Author: Garrett Russell Hamilton, Joshua Misa, Damodar Pokhrel, William St. Clair 👨🔬
Affiliation: University of Kentucky, Department of Radiation Medicine, University of Kentucky, Radiation Medicine 🌍
Purpose: Recurrent brain metastasis is very common, and patients who previously received whole-brain radiotherapy (WBRT) may receive an additional course(s) of stereotactic radiosurgery (SRS) to select metastasis. This project aims to analyze the feasibility of performing a second course of SRS on patients who have received SRS and WBRT for recurrent brain metastasis.
Methods: Twelve patients previously treated with WBRT (30Gy in 10 fractions) that received future SRS for brain metastasis were included in this study. Single- and multi-lesion SRS dose scheme varied from 18-30 Gy delivered in 1-5 fractions. EQD2 was calculated using an in-house script that calculates the spatial EQD2 voxel-by-voxel in 3D space (s-EQD2), retaining the spatial information across both plans. The s-EQD2 uses the Linear-Quadratic model, with an α/β=10 Gy for PTV structures and α/β=3 Gy for normal tissue structures. Mean EQD2 to whole brain (WB) tissue and PTVs were evaluated, in addition to maximum EQD2 to brain-PTV, brainstem, optic pathway, and spinal cord.
Results: Mean WB-EQD2 from WBRT course was calculated to be 34.0±1.1Gy. Average EQD2 to WB for WBRT plus SRS was 35.4±2.0Gy. Across the 12 re-treatment patients, average maximum EQD2 to brainstem was 39.1±2.0Gy, and average maximum EQD2 to 0.03 cc of the spinal cord was 35.7±2.5Gy. Average PTV dose to 0.03cc was 92.8±10.0Gy. Average EQD2 dose to Brain minus PTVs was 39.7±2.8Gy.
Conclusion: The resulting mean EQD2 to normal tissue structures including WB reveals that an additional SRS course may be feasible (if needed) for patients with new or recurrent lesions. An additional course of SRS may benefit patients by enhancing stereotactic dose to brain tumors, improving local-control rates and patient outcomes if dose to critical organs are managed. This s-EQD2 method (script) can be adopted for evaluating treatments at other treatment sites for reirradiation: recurrent head & neck tumors, abdominal/theocratic SBRT.