ESTRO 2024 - Abstract Book

S1039

Clinical - Gynaecology

ESTRO 2024

and OARs are well visualized on MRI scans, we hypothesize that daily adaptive radiotherapy using an MRLinac is a feasible alternative treatment option.

The first aim of the study group was to design a risk limiting dose prescription strategy. For this strategy, EDQ2 dose levels from the EMBRACE II study were used and an overall treatment time (OTT) < 50 days to limit the effect of tumor repopulation was considered. Furthermore, we agreed on a treatment planning approach in which the OAR dose constraints prevail over the target coverage in order to mitigate the risk of toxicity. A maximum of 3 fractions a week (ideally Mo/We/Fri) is intended to allow for normal tissue repair in between fractions. Within those restrictions, 6 fractions were possible, which widend the therapeutic window compared to BT (3-4 fractions). Resulting planning aims and constraints are given in Table 1. So far we used this treatment protocol to treat seven patients (UMCU: 3, OUH: 4), using the adapt-to-shape workflow (Monaco MRL, Elekta AB) where targets and OAR are recontoured at each fraction and the treatment plan is adapted accordingly. All patients gave their written consent for the use of clinical data for research purposes (MOMENTUM, NCT04075305). We report the achieved dose levels in the targets and OARs, as well as acute toxicity and target response on MRI (3 months). The achieved dose levels are based on the six daily adapted plans and accumulated using EQD2 summation with α/β = 10 for the targets and α/β = 3 for the OARs, assuming a homogeneous 25x1.8Gy background dose from EBRT on the elective fields.

Results:

For all seven patients, the reference (offline) and online plan data was collected. Dosimetric results expressed in EQD2, as well as CTV-HR volume during the reference scan and response at three months are given in Figure 2. For all patients OAR planning aims were met in the offline plans. Achieved offline and online target dose levels differed: D90% PTV-CTV-HR (offline: 77.4-86.6, online: 77.2-84.4), D98% PTV-CTV-HR (offline: 69.2-79.5, online: 70.3 78.5), D98% PTV-GTV (offline: 75.3-96.8, online: 74.7-93.5), all in EQD2,α/β=10. Inhomogenous dose levels were allowed, which resulted into an average offline D2%/D50%/D98% in the PTV-GTV over all patients of 107.0, 97.5, 80.9 Gy, EQD2,α/β=10, respectively. Six patients so far reached the first FU moment at 3 months. No grade 3 toxicity was reported, and in most toxicity was already present at baseline. MRI scans showed complete response (CR) in three, near CR in two and partial response in one patient.

Conclusion:

The proposed MRL treatment protocol for boosting CTV-HR and GTV in gynecological cancer if brachytherapy is not feasible appears to be achievable. For the first six patients treated, we did not observe adverse or unexpected events during and in the first months after treatment. Follow-up for all patients will be at least two years after treatment, the first outcome data are however are promissing. Using the OAR sparing planning approach, target