ARE ACOUSTIC NEUROMAS ENCAPSULATED TUMORS

In articles and chapters on the subject of acoustic neuroma, it is alm ost invariably stated that they are well-encapsulated tumors. During s urgical procedures, blunt mechanical dissection defines a natural subs urface cleavage plane that leaves intact a several millimeter thick ri nd of tumor surface. Occasionally, as a concession to neural integrity , less than complete resection is elected, leaving behind this ''capsu lar'' remnant, To clarify the nature of the surface of acoustic neurom as and to test whether this long held description is indeed correct, a microscopic analysis of 10 surgical specimens was performed. A wedge was harvested from the free surface of the tumor in the mid cerebellop ontine angle that included a large, undisturbed section of the tumor s urface, Histologic analysis showed that for most of the tumor surface only an extremely thin (3 to 5 mu m) layer of connective tissue envelo ps the tumor. Neoplastic Schwann cells, which extend essentially to th e margin of the tumor, were found to be somewhat flattened and compres sed in the vicinity of the surface, Although acoustic neuromas are sur rounded by a continuous layer of connective tissue, it is so exception ally thin (on average less than the diameter of a red blood cell) that its edge cannot be visualized intraoperatively by a surgeon, Because the pathologic definition of a capsule is a thick, enveloping layer of connective tissue that is both micro-and macroscopically evident, if must be concluded that acoustic neuromas are nonencapsulated, at least in the conventional sense of the term, The surface peel observed intr aoperatively is surgically produced during tumor debulking by cleaving of the looser central component from the more compressed portion of n eoplastic cells that lies immediately beneath the free margin of the l esion.