ESTRO 2023 - Abstract Book

S63

Saturday 13 May

ESTRO 2023

treatment volume (PTV) (per 100 mL; HR = 1.13, 95%CI 1.01-1.27, p =0.033) were found to be significantly associated with G4L. Multivariable classification analysis partitioned patients into five subgroups for whom the incidence of G4L was observed in 0%, 14%, 35%, 70%, and 100% of patients ( Figure 2 ). The benefit of PBT over IMRT was most pronounced in patients with an intermediate baseline ALC and large PTV (G4L in 35% versus 70%, respectively, p =0.011).

Figure 1. Distribution of absolute lymphocyte counts during concurrent CRT for patients stratified by treatment arm. G4L: grade 4 lymphopenia. *: p <0.05. **: p <0.01.

Figure 2. Decision risk tree model based on predictive factors for grade 4 lymphopenia (G4L). Conclusion

This is the first prospective and randomized evidence demonstrating that limiting dose scatter through using PBT instead of IMRT can significantly reduce the incidence of severe lymphopenia, especially in the intermediate-risk patients. The implication of this immune-sparing effect of PBT, especially in the recently adopted context of standard adjuvant immunotherapy, needs further examination in the currently ongoing phase III randomized trials. OC-0102 Clinical evaluation of heart dose changes during proton therapy of esophageal cancer R. Canters 1 , K. Van der Klugt 1 , V. Taasti 1 , J. Buijsen 1 , F. Vereijken 1 , T. Verstappen 1 , K. Limpens 1 , S. Hurkmans 1 , M. Van den Bosch 1 , F. Visser 1 , G. Vilches-Freixas 1 , M. Berbee 1 1 Department of Radiation Oncology (MAASTRO), GROW-School for Oncology, Maastricht University Medical Center, Maastricht, The Netherlands Purpose or Objective Following national consensus guidelines, patients with esophageal cancer are treated with proton radiotherapy using a model-based selection approach, based on a significant reduction of the mean heart dose (MHD) associated with improved 2-year survival. In this study, we assessed changes and trends in heart dose during intensity modulated proton therapy (IMPT) for esophageal cancer and the relation to anatomical variations. Materials and Methods We included 60 esophageal cancer patients, treated at our institute with neo-adjuvant (23x1.8 Gy) (n=54) or definitive (28x1.8 Gy) (n=6) chemo-radiotherapy. Clinical target volume (CTVs) were delineated on all phases of a 4D CT, and combined to an internal target volume (ITV). Treatment plans were robustly optimized with 5mm setup/3% range uncertainty combined with 3mm isotropic margin around the ITV, using 4, 3, or 2 beams IMPT (Figure 1). Each patient received weekly repeat CTs (reCTs) during the course of treatment, on which redelineation and dose recalculation were performed. Dose was summed for the complete course of treatment using deformable registration. To assess the factors influencing MHD changes and heart V40Gy, a multivariate linear regression analysis was performed using all parameters described in Table 1. Factors in multivariate regression of MHD changes or heart V40Gy p-value MHD p-value heart V40Gy Number of beams 0.06 0.001 Weight of oblique beams 0.08 0.50 Change in radiological heart depth in beam direction 0.02 0.70 Shift of the heart center of mass 0.43 0.52 Heart volume change <0.001 0.44