Feasibility of using a dual isocentre technique for treating cervical cancer on the 1.5T MR-Linac


 Objective: Patients treated for cervical cancer exhibit large inter and intra-fraction anatomical changes. The Unity MR-Linac (MRL) can image these patients with MR prior to and during treatment which enables daily plan adaptation. However, the MRL has a limited treatment field in the sup/inf direction of 22 cm which can restrict the treatment of patients who require longer treatment fields. Here we explore potential adaptive workflows in combination with a dual isocentre approach, to widen the range of cervix patients that can benefit from this treatment. Approach: Ten cervical cancer patients were retrospectively planned with a dual isocentre technique to deliver 45 Gy in 25 fractions. 5 node-negative and 5 node-positive patients were planned using the EMBRACE II protocol. A 2 cm overlap region between the two isocentres was positioned entirely in the nodal region. A treatment workflow was simulated to account for inter-fraction anatomical change. Isocentre shifts of 3 mm and 6 mm were applied to investigate the effect of intra-fraction motion. Main results: Dual isocentre adapted plans ensured significantly better coverage than non-adapted (recalculated) plans with a larger benefit seen for the node-negative cases. The difference to the reference plan for the V4275 cGy to the ITV was -0.8 cGy and -8.2 cGy for the adapted and recalculated plans respectively. Movements superiorly didn’t affect the coverage of the ITV by more than 1 %, but shifting it inferiorly caused the ITV coverage on the plan to reduce by ~2.4 % per mm. Significance: A dual isocentre technique for cervical cancer treatments and adaptive workflows have been demonstrated to recover the required plan quality for inter-fraction changes. This illustrates the feasibility of a dual isocentre technique for the MRL.