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Redistribution of Linear Energy Transfer Near Critical Structures Through the Introduction of Proton Track-End Objectives in Proton Plan Optimization

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E Traneus

E Traneus1*, J Oden1,2 , (1) RaySearch Laboratories, Stockholm, Stockholm, (2) Stockholm University, Stockholm, Stockholm


SU-I-GPD-T-158 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To explore the potential of controlling the dose-averaged linear energy transfer (LETd) in critical tissues by introducing objectives on the proton track-end distribution in intensity modulated proton therapy (IMPT) optimization.

Methods: IMPT plans of 2 Gy(RBE) per fraction in 30 fractions were created for a phantom case and an intracranial patient case using physical dose objectives and RBE = 1.1. The phantom case simulated a target wrapped around the brainstem whereas the target in the patient case was close to the brainstem and chiasm. Following this, the plans were re-optimized with the addition of proton track-end objectives. The proton track-end objective penalizes presence of track-ends in a certain region, e.g. a risk organ, and can is used in combination with regular dose based objectives. Comparisons of the physical dose distribution, proton track-end distribution, LET distribution and RBE-weighted dose distributions using several variable RBE models were performed.

Results: The original plans fulfilled the clinical goal assuming the generic RBE of 1.1. The introduction of proton track-ends objectives triggered a redistribution of the proton track-ends, which allowed for reductions of the LETd in the critical structures while the physical dose distribution was virtually unchanged for both cases. The median LETd was reduced from 4.7 to 4.1 keV/μm for the brainstem and from 6.9 to 6.1 keV/μm for the chiasm for the intracranial case. This allowed for a reduction of about 1–3 Gy(RBE) in D2% for both structures using various RBE models.

Conclusion: The introduction of proton track-end objectives in the IMPT optimization allow for LET reduction in critical structures without compromising the physical dose. Consequently, such a tool may mitigate the potential adverse effects associated with high LET values near the end of the proton range, independently of the RBE model of choice.

Funding Support, Disclosures, and Conflict of Interest: Erik Traneus is a senior researcher at RaySearch Laboratories and Jakob Oden is part-time employee as an industrial PhD student at RaySearch Laboratories.

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