Purpose: Magnetic resonance imaging-guided linear accelerator systems (MR-linacs) can facilitate the daily adaptation of radiotherapy plans. Here, we report our early clinical experience using an MR-linac for adaptive radiotherapy of gynecologic malignancies.
Methods and Materials: Treatments were planned with an Elekta Monaco v5.4.01 and delivered by a 1.5 Tesla Elekta Unity MR-linac. The system offers a choice of daily adaptation based on either position (ATP) or shape (ATS) of the tumor and surrounding normal structures. The ATS approach has the option of manually editing the contours of tumors and surrounding normal structures before the plan is adapted. Here we documented the duration of each treatment fraction; set-up variability (assessed by isocenter shifts in each plan) between fractions; and, for quality assurance, calculated the percentage of plans meeting the ;γ-criterion of 3%/3-mm distance to agreement. Deformable accumulated dose calculations were used to compare ATP plans with reference dose plans.
Results: Of the 10 patients treated with 90 fractions on the MR-linac, most received boost doses to recurrence in nodes or isolated tumors. Each treatment fraction lasted a median 32 minutes; fractions were shorter with ATP than with ATS (30 min vs 42 min, P<0.0001). The γ-criterion for all fraction plans exceeded >90% (median 99.9%, range 92.4%-100%), i.e., all plans passed quality assurance testing. The average extent of isocenter shift was <0.5 cm in each axis. The accumulated dose to the gross tumor volume was within 10% of the reference plan for all ATP cases. Accumulated doses for lesions in the pelvic periphery were within 1% of the reference plan as opposed to -5.8% to -9.6% for central tumors.
Conclusions: The MR-linac is a reliable and clinically feasible tool for treating patients with gynecologic cancer.
Quality assurance evaluation of spot scanning beam proton therapy with an anthropomorphic prostate phantom
