Rationale: Ectopic hamartomatous thymoma is a very rare soft tissue neoplasm that commonly occurs in the lower neck of adult men.
Patient concerns: A 32-year-old male presented 1-year history of the tumor lying in left supraclavicular fossa.
Diagnoses: Initial consideration of ultrasound and computed tomography was lipoma. After the operation, the pathologist diagnosed it as Ectopic Hamartomatous Thymoma.
Interventions: A complete resection was performed.
Outcomes: To date, the patient had no evidence of metastasis or recurrence for 26 months after the operation.
Lessons: Ectopic hamartomatous thymoma is rare. The mastery of the clinical and pathological features of the disease will contribute to the rapid diagnosis and treatment of the disease. In addition, it can be considered to update the name to "biphenotypic branchioma" in order to avoid conceptual confusion.