Introduction: Ectopic thymus is a rare condition, usually presented as a benign condition in which the thymic tissue is located at an unusual site along its migration.Case: A 5-week-old baby boy was admitted through our clinic with neck swelling. The swelling was over the right side of the neck, noticed since the age of 1 week. Based on the history, there are no alarming symptoms like difficulty in breathing or feeding and no fever. Patient is full term with an uncomplicated neonatal history. Family history was negative for malignancy. On physical examination, growth parameters were normal, and vital signs were normal. Examination of the neck revealed a right neck swelling in the submandibular region. The lesion was 3x4 cm in size and mobile, not fixed to the surrounding tissue, not tender, not pulsatile, and had no skin changes.An Ultrasound of the neck was done as an outpatient prior to current admission, showing findings in the right submandibular region, an oval-shaped, well-defined subcutaneous mainly hypoechoic mass measuring 3.7 x 2.4 x 3.8 cm with internal clear vascularity on color doppler. It is abutting the right parotid and right submandibular gland. No soft tissue edema. No collections. The left parotid, left submandibular gland and thyroid are unremarkable, no significantly enlarged lymph nodes noted. Impression given as Subcutaneous right submandibular lesion with internal vascularity likely neoplastic for further imaging and tissue diagnosis.Further imaging was carried out, MRI Neck Soft Tissue and Brain was performed, illustrating a Large, mildly enhancing soft tissue mass lesion seen inferomedially to the right parotid gland, abutting and superiorly displacing the right submandibular gland, extending obliquely inferomedially to the right lower neck, abutting the right thyroid lobe. It is displacing the right internal jugular vein laterally and posteriorly and partially encasing it. It is also noted slightly displacing the right common carotid artery medially and posteriorly. The mass is also noted to be abutting and laterally displacing the right sternocleidomastoid muscle with no evidence of invasion. No interconnection with the neural foramina. It measures about 4.6 x 2.5 x 2.4 cm in CC X AP X TR.The patient was then referred to King Fahad Medical City (KFMC) as a case of query neuroblastoma for further investigation and assessment by the hematology oncology team. There, MRI images were uploaded and assessed by another radiologist. The impression of a right neck mass with aggressive features and diffusion restriction with an adjacent enlarged lymph node qualified for lymphadenopathy suggestive of a neoplastic process, most likely rhabdomyosarcoma. MRI of the neck in KFMC was performed, with an impression of a Right anterior neck large mass with restricted diffusion, most likely representing a neoplastic process like sarcoma, for clinical assessment with histopathological result.Conclusion: This case highlights the importance of considering ectopic cervical thymus as a differential diagnosis for neck masses in infants. Although typically benign, further investigations are necessary to rule out other serious conditions and determine the appropriate management approach.
