Invasion Patterns of External Auditory Canal Squamous Cell Carcinoma: A Histopathology Study

Objectives/Hypothesis: To describe the histopathology of the invasion patterns of advanced-stage external auditory canal (EAC) squamous cell carcinoma (SCC). Study Design. Retrospective cohort study. Methods: Retrospective analysis of medical records of patients diagnosed with EAC SCC available at the Massachusetts Eye and Ear temporal bone (TB) collection. TBs underwent processing for histologic examination. Hematoxylin and eosin–stained slides were examined. Histologic findings were compared to premortem clinical data. Results: Nine TBs were identified. Male:female ratio was 6:3. The average age of diagnosis and duration of survival was 64 (46–80 years) and 2.3 years (1–50 months), respectively. All presented with T4 disease, most commonly due to petrous apex (PA) invasion and facial nerve (FN) weakness. The mastoid air cells system served as a tumor conduit to the tegmen mastoideum and overlying dura in four patients, posterior fossa dura in one patient, vertical segment of FN in four patients, and middle ear (ME) and lateral semicircular canal in five patients. The tumor did not penetrate the tympanic membrane, oval window membrane (fenestra vestibule), or round window (RW) membrane. Supra- and infralabyrinthine pneumatization patterns allowed direct routes to the PA. Translabyrinthine PA invasion was seen in two patients. The most common locus of otic capsule invasion was the cochlea. One patient had FN paralysis due to compression rather than invasion. Conclusions: SCC does not tend to extend from the ME to the inner ear through the RW and vestibule-stapedial ligament. Tumors tend to spread along the preexisting TB air-tract routes. Well-aerated TB, may facilitate extension to the PA. Level of Evidence: 4 Laryngoscope, 131:E590–E597, 2021.