Reducing pitfalls in composite plan evaluations of dose-weighted linear energy transfer (LETd) through a...

Objective.The dose-weighted linear energy transfer (LETd) increases significantly at the distal edge of the proton Bragg peak, leading to distinct biological responses compared to proximal regions of the depth-dose profile. Normal tissue complications have been linked to composite LETd distributions from clinical treatment plans, but the associations between LETd and complications remain inconclusive. This study introduces a field-by-field analysis method to provide more detailed insights into these relationships.Approach. LETd in composite plans was reformulated to account for weighted contributions from individual fields. Dose and LETd distributions for each field were calculated using the MCsquare Monte Carlo code. Field-specific dose volume histograms and LETd volume histograms (LETdVHs) were generated for regions with elevated LETd in 101 pediatric craniopharyngioma cases treated with two-field plans (n= 98) or three-field plans (n= 3).Main results.Individual fields contribute high LETd at the distal edge of the spread-out Bragg peak (SOBP) and lateral field edges. High LET particles delivered physical doses of several Gy exceeding the threshold for non-stochastic acute and late effects. Composite LETd distributions, however, show lower LET at the SOBP distal edge due to contributions from low LET particles in opposing fields. Information about high LET dose deposited by individual beams is diluted in composite plan LETd distributions.Significance.Evaluating LETd distributions based solely on composite plans can obscure the contribution of high-LET regions from individual fields to the biological response of tissues. A field-by-field assessment may provide a more accurate understanding of LETd's role in normal tissue effects and may improve the prediction of complications in proton therapy.