Retiform endothelial hyperplasia mimicking cavernous malformation as a late complication of Gamma Knife radiosurgery.

Objective: Gamma Knife radiosurgery (GKRS) is a powerful tool for the management of arteriovenous malformations; however, newly formed mass lesions resembling cavernous malformations are a rare late complication of GKRS. In this retrospective study, the authors tried to clarify the unique histological features of these mass lesions.

Methods: The authors retrospectively reviewed the clinical course of 889 patients who had undergone GKRS for arteriovenous malformations at their institute from 1991 to 2021. Among the 848 patients who had been followed up periodically with neuroradiological imaging, 37 developed a mass lesion mimicking a cavernous malformation and underwent surgical removal of the lesion. The median volume of the original nidus was 3.7 cm3 (range 0.07-30.5 cm3), and the median prescription dose was 21 Gy (range 12-25 Gy). The histological characteristics and radiological and clinical features of the 37 patients were investigated.

Results: Histological examination showed an organized hematoma and a structure termed "retiform endothelial hyperplasia" (RFEH) consisting of endothelium forming multiple lumen-like vascular channels mimicking cavernous malformations but lacking the subendothelial connective tissue that forms the typical vascular wall structure found in cavernous angioma and capillary telangiectasia. RFEH was detected a median of 10.8 years (range 3.2-27.4 years) after GKRS. Neuroimaging showed hematoma surrounded by massive brain edema in all 37 patients. Symptoms caused by mass effect of the lesion and perifocal edema worsened relatively rapidly but completely disappeared after surgery. No recurrence or morbidity occurred after the surgery.

Conclusions: The delayed formation of RFEH that is mimicking a cavernous malformation neuroradiologically but is histologically distinct from a vascular malformation is a potential complication of GKRS. Its progressive clinical course suggests that surgical removal should be considered for symptomatic patients and/or patients with an apparent radiological mass sign.