Purpose: Reirradiation of base of tongue (BOT) lesions is challenging due to the high dose gradient required to spare previously-treated nearby structures and the daily variation around the tongue region. This study investigates the feasibility of stereotactic radiotherapy for BOT lesions on an MR-Linac(MRL).
Methods: Three patients with locally-recurrent lesions were treated with stereotactic radiotherapy in 2020-2021. IMRT treatment plans(mIMRT) on MRL were generated in Monaco and compared to clinical RayStation VMAT plans(rVMAT) on Varian TrueBeam sTx. Simulation MR images were used 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: The CTV1 volumes for three patients were 12.7cm3, 23.1 cm3, and 63.4 cm3, respectively . mIMRT plans achieved comparable target coverage and uniformity as in rVMAT plans. Conformity and dose gradient were superior for rVMAT plans. All clinical goals were met in mIMRT plans for surrounding OARs (mandible, carotid, parotid and oral cavity), however mean doses of these OARs were higher than that in rVMAT plans. Volumes enclosed by 50% prescription isodose line were larger in mIMRT plans. dIMRT plans achieved similar clinical goals that used in reference plan for smaller lesion patients, but with higher CTV mean dose. All ATP plans were concluded within 3-minutes. For the patient with the largest lesion, the mandible V30 and oral cavity mean dose were >10% higher than those in reference plan and can be improved by an adapt-to-shape plan (>20 minutes) to account for anatomy changes.
Conclusion: MRL has the potential to be used for stereotactic radiotherapy to treat aggressive tumors in the tongue region. Daily adaptive planning can track target volume variation while maintaining nearby critical structure dose constraints.