Room: AAPM ePoster Library
Purpose: Advanced radiation therapy techniques contribute to enabling 85% of children diagnosed with cancer to survive 5 years or more, most of whom become long-term survivors. Proton therapy, delivered via pencil-beam-scanning (PBS) or passive-scattering (PS) methods, is the most widely available advanced radiation particle therapy. The theoretical advantages of proton therapy stem from its superior pattern of physical dose deposition, which is largely insensitive to delivery method. The biologic effectiveness of proton physical dose, however, purportedly rises with increasing ionization density, termed linear energy transfer. Nevertheless, the effect of proton delivery method on linear energy transfer, and thereby biologic effect, is poorly understood. The objective of this study was to determine the extent to which proton delivery method affects the dose-weighted average linear energy transfer (LET(D)).
Methods: We used Monte-Carlo methods to calculate the LET(D) for PS and PBS proton delivery methods in a virtual water phantom and voxelized patient anatomy with TOPAS Monte-Carlo transport code. Simulations in water were dosimetrically matched. Simulations in voxelized patient anatomy used the treatment and anatomic data from a population of patients treated for craniopharyngioma with PBS or PS proton therapy. We compared the distributions of LET(D) between sub populations (PS and PBS) via spatial and statistical analysis.
Results: Comparisons of the spatial LET(D) distribution in water revealed that PBS protons produced higher LET(D) than did PS protons for equivalent physical dose deposition. Comparisons of LET(D) between the 2 populations of patients revealed that PBS yielded a higher mean LET(D) magnitude than did PS in the brain, brainstem, tumor, and left and right optic nerves, but not in the left and right cochleae (p=0.06 and p=0.53, respectively).
Conclusion: Proton delivery methods affect LET(D). This suggests that the relation between proton physical dose and biologic effect is not generalizable across delivery methods.