Adaptation of dual isocentre plans for cervix treatments on the 1.5T MR- Linac
Chuter, Robert Hoskin, Peter J McWilliam, Alan
Chuter, Robert
Hoskin, Peter J
McWilliam, Alan
Citations
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Abstract
Purpose or Objective
Cervix patients exhibit large inter-fraction anatomical changes. The MR-Linac (MRL) can image these patients
with MR prior to & during treatment. Due to the improved image contrast and lack of imaging dose compared
to CBCT, daily plan adaptation can be achieved. However, the MRL has a limited treatment field in the Sup/Inf
direction of 22 cm. This can restrict the treatment of patients who require longer treatment fields such as
cervix patients with nodes. Here we explore adaptive workflows in combination with a dual isocentre
approach, to widen the range of cervix patients that can benefit from this treatment.
Materials and Methods
Four cervix cancer patients were retrospectively planned with a dual isocentre technique delivering 45Gy in 25
fractions. 2 node negative (NN) and 2 node positive (NP) patients were planned using the EMBRACE II study
protocol on research Monaco (Elekta AB, Sweden) with an MRL beam model and 1% statistical uncertainty. A 2
cm overlap region between the two isocentres was created, positioned entirely in the nodes (Fig 1a). To create a smooth dose gradient in the junction a structure-based optimisation method was used (Figure 1b).
A treatment workflow was simulated using an MR from the second week of treatment, with the plan adapted
to the nodal region first using an adapt-to-position (ATP) technique. The plan for the primary disease was fully
re-optimised via the adapt-to-shape (ATS) technique onto the nodal region plan using the bias dose method.
Additionally, the plans were recalculated without adaptation to simulate a standard workflow. The dosimetry
of the adapted and non-adapted plans were compared against the reference plan.
Finally an investigation into the effect of intra-fraction motion was performed. Relative shifts between the
two isocentres on the adapted plans were shifted superiorly (sup) and inferiorly (inf) by 3mm and 6mm, and
recalculated. Results
The mean PTV lengths of the NN and NP patients were 19.5 cm and 18.8 cm respectively. To allow room for
daily adaptation to anatomical changes none of these patients were eligible for the MRL. Using a dual
isocentre technique resulted in field lengths averaging 12.8 cm for isocentre 1 and 7.4 cm for isocentre 2,
allowing them to be treated on the MRL.
Fig 2 shows dual isocentre adapted plans give better coverage than non-adapted plans. This is true for both NN
and NP patients but a larger benefit is seen for nodal boost volumes in the NP cases, with the difference to
the reference plan for the D98% being 0.4 Gy and -3.5 Gy for the adapted and recalculated plans respectively.
Shifting the adapted plans sup didn’t affect the percentage coverage by more than 3% but shifting them inf
caused the PTV coverage on the plan to reduce by ~2.2% per mm for the NP and ~3.2% for the NN cases.Conclusion
A dual isocentre technique for cervix treatments can enable an adaptive workflow that recovers the required
plan quality. This illustrates the feasibility of a dual isocentre technique for the MRL
Description
Date
2021
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Chuter R, Hoskin P, McWilliam A. Adaptation of dual isocentre plans for cervix treatments on the 1.5T MR- Linac. Radiotherapy and Oncology. 2021;161:S1269-S71.