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. 2022 Jul-Aug;12(4):e296-e305.
doi: 10.1016/j.prro.2022.03.002. Epub 2022 Mar 9.

Clinical Implementation and Initial Experience With a 1.5 Tesla MR-Linac for MR-Guided Radiation Therapy for Gynecologic Cancer: An R-IDEAL Stage 1 and 2a First in Humans Feasibility Study of New Technology Implementation

David S Lakomy  1 , Jinzhong Yang  2 , Sastry Vedam  2 , Jihong Wang  2 , Belinda Lee  2 , Angela Sobremonte  2 , Pamela Castillo  2 , Neil Hughes  2 , Mustefa Mohammedsaid  2 , Anuja Jhingran  3 , Ann H Klopp  3 , Seungtaek Choi  3 , C David Fuller  3 , Lilie L Lin  4
Affiliations

Clinical Implementation and Initial Experience With a 1.5 Tesla MR-Linac for MR-Guided Radiation Therapy for Gynecologic Cancer: An R-IDEAL Stage 1 and 2a First in Humans Feasibility Study of New Technology Implementation

David S Lakomy et al. Pract Radiat Oncol. 2022 Jul-Aug.

Abstract

Purpose: Magnetic resonance imaging-guided linear accelerator systems (MR-linacs) can facilitate the daily adaptation of radiation therapy plans. Here, we report our early clinical experience using a MR-linac for adaptive radiation therapy 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 accumulated versus planned dose for patient treated with exclusively ATP fractions.

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 < .0001). The γ criterion for all fraction plans exceeded >90% (median, 99.9%; range, 92.4%-100%; ie, 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 5% 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 -1.6% to -4.4% for central pelvic tumors.

Conclusions: The MR-linac is a reliable and clinically feasible tool for treating patients with gynecologic cancer.

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Figures

Figure 1.
Figure 1.
Magnetic resonance linear accelerator workflow: Pretreatment planning, daily treatment setup, and adaptation and treatment. Central workflow in black, backup workflow in grey. Abbreviations: ATP, Adapt-to-point; ATS, Adapt-to-shape; MRI, magnetic resonance image; MRL, Magnetic resonance linear accelerator; sim, simulation.
Figure 2.
Figure 2.
Dose Accumulation Methods: Illustration of dose accumulation methods in adapt-to-position (ATP) plans. (A) ATP dose summation: direct summation of fractions doses (DFx1, DFx2, …, DFxn) to create summed planned dose (Dsum); (B) ATP dose deformation and accumulation: in each fraction (Fx1 for example), the fraction dose (DFx1) was first shifted to the corresponding daily MR coordinate space, creating a dose matrix DFx1′, using the iso-center shift obtained from MR-CT fusion, and then the deformable registration (DIR) between daily MR and simulation CT was used to deform the dose matrix DFx1′ to simulation CT space, creating dose matrix DFx1″, for accumulation. Summation of DFx1, DFx2, …, DFxn, creates the accumulated deformed dose (Ddef).
Figure 3.
Figure 3.
Percent difference between accumulated deformed dose and summed adaptive plan dose to GTV & OARs. Differences between the accumulated deformed dose and the summed adaptive plan dose, shown for the Dave (average dose to the volume) of thegross tumor volume (GTV) and the D1% (surrogate of max dose: 1% of the volume received at least this dose or higher) of various organs at risk (OARs) for 6 patients treated with solely ATP plans. Positive values indicate that the accumulated deformed dose exceeded the summed plan dose. The numbers on each bar refer to the patients as shown in Table 1. The various GTV targets for each patient were as follows: Patient 1, vaginal cuff; Patient 3, external iliac node; Patient 5, pelvic sidewall; Patient 7, left ischium; Patient 9, vaginal cuff boost; and Patient 10, uterine fundus. If an OAR was not considered in a particular patient, then no bar appears on the panel (e.g., for Patient 3, the bladder, femoral heads, rectum, and sigmoid colon were not considered as OARs).
Figure 4.
Figure 4.. CT reference plan and adapt-to-shape plans throughout treatment:
Comparisons of CT-based reference scans (left column) with magnetic resonance (MR) images of daily adapt-to-tumor-shape plans at fractions 5, 7, and 8 for Patient 6, a 67-year-old woman with recurrent ovarian cancer treated to 50 Gy with a conventional linear accelerator (linac) (not shown) and given a sequential 16-Gy pelvic boost with the MR-linac. Top row, axial views; middle row, coronal views; and bottom row, transverse views. The gross tumor volume (GTV) is outlined in in red; the planning target volume (PTV; i.e., GTV + a 2-mm expansion) in blue; the bladder in yellow; and the rectum in green. The GTV on the reference CT scan was 275 cm3; volumes on the daily adaptation scans were 264 cm3 at fraction 5, 266 cm3 at fraction 7, and 223 cm3 at fraction 8. These scans illustrate the high level of conformality possible with the ability of the MR-linac system to account for differences in bowel and rectal filling between daily fractions.
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