Esophageal duplication cysts are rare benign inherited lesions that are usually identified during childhood. Most occur in the posteroinferior esophagus, and adult symptoms are typically secondary to mass effect on nearby structures.1 Differential diagnosis includes bronchogenic cyst, neurenteric cyst, meningocele, pericardial cyst, cystic teratoma or thymoma, and lymphangioma. Clinical history, anatomic details and imaging appearance are typically sufficient for an accurate diagnosis, with surgical removal for treatment if required.2

“Ectopic parathyroid glands arising due to abnormal embryological migration are found in 1–3% of the population and these combined together account for 20–25% of the cases of parathyroid adenoma and clinically present as PHP. Majority of them are located in the anterior mediastinum near the thymus, but, rarely, some are present in the visceral compartment of the mediastinum or paraesophageal position or in the aortopulmonary window or close to the right pulmonary artery near the tracheal bifurcation which requires open thoracotomy or thoracoscopic removal.”
A challenging case of an ectopic parathyroid adenoma
Roopal Panchani et al.
Indian J Endocrinol Metab. 2012 Dec; 16(Suppl 2): S408S410.

“ Solitary parathyroid adenoma is a common cause of primary hyperparathyroidism, and is usually, located in a juxtathyroid position in the neck. Primary hyperparathyroidism often presents in an unusual manner, with 0.3-8% of the cases occurring due to an ectopically located parathyroid gland adenoma. An inability to locate the adenoma may delay the diagnosis of these cases and lead to further complications of hyperparathyroidism.”
“Ectopic Parathyroid Adenoma Presenting as a Mediastinal Mass.” 
Vaidya, Aneesha et al. 
Journal of clinical and diagnostic research : JCDR vol. 11,5 (2017): ED40-ED42.

”The parathyroid glands are arranged in two pairs, upper and lower. The lower pair originates from the dorsal wing of the third pharyngeal pouch, along with the thymus which originates from the ventral wing. Hence, the common descending route of the inferior parathyroid with the thymus, can explain their aberrant situation in the mediastinum. Ectopic parathyroid glands have been found to occur in various anatomic locations anywhere from the angle of the mandible to the mediastinum. The most common sites are the mediastinum, along the path of the vagus nerve and recurrent laryngeal nerve and within the thyroid parenchyma.”
“Ectopic Parathyroid Adenoma Presenting as a Mediastinal Mass.” 
Vaidya, Aneesha et al. 
Journal of clinical and diagnostic research : JCDR vol. 11,5 (2017): ED40-ED42.

”Hypercalcaemia with raised parathormone levels, or isolated hypercalcaemia, are diagnostic of primary hyperparathyroidism . Moran CA et al., described cases where patients were being reviewed for primary parathyroid tumour in the mediastinum, showed metabolic disturbances such as hypercalcaemia and hypophosphatemia. Thus, these derangements help to serve as a clue to suspect a parathyroid mass.Preoperative localization of the adenoma is critical, for which a Tc-99m-Methoxy Isobutyl Isonitrile (MIBI) scan has shown sensitivity of 80%-90%. The modality helps to reduce the requirement of repeated neck explorations, as was reported in a study by Abbas S et al., and proved useful in detecting the ectopic nature of the lesion in our case.”
“Ectopic Parathyroid Adenoma Presenting as a Mediastinal Mass.” 
Vaidya, Aneesha et al. 
Journal of clinical and diagnostic research : JCDR vol. 11,5 (2017): ED40-ED42.

Tumours of the mediastinum represent a wide diversity of disease state. The location and composition of a mass is critical to narrowing the differential diagnosis. The clinical spectrum of mediastinal masses can range from being asymptomatic to producing compressive symptoms. Although many of these masses have similar imaging appearances, clinical history, anatomical position and certain details seen at CT and MRI imaging allow correct diagnosis in many cases.
A diagnostic approach to the mediastinal masses.
Juanpere, Sergi et al.
Insights into imaging vol. 4,1 (2013): 29-52. doi:10.1007/s13244-012-0201-0