ESTRO 2021 Abstract Book

S243

ESTRO 2021

PH-0330 Introducing IMPT for mediastinal lymphoma: feasibility and NTCP model based selection of patients A. Niezink 1 , P. Klinker 1 , D.M. Busz 1 , M. Beijert 1 , S. Both 1 , P. Pisciotta 1 , E.W. Korevaar 1 , J.A. Langendijk 1 , A.P. Crijns 1 1 University Medical Center Groningen / University of Groningen, Radiation Oncology, Groningen, The Netherlands Purpose or Objective Since the end of 2019 we started to treat mediastinal lymphoma patients with intensity modulated proton radiotherapy (IMPT) using model-based selection (MBS). The objective of this study was to evaluate the feasibility of MBS and IMPT. Materials and Methods Patients referred for treatment of a mediastinal lymphoma with combined modality treatment according to the Dutch guidelines could be selected for proton therapy. Selection was based on the national indication protocol for proton therapy. This protocol contains guidelines for MBS. For mediastinal lymphoma cases this is based on difference in normal tissue complication probability (ΔNTCP) for life time acute coronary events (ACE) derived from a plan comparison (partial VMAT photon technique versus IMPT). The parameters in this NTCP model are mean heart dose (MHD), age, gender and the absence or presence of risk factors for ACE. The ΔNTCP threshold is 2% for ACE. For the current analysis the ΔNTCP thresholds for radiation pneumonitis and esophagitis were also calculated using the NTCP models used for selection of lung cancer patients. Finally, the reduction in mean breast dose (MBD) was investigated in female patients as well. Maximum target motion allowed for IMPT delivery was 15 mm. Robust IMPT plans were created using two to five beams depending on the target volume and a 3D robustly optimized planning technique (including 5 times layered rescanning). Online position verification consisted of daily (consecutive) body surface scanning, 2D kV imaging and cone-beam CT imaging. Weekly repeated (4D) CTs were acquired; the treatment plan was adapted in case of inadequate target coverage. Radiotherapy consisted of 30-36 Gy in 2 Gy fractions and chemotherapy consisted of 3-4 cycles of A(B)VD in case of Hodgkin lymphoma or 6 R-CHOP in case of primary mediastinal B-cell lymphoma. Results Eleven patients (58%) of the 19 patients referred for radiotherapy or planning comparison met the predefined ΔNTCP threshold and were eligible for IMPT (maximum target motion 11.7 mm, range 5-20 mm). Compared with partial VMAT, IMPT resulted in significantly lower MHD ( 10.0 Gy vs 6.1 Gy; p=0.003), mean lung dose (MLD) (8.8 Gy vs. 6.7 Gy; p=0.003), mean esophageal dose (MED) (16.9 Gy vs. 12.5 Gy; p=0.016), MBD-right (4.3 Gy vs. 2.5 Gy; p=0.025) and a non-significant reduction in MBD-left (5.5 Gy vs. 4.5 Gy; p=0.063). Using the NTCP model for ACE a mean NTCP of 7.84 (range 2.9 – 19.7) for partial VMAT and a mean NTCP for IMPT of 4.82 (range 0.70– 14.1). The dose reduction on the heart resulted in an average ΔNTCP with IMPT for ACE of 3.0% (range 2.0 – 5.6). Conclusion Model-based selection of mediastinal lymphoma patients for proton radiotherapy is feasible even in case of larger movement. IMPT results in a significant reduction of the MHD and a subsequent 3.0% reduction in estimated lifetime risk of ACE. Given de dose reduction in various other OAR, the actual benefit on other endpoints may be larger but cannot be quantified yet.

PH-0331 Patterns And Predictors Of Relapse In Merkel Cell Carcinoma :Results From A Population Based Study.