ESTRO 2024 - Abstract Book

S5956

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Material/Methods:

Between March and April 2023, RT therapists from eight different Dutch RT departments (including one proton treatment center) accepted the challenge to create a treatment plan, on the same anonymized case. The prescribed dose was 40.05 Gy, delivered in 15 fractions, to the Planning Target Volume (PTV) of right-sided breast and lymph node (LN) levels I to IV. The data set included a CT scan and delineated target volumes, including PTVs, and organs at-risk (OARs). Each department used the local protocols for the planning and delivery technique. This included local clinical (PTV coverage, clinical constraints to OARs) and technical considerations. The departments returned treatment plans for analysis. Dose parameters such as mean heart dose (MHD), mean lung dose (MLD, both lungs), and mean dose to the contralateral breast were determined. The various approaches were compared qualitatively, and differences in dose to OARs were analyzed quantitatively. Subsequently, a robustness analysis was performed for the seven photon plans. This analysis consisted of two scenarios, simulating patient shifts of +/- 5 mm perpendicular to the tangential fields in the original case. These shifts mimicked typical anatomical changes, such as swelling or shrinkage, of the breast contour that can occur during the course of treatment. It was assessed whether the shifted plans were still clinically acceptable, i.e. 95% of the clinical target volume (CTV) receives >98% (V95%>98%), and only a maximum of 3% of the CTV receives >107% of the prescribed dose (V107%<3%). Delivery techniques included intensity-modulated proton therapy (IMPT, one department), intensity-modulated radiotherapy (IMRT, one department), full volumetric modulated arc therapy (VMAT, two departments) or tangential fields combined with VMAT (hybrid, four departments), each with different choices in beam angles. Departments used various clinical constraints for OARs. MHD constraints varied from 1.5 to 3.5 Gy; for the MLD the range was between 6 and 15 Gy, and the mean dose constraint for the contralateral breast varied from 0.5 to 5 Gy. All techniques led to clinically acceptable plans with small variations in dose to OARs (Figure 1) and the PTV. Robustness analysis showed small differences in plans shifted by +/- 5 mm. One shifted plan showed small cold spots in the LN levels I and II (Table 1). Another plan displayed a small hotspot in LN levels III and IV in one scenario. All findings were within the 3% tolerance, except for the approach used in one hybrid plan. In this case, one scenario showed more than 5% of the volume exceeding the maximum dose limit of 107% of the prescribed dose. Results: