ESTRO 2025 - Abstract Book

S3488

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

Purpose/Objective: The dose-weighted linear energy transfer (LETd) increases at the distal edge of the proton Bragg peak inducing different biological responses compared to proximal aspects of the depth-dose profile. Analysis of normal tissue complications are often based on composite LETd distributions from clinical treatment plans. Associations between LETd and complications have been inconclusive [1]. This work aims to introduce a new, more informative field-by field method of analysis.

Material/Methods: The LETd TOT

i for a treatment plan is defined as

, where N is the total number of particles used in the treatment, D i

TOT is the total dose delivered to the voxel i , D n,i is

the dose contribution from the particle n with LET n,i .

The equation for LETd TOT

i can be rewritten in terms of contribution from J fields in a plan

, where LETd j,j is the LETd for field j , D j,i is the dose delivered by field j .

The LETd and doses for two- and three-field plans with equally weighted fields were calculated using MCsquare Monte Carlo code [2]. Three ROI for each field were defined. ROILET1 was the near distal edge of the spread-out Bragg peak (SOBP). ROILET2 was far distal edge of the SOBP. ROILET3 included the lateral edges of the entire field. The dose (DVH) and LETd (LVH) volume histograms were computed for the respective ROI to illustrate the field-by-field contribution of LETd and dose. The ROI were analyzed for 98 two-field plans and 3 three-field plans used for pediatric craniopharyngioma treatment. Results: When a composite plan analysis is performed, the contribution of a single field’s dose from high LET particles (>5 keV/µm) is diluted by dose from low LET particles (~ 2.5 keV/µm) contributed by other fields (Figure 1) shifting actual high LET volumes toward low LET volumes associated with the total plan. The calculated mean volumes of the high LET ROIs are 66 cm 3 (ROILET1) and 23 cm 3 (ROILET2) with LVH variability of 1 and 2 keV/µm, respectively, for 2 field plans.