To the Editor: In 2005, we (SP, S-CH) reported a child with a resected pulmonary congenital cystic adenomatoid malformation (CCAM) who later developed a botryoid embryonal rhabdomyosarcoma (RMS) at the site of the lung cyst 1. We have since shared our case with the International Pleuropulmonary Blastoma Registry. Together with those consultants, we want to correct the original report. The case is an example of pleuropulmonary blastoma (PPB) rather than RMS at the site of a lung cyst.
PPB Registry pathologists (DAH, LPD) reviewed the clinical case and histologic sections from all three surgeries. Clinical findings and pathology from the initial surgery support the diagnosis of PPB in its cystic early form, Type I PPB (Fig. 1, original report). Beneath the benign lining of the multilocular cystic lesion is a condensed layer of small proliferating cells with a rhabdomyosarcomatous phenotype. A year later, a predominantly solid neoplasm emerged, which had a blastemal appearance in areas and spindle elements intermixed with the rhabdomyosarcomatous component (Fig. 2, original report). The post-chemotherapy specimen reveals chemotherapy effect and residual rhabdomyoblasts, a nodule of primitive cartilage and areas with a chondroid appearance. The blastemal and chondroid areas differentiate PPB from typical embryonal RMS. Including the clinical progression from a cystic Type I lesion to the emergence of a complex solid Type II/III neoplasm, the overall findings are typical of PPB.
Type I PPB is a single or more often a multilocular cyst in peripheral lung parenchyma featuring a primitive mesenchymal-cell proliferation (cambium layer) in the stroma beneath a benign epithelial cyst lining 2 (Fig. 1, original report). The proliferation may be subtle in the absence of overt rhabdomyoblastic differentiation. It is often mistaken both clinically and radiographically for a benign congenital cyst, in particular CCAM type 4 3, as discussed elsewhere 4–6. Upon reevaluation, many such cases in the literature are diagnosed PPB (data available from the PPB Registry). Recurrences of Type I PPB are usually Type II and III disease. The solid components of Types II and III PPB are a collage of fibrosarcoma-like, blastemal, cartilaginous, rhabdomyosarcomatous, and anaplastic foci (Fig. 2, original report) 2.
Why is it important to identify lesions specifically as PPB? First, cystic Type I PPB is not a benign cystic malformation. It is the initial morphologic manifestation of a unique progression sequence into Type II and/or Type III PPB. Second, unlike RMS, there are strong genetic implications to the PPB diagnosis; in approximately 25% of cases there is additional neoplasia and dysplasia in PPB patients or their young relatives 7. Third, in contrast to other childhood sarcomas including RMS, PPB has a striking propensity to central nervous system metastasis 8,9.
We appreciate the opportunity to correct the interpretation of this case.