Mean Brain Dose Remains Uninfluenced by the Lesion Number for Gamma Knife Stereotactic Radiosurgery for 10+ Metastases

Objective

Gamma Knife (GK) stereotactic radiosurgery (SRS) is increasingly used as an initial treatment for patients with 10 or more brain metastases. However, the clinical and dosimetric consequences of this practice are not well established.

Methods

We performed a single-institution, retrospective analysis of 30 patients who received Gamma Knife SRS for 10 or more brain metastases in 1 session. We utilized MIM Software to contour the whole brain and accumulated the doses from all treated lesions to determine the mean dose delivered to the whole brain. Patient outcomes were determined from chart review.

Results

Our cohort had a median number of 13 treated lesions (range 10–26 lesions) for a total of 427 treated lesions. The mean dose to the whole brain was determined to be 1.8 ± 0.91 Gy (range 0.70–3.8 Gy). The mean dose to the whole brain did not correlate with the number of treated lesions (Pearson r = 0.23, P = 0.21), but was closely associated with tumor volume (Pearson r = 0.95, P < 0.0001). There were no significant correlations between overall survival and number of lesions or aggregate tumor volume. Fourteen patients (47%) underwent additional SRS sessions and 6 patients (20%) underwent whole-brain radiotherapy with a median of 6.6 months (range 3.0–50 months) after SRS. Two patients (6.6%) developed grade 2 radionecrosis following SRS beyond earlier whole-brain radiotherapy.

Conclusion

The mean dose to the whole brain in patients treated with Gamma Knife SRS for 10 or more brain metastases remained low with an acceptable rate of radionecrosis. This strategy allowed the majority of patients to avoid subsequent whole-brain radiotherapy.

Introduction

Brain metastases (BMs) represent an unfortunately common occurrence as improvements in screening techniques, diagnostic imaging, and systemic therapeutics have lengthened patient survival after primary cancer diagnosis.¹ Radiotherapy plays an important role in the treatment of BMs.² For patients with higher-burden intracranial disease, whole-brain radiotherapy (WBRT) has historically been the preferred therapeutic modality over stereotactic radiosurgery (SRS).³ However, SRS confers several advantages relative to WBRT, including significantly lower incidence and magnitude of neurocognitive decline,⁴ minimal hair loss, and completion in 1 to a few days as opposed to the 1–3 weeks needed for WBRT.

The decision to offer WBRT rather than SRS is sometimes driven by the belief that distant brain sites may be seeded with micrometastatic disease in patients with many BMs. Therefore, irradiating the entire brain may theoretically confer a survival advantage.⁵ However, the number of BMs has been shown not to be a clinically meaningful prognostic indicator of overall survival.⁶ Additionally, improvements in systemic therapy have prolonged overall survival after treatment of extensive intracranial disease.⁷ Hence, to avoid the toxicity associated with WBRT, more radiation oncologists are offering upfront SRS rather than WBRT for extensive BMs.⁸ Despite this trend, the clinical and dosimetric consequences of treating 10 or more BMs with SRS are not well known. This study endeavored to determine cumulative radiation doses to the brain in patients who underwent Gamma Knife (GK) SRS for 10 or more lesions in a single session and their associated clinical outcomes.