Dynamic Proton arc therapy for paediatric parameningeal rhabdomyosarcoma

Parameningeal rhabdomyosarcomas (PM-RMS) are pediatric tumors with a high risk of local failure and severe toxicity. Intensity modulated proton therapy (IMPT) offers improved healthy tissue sparing compared to conventional photon-based radiotherapy, and proton arc therapy (PAT) might offer further improvement in healthy tissue sparing for these patients, thus emphasizing the need for systematic studies specifically focusing on PM-RMS patients. Additionally, the relative biological effectiveness (RBE) of protons is known to vary with linear energy transfer (LET).

The aim of this study was to investigate the potential of LET-optimized PAT for PM-RMS treatment.

Volumetric modulated arc therapy (VMAT) plans, IMPT and PAT plans, both with and without LET optimization were made for 13 PM-RMS patients using Raystation version 2023B. The plans were evaluated based on the achievement of clinical goals, homogeneity index, delivery time, robustness, LET and variable RBE-weighted dose within the target and selected OARs.

LET optimization increased the mean LET in the target from 2.5 keV/µm [2.1, 2.8] to 3.4 keV/µm [2.7, 4.1] for IMPT and from 2.8 keV/µm [2.5, 3.1] to 3.4 keV/µm [2.7, 4.0] for PAT with minor changes in dose distributions. Target coverage and OAR doses were comparable between IMPT and PAT, while both had an expected improvement in OAR sparing compared to VMAT. Delivery times were 134.5 [95.3, 203.7] seconds for the IMPT plans and 197.4 [138.9, 244.6] seconds for the PAT plans before LET optimization, and were for both modalities reduced by around 2-5 seconds after LET optimization.

The dose distribution was similar across PAT and IMPT in this study, and superior compared to VMAT. LET optimization in the proton plans achieved LET improvements in both target and OARs without compromising dose.