1. Srikala Narayanan MD, Andre Furtado MD Division of Pediatric Neuroradiology Children’s Hospital of Pittsburgh of UPMC

2. None

3. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Radiologic evaluation of atypical extension of the thymus and ectopic thymus in the neck and posterior mediastinum. Radiologic evaluation of atypical extension of the thymus and ectopic thymus in the neck and posterior mediastinum. To help differentiate these from other pathologic masses on imaging. To help differentiate these from other pathologic masses on imaging. Learning Objectives

4. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionMaterials & Methods Retrospective review of atypical extension of thymus and pathology proven cases of ectopic thymus with review of the literature. Retrospective review of atypical extension of thymus and pathology proven cases of ectopic thymus with review of the literature. Presentation of most illustrative cases with discussion of the imaging findings and pathogenesis. Presentation of most illustrative cases with discussion of the imaging findings and pathogenesis.

5. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Case 1 17 month old male presenting with constipation, vomiting and a shallow sacral dimple. MRI brain and spine was obtained. Incidentally noted was a posterior mediastinal mass contiguous with the thymus. Illustrative cases

6. MRI of the cervicothoracic spine: Axial T1WI (A), Axial T2WI (B), Sagittal T1WI (C), Coronal T2WI (D), Sagittal T2WI (E) and post contrast Sagittal T1WI (F) images demonstrate a left posterior mediastinal mass in contiguity with the thymus with similar signal characteristics on pre-and pot contrast imaging. These findings and the relative absence of mass effect on the adjacent structures are consistent with an atypical posterior mediastinal extension of the thymus. B E A D F C

7. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Posterior Mediastinal thymus Illustrative cases Atypical extension of the thymus is a benign entity and should not be confused with pathologic mass. Atypical extension of the thymus is a benign entity and should not be confused with pathologic mass. Key diagnostic features: Posterior mediastinal mass with similar signal characteristics of the thymus, similar pattern of enhancement, contiguous with the thymus, lack of mass effect and absence of neural foramina or intra-spinal invasion. Key diagnostic features: Posterior mediastinal mass with similar signal characteristics of the thymus, similar pattern of enhancement, contiguous with the thymus, lack of mass effect and absence of neural foramina or intra-spinal invasion. Differential diagnosis: Atypical posterior mediastinal extension of the thymus can mimic posterior mediastinal tumors such as neurogenic tumors and lymphoma. Given the above key features it is easily differentiated from other masses. Differential diagnosis: Atypical posterior mediastinal extension of the thymus can mimic posterior mediastinal tumors such as neurogenic tumors and lymphoma. Given the above key features it is easily differentiated from other masses.

8. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Case 2a Illustrative cases 17 month old baby boy with incidentally noted thyroid nodules in spine MRI Coronal T1 (A) and axial T1 (B) MRI images reveal T1WI hypointense nodules in both lobes of thyroid (arrows). B A

9. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionIllustrative cases Case 2a Ultrasound images of the thyroid (C,D) confirm the nodules (arrows) have similar echotexture to the thymus as shown on image (E), suggestive of ectopic intrathyroid thymic tissue. C E B

10. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Case 2b Illustrative cases Ultrasound images of the thyroid (A,B,C ) demonstrate a nodule within the right lobe which has a characteristic dot pattern of echoes. This nodule has similar echotexture to the thymus as shown on image (D) and FNA confirmed ectopic intrathyroid thymic tissue. A C D B 23 month old with incidentally noted thyroid nodules on neck ultrasound

11. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Ectopic intra-thyroid thymus Illustrative cases These nodules were proven to be ectopic thymic tissue on needle biopsy/FNA. These nodules were proven to be ectopic thymic tissue on needle biopsy/FNA. Intra-thyroid nodules with similar echogenicity as the thymus demonstrating linear or dot like echoes. Intra-thyroid nodules with similar echogenicity as the thymus demonstrating linear or dot like echoes. Differential diagnosis: The differential diagnosis for thyroid nodules in children typically includes nodular goiter, lymphocytic thyroiditis, colloid cysts, follicular adenomas, and malignant thyroid nodules. Differential diagnosis: The differential diagnosis for thyroid nodules in children typically includes nodular goiter, lymphocytic thyroiditis, colloid cysts, follicular adenomas, and malignant thyroid nodules. Ectopic intra-thyroid thymus can mimic colloid nodules or even papillary carcinoma. Ectopic intra-thyroid thymus can mimic colloid nodules or even papillary carcinoma. US evaluation can be helpful in identifying the echogenicity and echotexture of thymic tissue. US evaluation can be helpful in identifying the echogenicity and echotexture of thymic tissue.

12. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Case 3 Illustrative cases 3 month old baby boy with neck mass Ultrasound images of the neck demonstrate a well defined mass lesion (arrows) with echo texture similar to the thymus (not shown).

13. A B C D E F MRI of the neck: Coronal T2WI (A,B), Coronal T1WI (C), Axial T2WI (B) and post contrast axial and Sagittal T1WI (F) images demonstrate a left neck mass (arrow) with retropharyngeal extension with signal characteristics similar to the thymus on pre-and post contrast imaging (short arrow).

14. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Cervical thymus Illustrative cases Well circumscribed mass in this 3 month old was surgically resected. Pathology: cervical thymus Well circumscribed mass in this 3 month old was surgically resected. Pathology: cervical thymus Imaging findings: Well circumscribed masse that mimic the signal intensity of normal thymic tissue on US, CT and MRI and that do not invade or displace adjacent structures. A large cervical thymus may exert mass effect on the adjacent structures and cause airway compression. Imaging findings: Well circumscribed masse that mimic the signal intensity of normal thymic tissue on US, CT and MRI and that do not invade or displace adjacent structures. A large cervical thymus may exert mass effect on the adjacent structures and cause airway compression. The differential diagnosis of a solid neck mass in infant: Adenopathy, fibromatosis colli, malignancy (most commonly neuroblastoma or rhabdomyosarcoma), teratoma, hemangioma, lipoma, thyroid masses, and ectopic thymus. Although lymphoma is extremely rare in the newborn and infant, it must be considered in an older child.

15. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Case 4 Illustrative cases Contrast enhanced CT of the neck: Axial (A), Coronal (B) and Sagittal (C,D) images demonstrate a cystic left neck mass (arrow) with few septations along the sternocleidomastoid muscle. A B C D

16. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Cervical thymic cyst Illustrative cases Cystic neck mass was surgically resected. Pathology revealed thymic tissue confirming a cervical thymic cyst. Cystic neck mass was surgically resected. Pathology revealed thymic tissue confirming a cervical thymic cyst. Imaging findings: Imaging findings: Cervical thymic cysts are typically large uniloculated or sometimes multiloculated cystic lesions located in the anterior aspect of the cervical thoracic junction or more cranially deep to the sternocleidomastoid muscle as in this case. Cervical thymic cysts are typically large uniloculated or sometimes multiloculated cystic lesions located in the anterior aspect of the cervical thoracic junction or more cranially deep to the sternocleidomastoid muscle as in this case.

17. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionIllustrative cases Differential diagnoiss: Thymic cyst is an uncommon differential diagnosis for a lateral cystic neck mass. The presence of thymic tissue within the lesion is required for pathologic diagnosis. More common cystic neck masses: -Lymphatic malformation (also called cystic hygroma or lymphangioma) -Teratomas, which may also contain fibrofatty or calcific elements. -Branchial cleft cysts -Thyroglossal duct cysts

18. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionIllustrative cases Axial (A), Coronal (B) and Sagittal (C) T1WI show cervical extension of thymus which is contiguous with the thymus in the superior mediastinum. Case 5a ABC

19. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionIllustrative cases Axial (A) and sagittal (B) contrast-enhanced CT demonstrating cervical extension of thymus which is contiguous with the thymus (arrow). Case 5b A B

20. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionIllustrative cases Case 5c Coronal (A,B) and Axial (C,D,E) T2WI in a patient with NF-1 show cervical extension of the thymus. This should be differentiated from neurofibroma. A B C D E

21. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Cervical Extension of Mediastinal Thymus. Illustrative cases Cervical extension of mediastinal thymus appear as anterior soft tissue structure at the thoracic inlet in continuity with the thymus due to incomplete mediastinal descent. It can mimic a lymph node or soft tissue mass at the thoracic inlet and should be carefully differentiated from lymph nodes or pathologic masses by its continuity with the thymus in the mediastinum and same attenuation/signal characteristics.

22. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionDiscussion EMBRYOLOGY Arrest of thymic tissue during its caudal migration may result in ectopic or accessory thymic tissue. -6th gestational week, thymic primordia arise from the third and fourth pharyngeal pouches. -7 th gestational week, the bud-like thymic primordia elongate forming thymopharyngeal ducts. They migrate caudally and medially to their final destination in the anterior mediastinum. -8th gestational week, the thymic primordia fuse at their lower poles. -10 th gestational week small lymphoid cells migrate from fetal liver tissue and bone marrow into the primordia, causing thymic lobulation. -14 th to 16 th gestational weeks, the thymus differentiates into cortical and medullary components.

23. Learning Objectives Materials & MethodsIllustrative casesDiscussion ConclusionDiscussion PATHOGENESIS -Unilateral failure of the thymic primordium to descend, which results in ectopic thymic tissue on one side of the neck with ipsilateral absence of a normal thymic lobe within the superior mediastinum -Ectopic cervical thymus may arise from a small rest of tissue left behind as the tail of the gland involutes within the thymopharyngeal duct, neck mass with normally positioned bilobed thymus -Masses that arise within the thymopharyngeal tract may be cystic or solid. Thymic cyst is thought to develop from persistent thymopharyngeal tracts and the degeneration of Hassall’s corpuscles within ectopic thymic remnants.

24. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion CATEGORIES Accessory Cervical Thymus. Solid cervical thymic tissue is sequestered from the main gland, along the normal descent path, with or without parathyroid. Previous terms include aberrant, ectopic, undescended, persistent, or accessory thymus. Cervical Thymic Cyst. Sequestered cystic cervical thymus is found along a normal path of descent, with or without parathyroid glands. It is a cystic version of accessory cervical thymus and may have fibrous band or a solid thymic cord connection to the pharynx or mediastinum. Undescended Cervical Thymus. This occurs when a solid lobe of thymus fails to descend entirely, with or without a parathyroid complex. It differs from accessory cervical thymus in that only half of the normally blobbed thymus is present in the mediastinum; conceivably, it may also become cystic. Persistent Thymopharyngeal Duct Cyst. This is the same as undescended cervical thymus; however, the thymic duct is cystic. The thymus is solid, with or without parathyroid complex, and probably represents undescended thymus. A variant would be the cervical cystic duct leading to the mediastinal thymus.

25. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Persistent Thymic Cord. This is the cervical prolongation of a solid thymic cord which is continuous with the mediastinal thymus. The cystic variant may overlap with the histology and clinical appearance of the cervical thymic cyst if a true connection to mediastinal thymus cannot be documented. Cervical Extension of Mediastinal Thymus. This appears as upward extension of the thymus through the thoracic inlet due to incomplete mediastinal descent. It may transiently present with increased intrathoracic pressure. Ectopic Thymus. This is the rare, solid thymic tissue in abnormal locations, for example, in the pharynx, trachea, or base of skull.

26. Learning Objectives Materials & MethodsIllustrative casesDiscussion Conclusion Ectopic thymus and/or its atypical extension can mimic pathologic mass or lymphadenopathy. Ectopic thymus and/or its atypical extension can mimic pathologic mass or lymphadenopathy. Key imaging findings which reflect well defined mass with contiguity or similar characteristics to thymus without invasion can prompt the correct diagnosis. Cystic changes and ectopic locations such as thyroid, trachea or skull may be rarely seen. Key imaging findings which reflect well defined mass with contiguity or similar characteristics to thymus without invasion can prompt the correct diagnosis. Cystic changes and ectopic locations such as thyroid, trachea or skull may be rarely seen. Familiarity with this benign entity is important to ensure proper management and prevent unnecessary invasive procedures. Familiarity with this benign entity is important to ensure proper management and prevent unnecessary invasive procedures.

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