Author: Omer Gal, Alonso N. Gutierrez, Matthew D. Hall, Rupesh Kotecha, Minesh P. Mehta, Andrew J. Wroe, Jen Yu ๐จโ๐ฌ
Affiliation: Miami Cancer Institute, Baptist Health South Florida ๐
Purpose: Linear energy transfer (LET) is directly related to relative-biological-effectiveness dose in proton therapy. This study aims to investigate how the treatment field hinge angles affect LET of intensity modulated proton therapy (IMPT) plans in brain tumor patients.
Methods: A spherical phantom and 36 patients with primary brain tumors treated with IMPT from 2017 to 2023 are included in this study. IMPT plans were created using 2-coplanar beams or 3-6 non-coplanar beams. The plans were robustly optimized with single-field-optimization. Dose and Dose-averaged LET, LETd, were calculated with Monte-Carlo algorithm. For phantom plans, 2-coplanar beams with a hinge angle from 5หto180ห in increments of 15หand with an additional vertex beam were modeled. For patient plans, to quantify the beam-angle-spread extent created by multiple non-coplanar beams with couch rotations in 3D a โbeam-spread-volumeโ was developed. This volume was defined as the sum of the projected area along each beam 1cm downstream from distal surfaces of CTV. This volume was normalized by a shell volume encompassing a 1cm outward expansion from CTV. The correlation between LETd of the CTV-1cm expansion and hinge angle, and normalized beam-spread-volume were investigated for phantom and patient plans, respectively.
Results: It was found that LETd decreased with increasing hinge angle. In phantom study the maximum LETd decreased from 7.7 to 3.9keV/ยตm with hinge angle increasing from 5หto180ห. Within 135ห, the addition of a vertex beam further reduced the maximum LETd from 6.4 to 4.9keV/ยตm. Similarly, for 36-patient plans, the maximum LETd decreased from 6.0 to 5.0keV/ยตm as the normalized beam-spread-volume increased based on the linear fit of the data.
Conclusion: Increasing hinge angle or spread extent of non-coplanar beams reduces LETd. The beam-spread-volume, a novel parameter, was created to quantify treatment field angle spread in 3D space. This can provide guidance for treatment planning to manage high LETd.