Proton pencil beam scanning and the brainstem in pediatric posterior fossa tumors: a European survey

Abstract

Purpose or Objective Radiation-induced brainstem injury after pencil beam scanning (PBS) proton therapy (PT) of pediatric posterior fossa tumors is a concern due to range and relative biological effectiveness (RBE) uncertainties. Recommendations for ‘best practice’ in passive scattering PT have been published. However, such guidelines are lacking for state-of-the-art PBS PT. This survey therefore aimed at evaluating the current PBS standard of care across European PT centers for the treatment of pediatric posterior fossa tumors, with special considerations for brainstem management. Materials and Methods In September 2021, a web-based survey approved by the radiotherapy working group of the SIOPE brain tumors group was distributed to all 18 European PT centers treating pediatric patients with PBS. The main items that were assessed included clinical experience of the participating centers, patient positioning and imaging protocols, definitions of organs at risk, beam arrangements, treatment planning process, dose constraints applied (including RBE/linear energy transfer (LET) considerations) and clinical follow-up – a survey consisting of approximately 50 questions. Results At the time of abstract submission, most centers (14/18; 78%) have responded to the survey. The clinical experience for treating pediatric posterior fossa patients with PBS varied across centers. All but one center reported treating patients under anesthesia, and twelve (86%) of the centers always treated patients in supine position. For brainstem delineation, most of the centers followed the European Particle Therapy Network definition and nine (64%) centers further distinguished between brainstem core and surface. Robust optimization was used in nine (64%) centers, commonly employing a 3%/3mm uncertainty settings. Eight (57%) centers reported manually editing the spot distribution to dismiss high-weighted spots towards the brainstem. Clinically, all centers used a fixed RBE value of 1.1. Eight (57%) centers have capabilities for LET calculations, six of them from Monte Carlo platforms with all but one center using the dose-averaged LET formalism. However, none of the centers were doing systematic LET calculation in all patients. For clinical follow-up, eleven (79%) centers were prospectively assessing image changes in the brainstem, as well as clinical toxicity. Eight of those are using the common terminology criteria for adverse events version 4.0 grading system (‘Central Nervous System necrosis’), while the CTCAE version 5.0 was used in other institutions. Conclusion Surveying clinical practice across European PT centers disclosed some variations in current standard of care for the treatment of pediatric posterior fossa tumors and the next step in this European-wide project will be a follow-up workshop. The feedback received so far as well as the enthusiasm of all participants in attending this workshop reflect the high interest on this topic in the pediatric PT community.