In adult patients, pulmonary sequestrations are a rare congenital malformation of the lung. However, they are the second most common congenital lung anomaly in pediatric respiratory disease because they are typically diagnosed during childhood.
A pulmonary sequestration is a lung anomaly lacking normal tracheobronchial connections and supplied systemic arteries. Pulmonary sequestrations are divided into two types, intralobar sequestration (ILS) and ELS, based on whether the visceral pleura does or does not separate the normal lobe, respectively. An ELS is rare; they usually occur between the lower lobe of the lung and diaphragm and manifest with multiple anomalies. CCAM is also a rare developmental lung anomaly in adolescence and adulthood, but it is the most common congenital lung lesion in childhood examined by routine ultrasound. Stocker divided CCAM that are hamartomatous lesions comprising cystic and adenomatous elements into three major types based upon the size of the cysts and their cellular characteristics . The present case involved type II CCAM comprising multiple cysts approximately 0.5 cm in diameter and solid areas that blended into adjacent normal tissue.
There are two major theories surrounding the source of lung anomalies: (1) they originate in the primitive foregut or lung bud and (2) they arise from the six pairs of aortic arches or venous radicals and their derivatives. Richard et al.  reported that proportion of ELS with frequently associated congenital cystic adenomatoid malformation type II was approximately 50%. Recently, the pathogenesis of CCAM, LLS, ELS, and lobar emphysema has been thought to have common origins in different stages of lung development and at different sites and times, under the influence of airway obstruction [3, 4]. We considered co-existence of these two diseases in the present case seems not to be coincidental. Congenital cystic adenomatoid malformation was considered a premalignant lesion, and right upper lobectomy was preferable. Therefore, careful long-term follow-up was required.
Among 133 cases of ELS reported by Savic et al. , only two were located in the right mediastinum. An ELS of the right superior portion of the mediastinum may be rarer. The arterial supply is normally provided by branches of the aorta, and the venous drainage is normally routed through the systemic veins, usually the inferior vena cava or azygos-hemiazygos systems. However, in this patient, the venous return was unique. The drainage vein returned to the right main pulmonary artery behind the dorsal pericardium.
Savic et al.  reported that only six of 66 patients who underwent surgery for ELS had received a correct diagnosis. A unique location can make a preoperative diagnosis even more difficult. However, carefully interpreting contrast-enhanced chest CT and three-dimensional CT images can ensure a correct diagnosis. An accurate diagnosis results in a safe and easy operation. Physicians should take great care in diagnosing a mediastinal tumor, regardless of whether it is in an atypical location [6–8].
Savic et al.  reported five patients with an ILS who died because of uncontrolled bleeding of the aberrant artery. In patients with ELS, massive intraoperative bleeding due to injured aberrant vessels has been reported . It is therefore very important to identify aberrant vessels to prevent an uncontrolled hemorrhage. It is very useful to examine contrast-enhanced chest CT images and to create a reconstructed three-dimensional CT image for safe surgery.
Pulmonary sequestration with CCAM has recently been reported more frequently in pediatric surgery [2–4]. Due to the presence of a mediastinal tumor with a cystic mass in the lung, a sequestration was also suspected in this patient.