ESTRO 2022 - Abstract Book

S256

Abstract book

ESTRO 2022

OC-0294 Proton therapy in mediastinal lymphoma patients in a nationwide cohort, using model-based selection

B. Ta 1 , S. Hutschemaekers 2,3 , A. Crijns 4 , P. Pisciotta 4 , R. Canters 1 , G. Vilches Freixas 1 , P. Cambraia Lopes 2 , A. Niezink 4

1 Maastro Clinic, GROW School for Oncology, Maastricht University Medical Center+, Department of Radiation Oncology (MAASTRO), Maastricht, The Netherlands; 2 HollandPTC, Department of Radiation Oncology, Delft, The Netherlands; 3 Haaglanden Medical Center, Department of Radiation Oncology, The Hague, The Netherlands; 4 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Since 2019 mediastinal lymphoma (ML) patients in the Netherlands are treated with intensity modulated proton therapy (IMPT) using model-based selection (MBS). MBS was performed according to the Dutch national indication protocol for proton therapy. For ML this is based on the difference in normal tissue complication probability ( Δ NTCP) for lifetime acute coronary events (ACE). The objective of this study was to evaluate dose advantages of both qualifying and non-qualifying patients. We evaluated the feasibility of MBS and the limitations of this approach by evaluating doses to other organs at risk (OARs) relevant for induction of secondary tumours of which currently no NTCP models are available. Materials and Methods All 61 ML patients referred for plan comparisons (VMAT vs IMPT) to one of the three Dutch proton centres were included in this study. In each patient, the estimated clinical benefit in terms of Δ NTCP is derived from both treatment plans using the ACE NTCP model which includes mean heart dose (MHD), age, gender and risk factors for ACE as predictors. The Δ NTCP threshold for MBS is a 2% reduction for ACE. The differences in MHD, lungs (MLD) and oesophagus (MED) were evaluated. Finally, the reduction in mean breast dose (MBD) was investigated in female patients. Robust IMPT plans were created using two to eleven beams depending on the target volume and a 3D robustly optimised planning technique was used. Radiotherapy consisted of 20-36 Gy in 2 Gy fractions and chemotherapy consisted of 3-4 cycles of A(B)VD or escBEACOPP for Hodgkin lymphoma or 6x R-CHOP for B-cell lymphoma. Results Thirty-two out of 61 patients (52%) referred for plan comparisons met the predefined Δ NTCP threshold and were eligible for IMPT (Table) . In patients qualifying for IMPT, IMPT resulted in a significantly lower MHD (8.9 Gy vs. 5.3 Gy; p <0.01), MLD (8.3 Gy vs. 6.0 Gy; p <0.01), MED (15.3 Gy vs. 11.2 Gy; p <0.01), MBD-right (2.9 Gy vs. 2.1 Gy; p =0.001) and MBD-left (3.6 Gy vs. 2.3 Gy; p =0.004). Using the NTCP model for ACE, the mean NTCP for VMAT was 12.1% (range 2.1 – 30.4%) compared to 8.6% (range 0.1– 27.4%) for IMPT, an average Δ NTCP with IMPT for ACE of 3.4% (range 2.0 – 9.8; p <0.01). In patients that did not qualify for IMPT, the dose to all OARs was also significantly reduced; MHD (7.3 Gy vs. 5.8 Gy; p<0.01), MLD (8.3 Gy vs. 6.2 Gy; p <0.01), MED (12.5 Gy vs. 10.5 Gy; p =0.004), MBD-right (3.3 Gy vs. 2.2 Gy; p =0.012) and MBD-left (4.6 Gy vs. 3.6 Gy; p =0.014) (Figure) .

Conclusion Model-based selection of ML patients for proton therapy is feasible. IMPT resulted in a significant MHD reduction and a subsequent 3.4% reduction in estimated lifetime risk of ACE. However, patients not qualifying for proton therapy could