What is Pleuropulmonary blastoma?
Pleuropulmonary blastoma (also known as PPB) is a rare tumor of the lungs or coverings of the lungs. Most cases occur in children under the age of five. PPB has some similarities to the more common pediatric cancers, embryonal rhabdomyosarcoma (a malignant tumor of skeletal muscle) and Wilms tumor (a tumor the primitive cells of the kidney). The majority of children with the earliest form of the disease (Type I PPB) are curable with surgery and chemotherapy. Children with the later forms (Type II or III PPB) have a much more difficult course. The website for the International PPB Registry is a great resource for learning more about PPB.

Pathology of PPB
PPB is a tumor that apparently originates from the connective tissue of the developing lungs, also known as mesenchyme. Malignant tumors of connective tissue are referred to generically as “sarcomas.”  PPB has three different but related pathologic forms that are based on the presence or absence of cysts. In its earliest form, PPB grows as an air-filled cyst in the lung with small numbers of tumor cells (Type I PPB). In later stages PPB progresses to a cystic and solid (Type II) or purely solid (Type III), aggressive malignant tumor. The cysts in Type I and II tumors typically have collections of tumor cells beneath the cyst lining called a cambium layer similar to that seen in embryonal rhabdomyosarcoma. The solid tumor component in Type II (cystic and solid) and Type III (purely solid) tumors can have a variety of tumor patterns. These areas can include rhabdomyosarcoma, blastema (similar to that seen in Wilms tumor), spindle cell sarcoma resembling fibrosarcoma or synovial sarcoma and chondrosarcoma (malignant cartilage). Marked cellular pleomorphism and atypical mitoses (anaplasia) is common in Type II and Type III tumors.

What causes PPB?
We do not know what causes PPB but it appears to have a strong genetic component.  PPB has early onset of disease occurring primarily in infants and children less than five years of age. Children with PPB can have more than one tumor in the lung and/or tumors in other organs. In some families, relatives may have PPB or other rare benign or cancerous tumors including rhabdomyosarcoma, cystic nephroma and Wilms tumor. A new study designed to understand why this tumor runs in some families is currently underway.

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Type I PPB can look benign on radiographs and on light microscopy. In this example, the cyst walls have a thin, discontinuous layer of primitive cells and focal nodules of cartilage (arrow).

Type I PPB - Cyst walls have a dense collection of primitive cells beneath a benign epithelium referred to as a cambium layer (brackets). A small nodule of primitive cartilage is forming in the upper portion of the cyst wall (arrow).

The solid portions of Type II and Type III PPB can have areas that resemble the blastema in Wilms tumor.

The solid areas of Type II and Type III PPB frequently have anaplasia. The anaplastic cells are the ones with the large, dark purple nuclei. An abnormal mitotic figure with excess chromosomes is seen just left of center (arrow).

Key References

• Manivel JC, Priest JR, Watterson J et al. Pleuropulmonary blastoma. The so-called pulmonary blastoma of childhood. Cancer 1988;62:1516-26.
• Delahunt B, Thomson KJ, Ferguson AF, Neale TJ, Meffan PJ, Nacey JN. Familial cystic nephroma and pleuropulmonary blastoma. Cancer 1993;71:1338-42.
• Priest JR, Watterson J, Strong L et al. Pleuropulmonary blastoma: a marker for familial disease. J Pediatr 1996;128:220-4.
• Priest JR, McDermott MB, Bhatia S, Watterson J, Manivel JC, Dehner LP. Pleuropulmonary blastoma: a clinicopathologic study of 50 cases. Cancer 1997;80:147-61.
• Hill DA, Dehner LP: A cautionary note about congenital cystic adenomatoid malformation (CCAM) Type 4. Letter to the Editor. Am J Surg Pathol 2004;28(4):554-5
• Hill DA USCAP Specialty Conference, case 1: Type I pleuropulmonary blastoma. Pediatr Dev Pathol. 2005;8:77-84.