These developmental cysts arising from the foregut appear most frequently as a posterior mediastinal cyst, but can also be intrapulmonary. These are rare lesions, but can compress the airway if large.

Aspirates yield thick proteinaceous to mucoid fluid with variable types of lining cells, including ciliated, squamous, or simple columnar epithelium (Fig. 6.7). Liquid-based cytology can be helpful for demonstrating the cyst lining cells and excluding thymic or neoplastic cysts (e.g., germ cell tumors with cystic change).

A cystic lesion in the mediastinum or lung of a child raises the differential diagnosis of thymic cyst, mediastinal goiter, lymphangioma, cystic pulmonary airway malformation (CPAM ; formerly congenital cystic adenomatoid malformation ), mesenchymal cystic hamartoma, and neoplastic cysts (e.g., cystic teratoma or other germ cell tumor with cystic change). In contrast to bronchogenic cysts, most of the congenital malformations and hamartomas in this age group are within the lung and are multicystic lesions with small cysts.

If small lymphocytes are identified from a cystic lesion in the mediastinum in the pediatric population, the possibility of a thymic cyst should be considered. Flow cytometry and/or immunoperoxidase stains can be used to support thymic origin by identifying immature and maturing T-cells that are positive for TdT, CD1a, and CD3.

Fine needle aspiration biopsy has not been routinely used for the diagnosis of infectious disease in the lung. Thus, there are few studies evaluating its effectiveness. One paper in the adult literature reported a range of sensitivity of 21–47 % for FNA and 52–94 % for core needle biopsy [3]. One study in the pediatric population reported a sensitivity of 88 %, but did not separate FNA from core needle biopsy [4]. Another noted that CT-guided biopsy had a high diagnostic yield of 60 %, but only yielded organisms already isolated from peripheral blood [1]. Occasionally, FNA performed for infectious disease of the lung may reveal an undiagnosed tumor of the chest wall or mediastinum.

Infections that present as discrete masses in the lung frequently show either abscess formation, granulomatous inflammation , or a cystic fungal ball. Aspirates show abundant histiocytes with a variable numbers of small lymphocytes and neutrophils in the background (Figs. 6.8 and 6.9). Stains for fungal elements and/or acid fast bacteria can be performed on aspirate smears and may be useful; however, if there are only rare yeast or fungal elements, the possibility of specimen contamination with Alternaria or other fungi or oropharyngeal contamination from Candida should be considered [5] (Fig. 6.10). Well-formed granulomas with clusters of epithelioid histiocytes are suggestive of fungal or mycobacterial infection (Fig. 6.11). Fungal balls typically show abundant granular debris with fungal hyphae present. BAL is commonly used for the cytologic evaluation of suspected infection but, in the setting of a mass lesion, may not yield representative material. Depending on the etiologic agent, BALs can show neutrophils, lymphocytes, and/or histiocytes, necroinflammatory debris and reactive epithelial cells. Gram, methenamine silver, AFB and Fite stains can be helpful for identifying microorganisms on cytospins, liquid-based preparations or cell blocks from BALs. Viral infections, such as cytomegalovirus and herpes virus, can also be seen in BAL specimens (Fig. 6.12).

Material should be submitted from the FNA or BAL for microbial cultures and special stains. When handling the specimen, care should be taken to avoid contamination. Since complications such as pneumothorax are common in lung FNA, few passes may be performed, and each pass should be triaged carefully. If the needle tip is not touched to the slide when the first drop(s) are taken for smears, the remaining specimen may be rinsed into sterile saline for microbiologic studies. If the smears show anything other than an infectious process, this rinse can be used for other ancillary studies (flow cytometry, cell block, etc.). In addition, if there is abundant hemodilution on subsequent passes, unstained smears could be prepared from the best pass and sent for special stains [5].

Neoplastic disease, metastatic or primary, may mimic an infectious lung mass radiologically. Although atypical bronchial cells and/or pneumocytes can be present in the setting of infection, cytologic features of malignancy, such as increased nuclear to cytoplasmic ratios, nuclear pleomorphism, nuclear membrane irregularity, and hyperchromasia are absent. Lymphocytic infiltrates associated with infectious processes are polyclonal, in contrast to the monoclonal proliferations associated with non-Hodgkin lymphomas.

Primary lung carcinomas are exceedingly rare in the pediatric population. When atypical squamous or glandular cells are identified in an FNA or exfoliative specimen from the lung of a child or adolescent with a mass lesion, reactive atypia in metaplastic squamous cells, bronchial cells or pneumocytes secondary to infection should be excluded before considering malignancy.

Pulmonary alveolar proteinosis (PAP) is a rare disorder associated with impaired or dysfunctional surfactant clearance. Some cases are congenital, due to mutations in surfactant protein-B, ABCA3 or GM-CSF receptor genes. This is in contrast to adults, in which most cases are acquired and due to autoantibodies to GM-CSF. Most other pediatric cases are secondary to infection, drugs, foreign material, treatment for malignancy, or idiopathic causes. PAP is characterized by accumulation of lipid-rich proteinaceous material in alveolar spaces leading to respiratory distress or failure. On imaging, patients have diffuse, bilateral, pulmonary infiltrates or opacities, without features of fibrosis or chronic lung disease. BALs can be performed in these patients for diagnosis and therapeutic purposes.

The BALs from patients with PAP have abundant, acellular, PAS-positive diastase-resistant, lipid-rich proteinaceous material, often in large globules, and pulmonary macrophages containing similar material. Degenerating macrophages, lamellar bodies, cholesterol crystals, and reactive pneumocytes can also be present, but inflammation is usually absent (Fig. 6.13). PAS and PAS-D stains can be performed on additional cytospins, liquid-based preparations or sections from a cell block to support the diagnosis.

The other possibilities to consider are mucus plugs or alveolar casts from Pneumocystis jirovecii infection. Mucus plugs lack the intense PAS-positivity seen in PAP but stain with mucicarmine or Alcian blue , while methenamine silver stains can help to exclude Pneumocystis jirovecii infection.

Electron microscopy shows that the lamellar bodies are proteinaceous surfactant material.