Pleuropulmonary blastoma

Pleuropulmonary blastomas (PPB) are a rare, variably aggressive, childhood primary intrathoracic malignancy. In up to 25% of cases, the mass can be extrapulmonary with attachment to the parietal pleura. 

Epidemiology

PPB is encountered in childhood, mostly in the first years of life (90% in those between 0-2 years old). 

Associations

PPBs are associated with type 4 congenital pulmonary airway malformations (CPAM). However, it is likely that any coexistence is related to underlying genetic predisposition rather than the original belief that CPAM undergoes malignant transformation.

PPB is associated with PPB family tumor and dysplasia syndrome in 33% of cases. Many of these patients have a mutation of the DICER-1 gene. In 10% of cases, patients with PPB may also present with multilocular cystic nephroma, and, very rarely, Wilms tumor ^7-9.

Pathology

PPB comprises both mesenchymal and epithelial components resembling fetal lung. Bilateral occurrence is extremely rare ¹⁴.

Classification

This classification is a continuum from the less malignant to the most malignant lesion:

• cystic: type 1 (prenatal and 10 months old), 14%
• mixed: type 2 (mean age 34 months), 48%
• solid: type 3 (mean age 44 months), 38%

Type 1 PPBs are impossible to differentiate from types 1 and 4 CPAM. Thus, it must be included in the differential diagnosis, particularly if the patient is known for another type of blastoma, as 25% of PPB appear in families with a history of blastomas.

PPBs are usually right-sided, pleural-based, without chest wall invasion or calcifications. It can sometimes present with pneumothorax. Types 2 and 3 PPBs are associated with CNS and bone metastases.

Radiographic features

Often late presentation at radiographic diagnosis. Unilateral lung whiteout on plain film with mediastinal shift to the opposite side. Usually, there are no adjacent rib erosions or calcifications.

Ultrasound

A poor imaging modality for this diagnosis. Non-specific and may show a large region of consolidation without sonographic air bronchograms ³.

CT

Usually seen as a large mass in the thorax with mixed solid-cystic heterogeneous low attenuation, pleural effusion (not dominant abnormality), contralateral mediastinal shift, and lack of chest wall invasion ^3,4.

• type 1 lesions manifest as a single cyst or a multicystic lesion (often air filled)
• type 2 lesions show air- or fluid-filled cavities with possible air-fluid levels along with solid internal nodules
• type 3 neoplasms are solid lesions that show low attenuation at CT and homogeneous or heterogeneous enhancement

PET/CT

Type 3 PPB shows increased 18F-fluorodeoxyglucose (FDG) uptake at PET/CT ¹³

Treatment and prognosis

Type 1 tumors have a good prognosis. Complete surgical resection is often the treatment of choice, as it is with CPAM. Tumors larger than 5 cm, just like type 1 and 2 PPBs carry a worse prognosis ²_.

Differential diagnosis

The differential diagnosis for type 1 PPB includes other cystic lesions:

• types 1 and 4 CPAM: indistinguishable from type 1 PPB; however, congenital lung malformations are often detected during the second trimester fetal US, whereas PPB are usually discovered postnatally ¹³
• large bronchogenic cyst/lung cyst/pneumatocele/pulmonary interstitial emphysema

The differential diagnosis for type 2 and 3 PPB, particularly when they are locally aggressive, includes more common tumors ¹³:

• neuroblastoma
• intrathoracic soft tissue sarcoma
• PNET of thorax
• FLIT (fetal lung interstitial tumor) ¹⁰

See also

• sarcomatoid carcinoma of the lungs