Treatment‐planning approaches to intensity modulated proton therapy and the impact on dose‐weighted linear energy transfer

Abstract

Purpose We quantified the effect of various forward‐based treatment‐planning strategies in proton therapy on dose‐weighted linear energy transfer (LETd). By maintaining the dosimetric quality at a clinically acceptable level, we aimed to evaluate the differences in LETd among various treatment‐planning approaches and their practicality in minimizing biologic uncertainties associated with LETd. Method Eight treatment‐planning strategies that are achievable in commercial treatment‐planning systems were applied on a cylindrical water phantom and four pediatric brain tumor cases. Each planning strategy was compared to either an opposed lateral plan (phantom study) or original clinical plan (patient study). Deviations in mean and maximum LETd from clinically acceptable dose distributions were compared. Results In the phantom study, using a range shifter and altering the robust scenarios during optimization had the largest effect on the mean clinical target volume LETd, which was reduced from 4.5 to 3.9 keV/μm in both cases. Variations in the intersection angle between beams had the largest effect on LETd in a ring defined 3 to 5 mm outside the target. When beam intersection angles were reduced from opposed laterals (180°) to 120°, 90°, and 60°, corresponding maximum LETd increased from 7.9 to 8.9, 10.9, and 12.2 keV/μm, respectively. A clear trend in mean and maximum LETd variations in the clinical cases could not be established, though spatial distribution of LETd suggested a strong dependence on patient anatomy and treatment geometry. Conclusion Changes in LETd from treatment‐plan setup follow intuitive trends in a controlled phantom experiment. Anatomical and other patient‐specific considerations, however, can preclude generalizable strategies in clinical cases. For pediatric cranial radiation therapy, we recommend using opposed lateral treatment fields to treat midline targets.

The LET trilemma: Conflicts between robust target coverage, uniform dose, and dose‐averaged LET in carbon therapy

“…In addition, a structure called "CTV-GTV" is created as the CTV with the GTV subtracted, for each of the cases. Two parallel opposed beams from 90 • and 270 • are used, as recommended by Faught et al 15 We also performed experiments (not shown) with 4, 8, and 16 beams with similar results for the effect on LET d . An ROI called "Entrance" for gauging the entrance dose is created as a 2 cm long cylinder with radius 0.5 cm aligned with the beam central axis 1 cm from the patient surface to the left.…”

“…Faught et al 15 studied the effects on LET d for various treatment planning approaches for IMPT under uncertainty. Using dose-based planning, without any LET d -based functions included in the optimization, they observed that when the magnitude of the range uncertainty included in the optimization was increased, the LET d in the target decreased.…”

“…Faught et al 15 . studied the effects on LET d for various treatment planning approaches for IMPT under uncertainty.…”

A Systematic Review of LET-Guided Treatment Plan Optimisation in Proton Therapy: Identifying the Current State and Future Needs

“…Studies investigating LET changes with beam arrangement have shown that LET max (Appendix B) increases significantly as the angle between the two beams approaches 0 • due to a build-up of high LET in a small region [10,63,64]. Faught et al [10] observed an LET max increase of 3.3 keV/µm immediately outside the target boundary when a beam angle of 60 • is used compared to 180 • , while inside the target LET max increased by 1.2 keV/µm and LET mean (Appendix B) remained unchanged. Fjaera et al [64] made similar observations in that narrowing the angle between two lateral beams increased LET within the brainstem; however, with the addition of a third vertex beam, LET mean decreased.…”

“…A universal RBE of 1.1 is currently adopted in a clinical setting [4,5], despite increasing to values in excess of 2 [6] in the distal Bragg Peak region. Clinically, the distribution of RBE for a treatment plan is complicated further by the use of the spreadout Bragg Peak (SOBP), due to the higher degree of modulation compared to a pristine Bragg Peak (e.g., IMPT) [7][8][9][10]. By disregarding RBE exceeding 1.1 in the distal region, we underestimate the biological effect inside critical structures, potentially having a negative effect on treatment outcomes [11].…”