Proton therapy in breast cancer: how do different beam arrangements affect linear energy transfer?
Settatree, S. Bertolet, A. Carabe, A. Lines, D. Harrold, N. Harris, E. Kirby, A. M. Gulliford, S.
Settatree, S.
Bertolet, A.
Carabe, A.
Lines, D.
Harrold, N.
Harris, E.
Kirby, A. M.
Gulliford, S.
Citations
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Abstract
Purpose or Objective
It is considered that protons have a variable relative biological effectiveness (RBE) which is related to linear energy transfer
(LET). LET is variable along the proton track with highest values distally. For a breast and internal mammary nodal proton
beam therapy (PBT) plan, this could affect the dose to organs at risk (OARs) such as heart, left anterior descending
coronary artery (LAD) and ribs. This study compares dose-averaged LET (LETd) distributions in OARs from different beam
arrangements with the aim of identifying which could mitigate the potential clinical effects of high LETd.
Materials and Methods
Five left-sided breast patients comprising a spectrum of chest wall shapes were identified. Three patients had varying
degrees of pectus excavatum (Haller Index 2.7-4.9). Breast/ reconstructed chest wall, axilla levels 1-4 and internal
mammary clinical target volumes were contoured according to ESTRO consensus [1].
Four pencil beam scanning PBT plans were prepared for each patient using the beam arrangements described in Table
1. The prescription was 40.05 Gy in 15 fractions. All plans were optimised to achieve 95% dose coverage of 95% of all
target volumes and satisfied uncertainty parameters of 5mm (set up) and +/-3.5% (range). Treatment plans were calculated using an analytical pencil beam algorithm (Varian Eclipse). LETd was calculated using a
bespoke script developed at the University of Pennsylvania [2,3]. Using summary statistics for CTV and OARs at the distal edge of beam (LAD and rib), the differences in RBE-weighted dose
(using fixed RBE 1.1) and LETd distributions between different beam arrangements were analysed by Friedman’s Test.
Results
All plans were clinically acceptable in terms of CTV coverage and OAR doses: mean heart dose ≤2.2 Gy, mean left lung dose
≤12 Gy. There were no statistically significant differences in dose to mean CTV, dose to maximum LAD and dose to rib
(0.5cc) between different beam arrangements. For the corresponding LETd calculations there were statistically significant
differences between the beam arrangements (Figure 1). Single beam plans demonstrated higher LETd in these distal OARs Results demonstrate that the distribution of LETd varies between beam arrangements. Single beam plans showed the
highest LETd in rib and LAD. In conclusion, to reduce the potential for high LETd contributing to an increased risk of rib
fracture, our data support consideration of additional beams and varied beam angles. These considerations should be
weighed against any disadvantages in dosimetry or plan robustness.
References
[1] Offersen B V. et al Radiother Oncol 2015;114:3–10. doi:10.1016/j.radonc.2014.11.030
[2] Bertolet A et al Med Phys 2020;47:2495–505. doi:10.1002/mp.14108
[3] Bertolet A et al Phys Medica 2021;81:69–76. doi:10.1016/j.ejmp.2020.11.024
Description
Date
2021
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Settatree S, Bertolet A, Carabe A, Lines D, Harrold N, Harris E, et al. Proton therapy in breast cancer: how do different beam arrangements affect linear energy transfer? Radiotherapy and Oncology. 2021;161:S1630-S1.