Abstract
Brain metastases can effectively be treated with surgical resection and adjuvant stereotactic radiotherapy (SRT). Navigated transcranial magnetic stimulation (nTMS) has been used to non-invasively map the motor cortex prior to surgery of motor eloquent brain lesions. To date, few studies have reported the integration of such motor maps into radiotherapy planning. The hippocampus has been identified as an additional critical structure of radiation-induced deficits. The aim of this study is to assess the feasibility of selective dose reduction to both the nTMS-based motor cortex and the hippocampi in SRT of motor-eloquent brain metastases. Patients with motor-eloquent brain metastases undergoing surgical resection and adjuvant SRT between 07/2014 and 12/2018 were retrospectively analyzed. The radiotherapy treatment plans were retrieved from the treatment planning system (“original” plan). For each case, two intensity-modulated treatment plans were created: the “motor” plan aimed to reduce the dose to the motor cortex, the “motor & hipp” plan additionally reduce the dose to the hippocampus. The optimized plans were compared with the “original” plan regarding plan quality, planning target volume (PTV) coverage, and sparing of organs at risk (OAR). 69 plans were analyzed, all of which were clinically acceptable with no significant differences for PTV coverage. All OAR were protected according to standard protocols. Sparing of the nTMS motor map was feasible: mean dose 9.66 ± 5.97 Gy (original) to 6.32 ± 3.60 Gy (motor) and 6.49 ± 3.78 Gy (motor & hipp), p<0.001. In the “motor & hipp” plan, dose to the ipsilateral hippocampi could be significantly reduced (max 1.78 ± 1.44 Gy vs 2.49 ± 1.87 Gy in “original”, p = 0.003; mean 1.01 ± 0.92 Gy vs. 1.32 ± 1.07 Gy in “original”, p = 0.007). The study confirms the results from previous studies that inclusion of nTMS motor information into radiotherapy treatment planning is possible with a relatively straightforward workflow and can achieve reduced doses to the nTMS-defined motor area without compromising PTV coverage. Furthermore, we demonstrate the feasibility of selective dose reduction to the hippocampus at the same time. The clinical significance of these optimized plans yet remains to be determined. However, with no apparent disadvantages these optimized plans call for further and broader exploration.
Highlights
Brain metastases can effectively be treated with surgical resection and/or stereotactic radiotherapy (SRT)
Even though this introduces some heterogeneity in the study collective, this wide selection of planning methods is representative of routine radiotherapy practice and differences could be identified in the amount of sparing achieved depending on the original planning technique
As CyberKnife and GammaKnife availability is restricted to a small selection of specialized centers, whereas linear accelerators with stereotactic capability are much more wide-spread, an evaluation of motor cortex and hippocampus sparing achievable with linac stereotactic radiotherapy will be useful for a wider range of patients and treatment centers
Summary
Brain metastases can effectively be treated with surgical resection and/or stereotactic radiotherapy (SRT). Lesions located within or adjacent to critical motor areas pose a challenge to both the neuro- and radiosurgeon. Preoperative neurosurgical planning and surgical resection itself predominantly aim to identify and preserve critical motor areas. In the past decade, navigated transcranial magnetic stimulation (nTMS) has been used to non-invasively map the motor cortex prior to surgery of motor eloquent brain lesions. These preoperative motor maps appear to facilitate better resection rates while maintaining neurological function [1,2,3,4,5]
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