Anaplastic meningioma (also known as malignant meningiomas) is defined by several criteria including:
1) Invasion of adjacent brain parenchyma or skull. (see invasive meningioma)
2) Numerous mitosis (> 5/high-powered field)
3) Elevated proliferative index (>3%) as assessed by either 5-bromodeoxyuridine or KI-67 staining
5) Increased cellularity
6) Nuclear pleomorphism
Anaplastic meningiomas are uncommon, accounting for only ~1% of all meningiomas 1).
Generally, it is not possible to confidently distinguish benign (WHO grade I) and atypical (WHO grade II) from anaplastic (WHO grade III) meningiomas on general morphology. The most reliable feature in suggesting a non-grade I tumour is the presence of lower ADC values (reflecting higher cellularity) 2) 3).
Importantly presence of vasogenic oedema in adjacent brain parenchyma is not a predictor of atypical or anaplastic histology 4).
Brain invasion, although by definition denoting at least a grade II tumour, is also surprisingly difficult to predict on MRI.
There are, some CT or MRI trends that point in favor of malignant meningioma:
1) the absence of visible calcium aggregates 5).
Malignant progression with the accumulation of mutations in a benign meningioma can result in an atypical meningioma and/or anaplastic meningioma. Both tumors are difficult to manage and have a high recurrence and poor survival rates. The extent of tumor resection and histological grade are the key determinants for recurrence.
Anaplastic meningiomas are aggressive tumors, with a median overall survival time of 15 months 10).
They have a higher rate of recurrence and metastases accompanied by a significantly shorter survival rate compared to benign variants. Meningioma cancer stem cells (CSCs) have been previously shown to be associated with resistance and aggressiveness. However, the role they play in meningioma progression is still being investigated 11).
Baeesa et al. from the Division of Neurosurgery, Department of Surgery, King Abdulaziz University Hospital, Faculty of Medicine, Jeddah, Saudi Arabia, report a 29-year-old man who underwent a resection of a grade I meningioma in 2011. The patient had multiple local recurrences of the tumor that exhibited an aggressive change in behavior and transformation to grade III meningioma and developed extracranial metastases to the cervical spine. He underwent multiple operations and received radiotherapy. Analysis of the meningioma indicated the presence of CSCs markers before metastases and showed elevated expressions of associated markers in the metastasized tissue. Also, and similar to the patient’s profile, the pharmacological testing of a primary cell line retrieved from the metastasized tissues showed a high level of drug tolerance and a loss of ability to initiate apoptosis.
Malignant progression of grade I meningioma can occur, and its eventuality may be anticipated by detecting CSCs. We included a comprehensive literature review of relevant cases and discussed the clinical, diagnostic and management characteristics of the reported cases 12).
A patient had an intracranial malignant meningioma and developed a symptomatic osteolytic contrast-enhancing lesion in the left C-1 lateral mass suspicious for metastases. The authors performed a minimally invasive posterior resection of the lesion with vertebroplasty of C-1. Histopathology verified metastases of the malignant meningioma. The surgical procedure resulted in prompt and permanent pain reduction until the patient died 18 months later. Given the very limited life expectancy in this case, the authors did not consider occipitocervical fusion because of their desire to preserve the range of motion of the head. Therefore, they suggest minimally invasive tumor resection and vertebroplasty in selected palliative tumor patients 13).