Conceptual Volumetric Comparison of Preoperative versus Postoperative Stereotactic Radiosurgery for Brain Metastases

Introduction: Stereotactic radiosurgery (SRS) for operative brain metastases (BrM) is usually administered following resection. The timing of SRS delivery relating to surgery is in question because a potential failure mechanism is the development of leptomeningeal disease (LMD), occurring in 6.1% of patients receiving postoperative SRS. While we await the results of two active clinical trials, we aim to bridge the gap by conceptually investigating pre- and postoperative SRS in the planning stage.

Methods: We queried our institutional database for patients who underwent postoperative SRS for BrM between January 2014 and January 2021. Exclusion criteria included prior radiation delivered to the same region, age < 18 and diagnosis of LMD. Once identified, an SRS plan based on preoperative imaging was designed.

Results: We identified 37 patients with a median age of 59 years. Nineteen patients were male and the most common histologies were colorectal cancer and NSCLC. Twenty patients had solitary BrM and treatment volumes ranged from 2cm3 – 84.15cm3 (median 15.5cm3) for postoperative cavities and 3.3 cm3 – 60.2 cm3 (median 9.78cm3) in preoperative plans. In a paired comparison, seventeen patients had increased postoperative treatment volume, fourteen had decreased treatment volume, and the remainder were stable. When comparing the volume of surrounding brain receiving 12Gy or more, 65% of patients had higher volumes in postoperative plans (N=24, p< 0.0001) while the remainder had a decrease (N=13, p=0.0002).

Conclusion : Our results suggest that while treatment volume is variable between preoperative and postoperative plans, we observed a higher volume of brain receiving 12Gy in postoperative SRS, a predictor of radiation necrosis. This is likely secondary to no margin beyond tumor requiring coverage, increased conformality to tumor vs. irregular and heterogenous postoperative cavity and treatment of surgical tract. While conceptual, this study has the strength of paired plans and demonstrates the feasibility and potential advantages preoperative SRS.