Extremely rare case of retroperitoneal "ancient" schwannoma with significant cystic change embedded in the psoas muscle.

Abstract

Schwannoma is a tumor derived from the Schwann cells in the peripheral nerve system. Schwannoma located in the retroperitoneal cavity is rare and unusual. Less than 50 cases of schwannoma in the psoas muscle have been published.1 In 1951, “ancient schwannoma” was termed by Ackerman and Taylor, a type of schwannoma with degenerative features acquired inside the tumor with increasing age, such as cystic necrosis or hemorrhage.2 The almost completely cystic change in our case often leads to the misdiagnosis of malignancy. This 64-year-old woman presented with a palpable abdominal mass in the left lower quadrant. On abdominal examination, nothing was significant except a firm, immobile, non-tender globular mass in the suprapubic area, roughly three centimeters left to the umbilicus. Abdominal computed tomography showed a heterogeneous mass, about the size of 5 × 5.5 × 6 cm, adjacent to the aortic bifurcation and ureter (Figure 1A,B). There was no sign of vascular encasement of the abdominal aorta, with less than 180 degrees of tumor contact of the vessel circumference. Initial examination after laparotomy revealed the tumor was relatively detached (Figure 1C). Cystic fluid drainage was performed to facilitate complete resection because the tumor was deeply seated. The tumor was removed en bloc from the psoas muscle with a 1 cm surgical margin, while a flexible soft tissue part was found inside the tumor. (Figure 1D). Grossly appearing, retroperitoneal schwannoma is solitary, well-circumscribed, firm, and encapsulated. Microscopically, retroperitoneal schwannoma is composed of two different kinds of Antoni A and Antoni B cells, presenting in an alternating pattern. Infiltrating tumor growth into the psoas muscle was noted in our case (Figure 1E,F,G). Immunohistochemically, intense staining of the S-100 protein is pivotal for the exact diagnosis of retroperitoneal schwannoma. (Figure 1H). Awadh AlQahtani et al. stated there was neither a gender predilection (23 females versus 20 males) nor the significant distribution of ages (ranged from 19 to 77) among the 46 published cases of intra-psoas schwannoma.1 Three cases were malignant (6.5%), while the other 43 cases were benign (93.5%).1 Size variation was remarkable since the largest reported size was 42 × 16 × 16 cm, comparing to the fairly small size of 5 × 5.5 × 6 cm in our case. The lesion may reach a significant size at the time of diagnosis due to the non-restrictive and expandable property of the retroperitoneal space. Ancient schwannoma is an uncommon variety of schwannoma. Variable theories are proposed to explain the degenerative features of ancient schwannoma. One hypothesis involves the frequently encountered hyalinized vessels within the Antoni B cells, which may contribute to ischemia, susceptible hemorrhage, and subsequent cystic formation.3 CT-guided or ultrasound-guided fine-needle aspiration remains debatable for the accurate diagnosis of schwannoma. No specific imaging modalities or findings can lead to a definitive preoperative diagnosis of schwannoma.4 Area of cell pleomorphism or degenerative change taken from fine-needle aspiration can sometimes be misinterpreted as malignancy, let alone the risks of unwanted contamination, spillage, infection, or tumor seeding.4 Complete surgical resection is the treatment of choice since malignancy cannot be excluded by preoperative or perioperative evaluation. However, considerable controversy arises over “What is the optimal distance between tumor and resection margin?” because tumor-free margins can inevitably sacrifice adjacent healthy tissue.5 In our case, the nearby aorta and ureter were uninjured by careful looping before the meticulous dissection of the psoas muscle and tumor. All authors declare no conflict of interest.