Retroperitoneal Bronchogenic Cysts: A Case Series

Introduction

Bronchogenic cysts (BCs) are congenital malformations of the ventral foregut, caused by abnormal sprouting of the original tracheobronchial tree during early embryonic development [1,2]. Most BCs are above the diaphragm, typically in the mediastinum or lungs, but can also occur in atypical foregut locations [3,4]. Retroperitoneal BCs are exceptionally rare, with most located in the subdiaphragmatic space, especially the left adrenal region [5].

Most RBCs are asymptomatic and detected incidentally on imaging. Only a minority of patients present with nonspecific symptoms such as pain. As the treatment of retroperitoneal masses varies depending on the nature of the lesion, it is crucial to have a definite diagnosis before surgery. However, to date, only over 100 cases of RBCs have been reported [6,7]. Due to nonspecific imaging features and limited clinical awareness, preoperative diagnosis remains challenging.

Here, we report 6 cases of surgically resected retroperitoneal bronchogenic cysts, all confirmed histologically. This study aims to delineate the key imaging features of RBCs to enhance preoperative diagnosis, and we share our experience with RBC treatment.

Case Report

We retrospectively identified patients with RBCs admitted to the First Hospital of China Medical University through the electronic medical system. No identifying information was used for these cases. Data on demographics, presentations, imaging features, preoperative diagnoses, and surgical information were collected and presented in Table 1.

Six patients (median age: 36.5 years; 4 males) underwent RBC resection at our institution. All RBCs were left-sided, with 1 symptomatic case (abdominal pain). Preoperative evaluations included contrast-enhanced CT (CECT) in all patients, ultrasound in 4 patients, and MRI in 1. On non-contrast CT (NCCT), the contents of the cysts showed relatively high attenuation (9–56 HU), while on CECT, the contents showed no or mild enhancement (Figure 1A, 1B). Calcifications were observed in 3 cases. On MRI, the RBC demonstrated a low to medium-intensity signal on T1-weighted images and a high signal on T2-weighted images (Figure 1C, 1D). Although the border of the cyst was irregular, it was well-demarcated from the left diaphragm on T2-weighted images. On ultrasound, 1 RBC was anechoic with septa, while 3 RBCs were hypoechoic. In Case 2, the RBC showed multiple hyperechoic spots and septa. No positive relation was observed between age and the width of RBCs (Figure 2).

Only 1 RBC was correctly identified preoperatively. This fusiform mass, with an ill-defined border, demonstrated uneven attenuation on NCCT and showed mild enhancement on CECT. The mass was adherent to the left diaphragm, and a small calcification was present on its wall. Others were mistaken as ganglioneuromas (n=2), adrenal mass (n=1), or indeterminate retroperitoneal lesions (n=2). Endocrine tests (serum cortisol, plasma free metanephrines, serum/plasma aldosterone concentration, and plasma renin) performed on 4 patients were within normal limits. Only Case 4 was tested for AFP, CEA, CA-125, and CA-19-9, and the results were normal. All patients underwent resection of the RBC via the retroperitoneal approach. During the surgery for Case 4, the cyst was found in the retroperitoneum and adhered to the diaphragmatic muscle, clearly demarcating from the left adrenal gland and upper pole of the left kidney (Figure 3A). Macroscopically, RBCs were often unilocular and filled with white-to-brown mucus (Figure 3B). Histologically, RBCs can contain pseudostratified ciliated columnar epithelium, hyaline cartilage, smooth muscle, and seromucous glands (Figure 3C). All patients underwent uneventful recovery after surgery. Follow-up CT scans performed at 6 months for 3 patients showed no evidence of recurrence. One patient reported being asymptomatic 25 months after surgery and another patient was asymptomatic 2 months after surgery. One patient was lost to follow-up.

Discussion

The pathogenesis of RBC is unknown. Sumiyoshi et al proposed the following theory: in an early embryonic stage, the thoracic and abdominal cavities are connected by the pericardio–peritoneal canal. When the canal is later separated by the fusion of the pleuroperitoneal membranes (which form the diaphragm), a portion of the tracheobronchial tree could be pinched off and migrate, resulting in an RBC [2].

RBCs were historically difficult to diagnose due to their rarity, but increased imaging use has improved recognition and characterization. RBCs have been reported to occur equally in men and women. Although most RBCs are asymptomatic, expanding cysts can become infected, perforated, or large enough to compress adjacent organs. The most common concern of patients with RBCs is pain or discomfort [7]. Other symptoms include nausea, vomiting, and infection. As most of these symptoms are nonspecific, it is challenging to make a definitive diagnosis based solely on these symptoms.

Most RBCs are detected incidentally, with CT and MRI being the primary diagnostic modalities. A typical RBC can have several distinct features on cross-sectional imaging. Most RBCs are located on the left in the subdiaphragmatic triangle defined by the left diaphragmatic crus, splenic vein, and the left adrenal gland (Figure 4A, 4B). A large proportion of RBCs adhere to the left diaphragm or the left diaphragmatic crus [8]. Occasionally, the wall of RBCs is part of the diaphragm, which could be a key feature to distinguish RBC from other retroperitoneal masses. The attenuation value of RBCs on NCCT can vary depending on the contents of the cysts. Hyperattenuation is likely due to hemorrhage or thick proteinaceous mucin. Although no enhancement is reported in most cases, mild enhancement is not uncommon, as demonstrated in our series.

On MRI, RBCs appear as well-defined, thin-walled cystic lesions exhibiting varying signal intensity on T1-weighted sequences depending on the content of the cyst fluid. Most RBCs are isointense to hyperintense compared with skeletal muscle. These cysts are typically hyperintense on T2-weighted imaging and display thin enhancing walls on post-contrast sequences. As opposed to a CT scan, T2-weighted MRI can show the cysts are well demarcated from surrounding tissues, including the diaphragm. Additionally, RBCs are often mistaken for solid masses due to their intermediate to high signal intensity on CT scans. However, T2-weighted MRI can help reveal the cystic nature of RBCs.

Although widely used, ultrasound is not the preferred imaging modality for RBCs due to the difficulty of accessing the retroperitoneal space, interference from gases in the gastrointestinal tract, limited resolution, and varying echogenicity [9].

18F-fluorodeoxyglucose (18F-FDG) PET/CT was not a routine imaging method for RBCs due to its high cost. Previous studies found that almost no FDG uptake was observed in RBCs [10,11]. Yoon et al reported that 18F-FDG PET/CT can assist in detecting RBC complications, such as infection or malignancy transformation [11].

Decades ago, when reports of RBCs were limited, it was difficult to diagnose RBCs radiologically. Even today, nonspecific imaging features and a physician’s unfamiliarity with this rare condition complicate diagnosis based solely on radiological findings. An RBC can be misdiagnosed as an adrenal tumor, a pancreatic cyst, or solid lesions, including metastatic tumors, lymphoma, lymphangioma, ganglioneuroma, lipoma, sarcoma, teratoma, and dermoid cyst [12]. In our series, only 1 mass was diagnosed as RBC preoperatively. One mass was mistaken for an adrenal tumor due to its proximity to the left adrenal gland, while 2 others were diagnosed as ganglioneuromas. Differentiating between RBCs and ganglioneuromas poses the greatest challenge due to their shared imaging features, including location, calcification, and mild enhancement. However, a left-sided predominance, adherence to the diaphragm, and an intact adrenal contour suggest the diagnosis of an RBC. The presence of multiple imaging features listed above should raise the suspicion of RBC.

Due to the rarity of RBCs, there are currently no established guidelines for managing them. Surgical resection was considered the gold standard for RBCs management since histopathologic examination of the resected specimen is the most reliable way to confirm the diagnosis of RBCs.

Other reasons in favor of surgical intervention are also presented below. Most authors believe RBCs will grow and can become symptomatic. Larger RBCs are more difficult to resect and can increase postoperative morbidities. Moreover, although generally benign, malignant transformation of BC has been reported [13,14].

With the emergence of minimally invasive surgeries, resection of bronchogenic cysts has transitioned from open surgery to laparoscopic surgery. In our series, all patients underwent minimally invasive surgeries via the retroperitoneal approach. Although resection through the transperitoneal approach was reported to be successful [7], due to the retroperitoneal location of RBCs, surgical resection through the retroperitoneal approach provides a more direct access. Most authors recommend complete resection of the cyst or ablation of the cyst wall to prevent recurrence and malignant change.

Although aspiration of BCs in the mediastinum has been examined as an adjunct to traditional surgical resection, it is associated with infection and recurrence [15]. Thus, it is still premature to recommend aspiration for RBC treatment.

Despite the advances in knowledge of RBCs, questions remain regarding their diagnosis and treatment. Firstly, the risk of malignant transformation has been advocated as an indication for surgical resection. However, to date, only 1 malignant transformation of RBCs has been reported (<1% of all reported RBCs) [14]. Another reason for surgery is that the growth of RBCs can complicate delayed surgical intervention. However, the natural history of RBC growth is unclear. No positive age–size correlation was observed in our series (Figure 2). This may be indirect evidence that the volume of RBCs does not increase with age. Therefore, when preoperative RBC diagnosis is possible, the optimal management (surgical intervention versus conservative treatment) for small asymptomatic RBCs remains to be determined. Lastly, the diagnostic and therapeutic value of aspiration for RBCs in selected patients requires further investigation. As RBCs are rare, a carefully designed multicenter study may eventually help resolve these clinical questions.

Conclusions

Consider RBCs in left retroperitoneal masses with diaphragmatic adherence, especially when multiple characteristic imaging features are present. Minimally invasive surgical resection remains the gold standard for treatment. Prognosis is typically excellent after complete surgical resection. Future multicenter studies with long-term follow-up are needed to determine the best therapeutic strategy for small asymptomatic RBCs.