTV1(21.6cm3) and CTV2(34.3cm3). The mean registration error in Monaco was 0.3mm(range: 0.0 – 0.7mm). mIMRT plans achieved comparable target coverage and uniformity to rVMAT plans. Conformity and dose gradient were superior for rVMAT plans. All clinical goals were met in mIMRT plans for non-adjacent OARs, however mean doses of these OARs were inferior to rVMAT. Volumes enclosed by 50% prescription isodose line were larger in mIMRT plans. dIMRT plans achieved same clinical goals that used in corresponding mIMRT plans on simulated 2mm shift images but not on all 4mm images. This reflects the features of current MR-Linac with coplanar beams, sagittal MLCs, and 7mm MLC width. All ATP plans were concluded within 3-minutes. Brainstem and cochlea goals were not met for Patient-1 with 4mm lateral shift and required an Adapt-to-Shape plan(>20-minutes).
Conclusion: MR-Linac has the potential for stereotactic radiotherapy to treat aggressive tumors in the skull base region. Daily adaptive planning can track lesion changes while maintaining nearby critical structure dose constraints.
Not Applicable / None Entered.