ESTRO 2024 - Abstract Book

S3573

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

1449

Digital Poster

Brachytherapy versus MRLinac Boost in locally advanced cervical cancer: a dosimetric comparison

Astrid van Lier 1 , Petra Kroon 1 , Jochem Hes 1 , Katelijne van Vliet - van den Ende 1 , Ronald Zweemer 2 , Gonda Sikkes 1 , Judith Roesink 1 , Raquel Davila Fajardo 1 , Femke van der Leij 1 , Ina Schulz-Jürgenliemk 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, Netherlands. 2 UMC Utrecht, Department of Gynaecological Oncology, Utrecht, Netherlands

Purpose/Objective:

In some cases it is difficult to fulfill target dose aims when treating locally advanced cervical cancer (LACC) with a brachytherapy (BT) boost. In this planning study we investigate for those cases if a MR Linac (MRL) boost might have had dosimetric benefit.

Material/Methods:

Six patients were retrospectively selected from our clinical database based on suboptimal dose distribution for the BT treatment, i.e. target dose aim not achieved after 2 applications/4 fractions (Geneva, Venezia or Utrecht applicator with 2-6 needles). To simulate BT treatment planning, we compared BT plans based on the first application and re-optimized for this study (Oncentra Brachy, Elekta AB), such that 4 identical fractions would not have violated the organ-at-risk (OAR) hard dose constraints (as defined in EMBRACE II, NCT03617133). For simulating MRL treatment planning, MRI scans collected at the day of BT but without BT applicator were used. Contouring was performed according to BT delineation guidelines. A PTV margin of 3 mm was added to GTV and CTV-HR, and a MRL plan (Monaco, Elekta AB) created with OAR hard constraints being dose limiting. The total MRL plan is based on a 6 fractions; we recalculated the dose constraints and target doses based to match EQD2 as used in EMBRACE II (Table 1). This strategy is also used in the MARGARITA study (NCT05937958). We compared the achieved dose levels to the GTV and CTV-HR and various dose volumes. All dose levels are reported in EQD2, taking the difference in BT and MRL fractionation scheme into account. A homogeneous background dose of 25x1.8 Gy from elective field radiotherapy was assumed.

Results:

For two patients (3 and 5) BT plans could be improved such that all target dose planning aims were marginally met (Table 1). Also the MRL plans of these patients met the dose planning aims. In patients 2 and 6 the MRL plan target dose levels were adequate, whereas this could not be achieved by replanning BT. In the remaining patients (1 and 4) both BT and MRL plans failed to meet all target dose level aims. In patient 1 the MRL dose levels were clearly less favorable compared to BT, whereas for patient 4 slightly higher doses were achieved in the CTV-HR with MRL, but with a lower GTV dose. Overall the variation in target dose was smaller for the MRL plans, meaning less outliers were seen. When comparing the dose distributions using dose volumes, it became obvious that in BT plans high dose volumes were larger, whereas the low dose volumes (V50Gy, V60Gy) were smaller compared to MRL plans (Table 2). V75Gy and V85Gy were fairly equal between the techniques.