Hydatid cyst also known as cystic echinococcosis is endemic in the Middle East, Mediterranean countries, India, Africa, South America, New Zealand, and Australia. The annual incidence rate of cystic echinococcosis worldwide ranges from 1 to 200 cases per 100,000 inhabitants [1,2,3, 5]. The increase in the frequency of hydatid cysts up to 200 times in endemic regions makes it necessary to keep cystic echinococcosis in mind in the differential diagnosis of lung and liver pathologies in these regions.

Although some specific radiological findings have been described in the literature, it is not always easy to radiologically diagnose pulmonary cyst hydatid [6,7,8]. Particularly, the radiological findings of complicated hydatid cysts are extremely diverse. While uncomplicated hydatid cysts present smooth, hyperdense cyst walls and well-circumscribed fluid attenuation radiological findings are extremely heterogeneous in complicated ones [4, 8]. Therefore, hydatid cysts can be confused with benign parenchymal lesions (like infective lung lesions, bronchiectasis, and congenital lung malformations) or malignant diseases. This makes it necessary to consider hydatid cyst in the differential diagnosis of all cystic lung lesions.

Because of the elasticity of the lungs and intrapleural negative pressure, lung hydatid cysts tend to grow rapidly, and, in some patients, they can reach gigantic sizes. The risk of perforation of the hydatid cyst is directly proportional to its size. The radiological signs of the perforation in hydatid cysts have been named in different ways. The most frequently used of these are [9, 10]: inverse crescent sign, air bubble sign, ring enhancement sign, Cumbo sign, serpent sign, water lily sign and incarcerated membrane sign. These expressions are the names given to the radiological findings of different stages of hydatid cyst perforation. However, perforated hydatid cysts may not have any of the specific radiological signs mentioned above and may present with radiological findings that mimic very different lung pathologies.

Surgical treatment is curative in most cases of pulmonary hydatid cysts. Although different surgical techniques have been described in its treatment, cystotomy plus capitonnage is the most preferred method. The patient should be carefully evaluated in terms of possible differential diagnoses in the preoperative period in order to make the appropriate preoperative preparation and to choose the correct surgical technique [11,12,13].

Especially when hydatid cysts of the lung become complicated after perforation, they may present as thickening and irregularity in the cyst wall, or as parenchymal opacities.

In malignant lesions, the cavity wall is usually thicker and irregular [14, 15]. However, although rare, lung cancer may present with thin-walled cavitary lesions [16]. In regions where hydatid cyst is endemic, it is not easy to distinguish between complicated hydatid cyst and lung cancer during the preoperative period. The incidence of cavitation in primary lung cancers varies between 2 and 25%. The lung cancer types in which cavitary parenchymal lesions are most frequently observed are squamous cell lung cancer, adenocarcinoma, and large cell carcinoma, respectively [14, 15, 17, 18].

In our study, there were five patients with lung cancer radiologically mimicking complicated hydatid cysts. The histological subtype was squamous cell carcinoma in four, and large cell carcinoma in one patient. In these cases, the cavity wall was thin and relatively regular. All of these patients underwent bronchoscopy in the preoperative period, but no pathological diagnosis could be made. One of the most important problems in the management of patients radiologically mimicking complicated hydatid cysts is the risk of incomplete preoperative staging. Therefore, we recommend performing bronchoscopy in all patients presenting with a cavitary lung lesion, evaluating the radiological features of the cavity by an experienced radiologist, and investigating the lung cancer risk characteristics of the patient. As in our study, if a preoperative diagnosis cannot be made, pathological studies should be performed in the intraoperative period.

Perforated hydatid cysts may present only with heterogeneous increased parenchymal opacity without displaying cystic or cavitary features on thorax CT. Considering the possibility of lung cancer in patients with increased parenchymal opacity that cannot be regressed with antibiotic treatment, advanced diagnostic interventions such as transthoracic fine needle aspiration biopsy, brain MRI and PET-CT may be required. In our study, further investigation was performed with a preliminary diagnosis of lung cancer in two patients with heterogeneous parenchymal opacity increase that did not regress with antibiotic therapy, but surprisingly, a hydatid cyst was diagnosed in the intraoperative period. It is possible to encounter similar cases in the literature [19,20,21]. Çobanoglu et al. [21] reported clinical and radiological features of seven patients with tumors mimicking hydatid cyst and emphasized the diagnostic difficulties due to the variety of radiological features that hydatid cyst may show.

The perforated hydatid cyst may become infected and form an abscess, displaying a common radiological pattern with infective lung lesions [22, 23]. Surgery is indicated for lung lesions that persist despite medical treatment or in the case of recurring abscesses. In our study, a lung abscess associated with bronchiectasis was detected in two patients radiologically mimicking complicated hydatid cysts. Recognizing the clinical and biochemical signs of infection and arranging appropriate medical treatment will prevent unnecessary surgical interventions. It may be more difficult to elucidate the etiology in cases where empyema develops by opening the abscess cavity into the pleural space. In a study by Aribas et al. [24], 145 patients hospitalized for hydatid cysts were reviewed and pleural complications were detected in 40 patients. They found that empyema was to be one of the most common pleural complications with a rate of 7.6%.

We included two patients who underwent tube thoracostomy with the diagnosis of empyema and were diagnosed with hydatid cyst after surgical treatment due to prolonged air leak and suspected parenchymal opacities in the follow-ups. Our recommendation in cases of empyema is the careful evaluation of the lung parenchyma with thorax CT after the drainage of the purulent pleural contents and appropriate antibiotic therapy. In the presence of persistent cavitary or cystic parenchymal lesion despite pleural drainage and appropriate medical treatment, the possibility of a complicated hydatid cyst should be kept in mind.

Another group of diseases that may cause difficulties in the differential diagnosis is congenital lung malformations which represent a broad spectrum of pathology affecting the airway or lung parenchyma [25]. The most common congenital lung malformations are bronchogenic cyst, congenital cystic adenomatoid malformation, pulmonary sequestrations, and congenital lobar emphysema. Lesions may present radiologically as a single thin- or thick-walled cyst, multicystic, or solid lesions with heterogeneous density [25, 26]. Cysts may be filled with air or fluid. These radiological findings of congenital lung malformations show great similarities with pulmonary hydatid cysts. In our study, intraparenchymal bronchogenic cysts were detected in four patients who radiologically suggested the diagnosis of pulmonary hydatid cysts with their thin-walled cystic lesions. In these cases, the absence of laminated membrane in the cyst during surgery and the revealing presence of cartilage tissue in the cyst wall in the frozen section enabled the diagnosis of bronchogenic cyst. Making a definitive diagnosis during the operation is very important in terms of directly affecting the surgical technique to be applied.