Author: Michael Confer, Mark A. Newpower 👨🔬
Affiliation: University of Oklahoma Health Sciences Center 🌍
Purpose: A pediatric patient recently presented to our clinic for treatment for chordoma, which was near the brainstem. The clinician elected to treat this patient using intensity modulated proton therapy (IMPT). When treating targets near organs at risk (OARs) such as the brainstem, enhanced relative biological effectiveness (RBE) related to increased dose averaged linear energy transfer (LETd) is of utmost concern. Recently, treatment planning systems have gained the ability to compute LETd along with dose, and to review the LETd distribution using the LET volume histogram (LVH). This feature allows planners to make clinical decisions based on the LETd distribution. We present a case report where we selected a beam arrangement based on direct comparison of the brainstem LVH for a pediatric chordoma patient.
Methods: The patient was planned in RayStation 12A. Two plans were developed and compared: brainstem avoid (BA) with four obliques and one vertex beam, and a brainstem shoot through (BST) with two obliques, one posterior passing through the brainstem and one vertex beam. During the planning process, the LVHs and dose volume histograms were considered.
Results: Both plans had adequate target coverage and organ at risk (OAR) sparing in terms of RBE=1.1 dose. The brainstem maximum point dose was below constraints for the two plans (BA: 4000 cGy(RBE), BST: 3465 cGy(RBE). For the low to medium dose volume of the brainstem, the BA plan was superior to the BST. The BST plan had a significantly superior LVH in terms of maximum (BA: 9.8 keV/µm, BST: 4.2 keV/µm) and average LETd (BA: 6.0 keV/µm, BST: 2.0 keV/µm).
Conclusion: Our clinical team decided to treat the patient with the BST plan due to the superior LVH as compared to the BA plan. This case study demonstrates how clinical decisions could be made based on the LVH.