Can a dual isocentre technique enable cervix treatments on the MR-Linac?

Abstract

Purpose or Objective Cervix patients can exhibit large inter and intra-fraction anatomical changes due to variation in bladder and rectum filling. The MR-Linac (MRL), which combines a 7MV linear accelerator with a 1.5T MR scanner can image these patients prior to and during treatment and enables daily adaptation due to improved image contrast compared to CBCT. The MR-Linac has a limited treatment field in the Sup/Inf direction of 22 cm at isocentre which can restrict the treatment of patients where nodal disease extends outside this limit. Here we investigate the feasibility of a dual isocentre technique to enable cervix patients with large nodal volumes to be treated, exploring potential adaptive workflows. Material and Methods Four cervix cancer patients were retrospectively planned with a dual isocentre technique delivering 45Gy in 25 fractions. Research Monaco v5.19.02 (Elekta AB Stockholm, Sweden) with an MR-Linac specific beam model and 1% statistical uncertainty was utilised. A 1 cm overlap region between PTV1 (primary) and PTV2 (elective nodal volume) was created, positioned entirely in PTV2 (Figure 1). The plan consisted of 14 step-and-shoot IMRT beams, 7 beams per isocentre. The plans met dose constraints for the EMBRACE II study. To test robustness to small movements between treatments of each plan, PTV1 isocentre was shifted superiorly and inferiorly by 3mm and 6mm. The plan was recalculated and dosimetric changes evaluated. A potential treatment workflow was simulated, with the plan to the nodal region delivered first as the interfraction movement is likely to be less. The plan for the primary disease was then re-optimised (post PTV1 isocentre shift 3mm or 6mm) onto the nodal region plan using the bias dose. Plan dosimetry was evaluated compared against the initial reference plan. Results For the four patients included, combined PTV lengths of 18.6–21.6 cm were required. Therefore to ensure coverage three patients would not have been suitable for the MRL. Using a dual isocentre technique resulted in two fields ranging from 10.8–15.3 cm for PTV1 isocentre and 6.3–10.5 cm for PTV2 isocentre. Shifting PTV1 isocentre and recalculating 6 mm sup increased the maximum dose to 1cc of CTV1 by 17.2% on average over the four patients. Similarly, shifting the PTV1 isocentre 6 mm inf decreased the minimum dose to 1cc of CTV1 by 17.8% averaged over four patients. The results of re-optimising onto the nodal dose with shifted PTV1 regions is shown in Figure 2. This shows that reoptimising the plan rather than just recalculating maintains plan quality. However, this method does struggle to maintain dose coverage to PTV2 for large shifts in the junction region. Conclusion The dual isocentre technique for cervix and nodal treatments can feasibly give good quality plans whilst ensuring that the treatment fields can be treated on the MR-Linac. This preliminary work has investigated the effect of potential intra-fraction motion and possible plan adaptation workflows.