Clinicopathological features of low-grade malignant endolymphatic sac tumors

Low-grade malignant endolymphatic sac tumor (ELST) is a rare neoplasm, occurring in the inner ear and invading the temporal bone. This study aims to investigate the clinicopathological features of low-grade malignant ELSTs.

The clinicopathological data of 21 patients with low-grade malignant ELSTs were collected and analyzed.

The patients were aged 16-71 years, with an average age of 40.3 years and a median age of 39 years, and the male to female ratio was 1:1.6. There were 13 cases (61.9%) of ELSTs occurring on the left side, 7 cases (33.3%) on the right side, and 1 case (4.8%) on both sides. Blood types O and B were noted in 71.4% of the patients. Immunohistochemistry showed that CK, EMA and Vim were all positive, and S-100 (71.4%, 10/14), CD56 (75.0%, 9/12), NSE (50.0%, 2/4), and GFAP (11.1%, 1/9) were also positive, while Syn, CgA, TTF-1, TG, CD34, and calcitonin were negative. The Ki-67 index was 4.3% on average. Histologically, cells were arranged in a papillary shape often with branches and abundant fibrous axial vessel. Some cells had an expanded different-sized thyroid-follicle-like structure, with the follicles containing red-stained colloids and scallop-like secretary vacuoles. There were expanded cavities. Some cases were in a glandular arrangement, and a few in a nest-like, gland-cystoid arrangement. Most tumors were coated with a monolayer of cubic epithelium, a few cells were flat or columnar, with translucent cytoplasm and light staining. The nuclei were oval, nucleolus was not obvious, chromatin was delicate, and a few nucleoli were small. The tissue was prone to bleeding, with fresh and old bleeding. Approximately half of the patients had necrotic bones, and in some cases the tumor tissue had destroyed the surrounding bone. The background fibrous tissue showed hyperplasia with hyaline degeneration, some had calcification and formation of sandy-gravel bodies. The clinical manifestations were hearing reduction or loss, followed by tinnitus, and accompanied by varying degrees of cranial nerve injury. No patients died during follow-up.

Low-grade malignant ELSTs occur most frequently on the left side, with a female preponderance. The disease progressed slowly, with no death, and but relapse in two patients in this series. These tumors are often misdiagnosed.