ESTRO 2024 - Abstract Book

S6000

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

2351

Digital Poster

Dosimetric comparison between partialVMAT and fullVMAT plans in mediastinal lymphoma patients

Petra Klinker, Pietro Pisciotta, Christina T. Muijs, Hans H.G. Verbeek, Dianne M. Busz, Johannes A. Langendijk, Anne P.G. Crijns, Anne G.H. Niezink

University Medical Centre Groningen, Radiation Oncology, Groningen, Netherlands

Purpose/Objective:

Over the years, planning techniques for mediastinal lymphoma patients have been improved consistently and parVMAT and fullVMAT are the standard techniques applied for photon treatments. Given the target proximity to heart, lungs and breasts, optimal treatment planning strategies are essential to avoid late complications. Furthermore, the importance of plan robustness for moving target is emphasized by the relatively short treatment schedules (10 to 18 fractions), leaving limited time for plan adaptation. The aim of this study was to compare parVMAT and fullVMAT plans in mediastinal lymphoma patients for organs at risk (OARs) doses and plan robustness.

Material/Methods:

Eleven patients with mediastinal lymphoma treated between June 2021 and March 2023 were included in this study. The treatment targets were classified based on location to the heart: anteriorly, posteriorly, left, right or upper mediastinum. Patients were treated in a supine position using a 5-points mask in free breathing, facilitating matching with pre-treatment PET and 4DCT scans in treatment position for involved node radiotherapy. The parVMAT plans consisted of 70% open AP/PA fields and a 30% VMAT to ensure a homogenous dose distribution. The beam angles of the arc(s) for parVMAT and fullVMAT were determined based on target volume’s location and extent. Plans were only clinically accepted if the target coverage was V99% > 95% of the prescribed dose (PD). The study compared the mean dose to the heart (MHD), lungs (MLD) and left and right breast (MBD_L and MBD_R), V5% and V20% of the lungs for both treatment techniques. Plan robustness was further evaluated during treatment through weekly repeat scans (rCTs) using 2-mm setup uncertainty to account for additional uncertainties associated with positioning and target motion. Each plan was recomputed on the rCTs and adequate coverage was defined as V94% > 98% of the PD. For patients not meeting this criterion, a summed dose calculation (7 fractions on rCT1week and 8 fractions on rCT2week) was performed to assess target coverage post-treatment.

Results:

The treatment target was located in the upper mediastinum in 3 patients, all others were located close to the heart. In 4 patients targets were located at one side of the heart (36.4%), in 3 patients at two sides of the heart(27.3%), and in 1 patient targets were located at 3 sides of the heart (9.1%). The fullVMAT plans resulted in a lower MHD in 10 out of 11 patients (mean MHD reduction 1.29 Gy), with ≥ 1.5 Gy in 4 of these patients. However, the MLD and V5% of the lungs were higher in fullVMAT plans in 8 and 9 patients respectively (Figure 1). In all six female patients, MBD_R was higher in fullVMAT while the MBD_L was lower in 5 out of 6 patients. Target coverage was adequate in all patients for parVMAT plans and in 9 of 11 patients for the fullVMAT plans. In two patients the fullVMAT plans were less robust to