Differential biological effects of scattered versus scanned clinical proton beams on Ewing sarcoma models

Proton therapy (PT), first delivered passively using the Double Scattering (DS) technique, has shifted to active delivery through the Pencil Beam technique (PBS). However, the biological effectiveness of DS versus PBS proton beams on cancer cells, remain largely unknown. Given that PBS-PT is now commonly used to treat young patients with Ewing sarcoma, this point is of major importance in clinical practice and raises the question of the same effectiveness of both proton modalities on tumor control.

This study aims to compare the biological effects of clinical PBS vs. DS proton beams on various Ewing sarcoma cell lines. For this, we used a clinical Proteus 235 isochronous cyclotron capable of delivering both PBS and DS treatment modalities. Three different cancerous Ewing cell models were irradiated either in the plateau or in mid Spread-Out Bragg Peak (SOBP). We performed cell survival assays (2D monolayers and 3D spheroids), along with Reactive Oxygen Species (ROS) production (CellRox deep red assay) and quantification of 53BP1 foci 1 h and 18 h post-irradiation by immunofluorescence.

No significant differences were observed in cell survival (either in 2D or in 3D) at the plateau or mid-SOBP between the two proton treatment modalities. However, higher ROS production and a greater number of residual 53BP1 foci were observed in PBS compared to DS technique.

Both PBS and DS proton irradiation modalities exhibit the same in vitro biological effectiveness on Ewing sarcoma models. However, the question of the possible influence of the ROS/53BP1 increase on the onset of side effects or late toxicities when using the PBS irradiation technique, may arise.