Natural History and Histopathology of Expanding Cysts and Hematomas after Stereotactic Radiosurgery for Arteriovenous Malformations of the Brain: A Case Series.

OBJECTIVE: To review the clinical course and histopathologic findings in cases involving the formation of expanding cysts and/or hematomas after gamma knife surgery (GKS) for arteriovenous malformations (AVM).

METHOD: This paper presents a single-center retrospective review of 18 patients who presented cyst and/or hematoma expansion after GKS for AVM between 1993-2023. Expanding cysts and hematomas were respectively defined as well-demarcated cavities filled with fluid or well-marginated heterogenous hematomas presenting expansion proximal to or in the location of the original AVM. Patient demographics, AVM characteristics, history of interventions and surgeries, as well as imaging and histopathologic features of expanding cysts/hematomas were collected for analysis.

RESULTS: Among 1,072 AVM patients treated using GKS, 18 presented expanding cysts/hematomas over a total follow-up period of 16,757 patient-years (0.11 cases per 100 per patient-year). Time to cyst/hematoma identification was 4-13 years after the initial GKS with a mean duration of 8.6 years. Among the cases examined, 7 (38.9%) presented mainly hematoma, 10 (55.6%) presented mainly cysts, and 1 presented roughly equal components of both. Among the 18 patients, 13 (72.2%) underwent craniotomy to treat cyst/hematoma expansion. All specimens presented similar histopathologic characteristics, including organizing hematoma with fresh and old hemorrhage, fibrinoid necrosis of the vessels, gliosis of normal brain tissue, infiltration of hemosiderin-laden histiocytes, and extravascular protein leakage.

CONCLUSION: Our findings suggest that the formation of the two complications may be attributed to a common mechanism involving radiation-induced vascular damage in brain tissue adjacent to the AVM and subsequent chronic inflammation and capillary dilatation.