How are fetal lung masses evaluated sonographically?
Aside from determining whether or not a fetal lung mass is present, the type of mass is important. Most CPAM lung lesions are more echogenic than normal lung, and many will have cysts. The cysts can vary in size from subcentimeter to several centimeters.
The current classification scheme for these masses is based on whether or not the lesions contain macroscopic cysts.
Once a mass is identified and the location within the thorax documented, determination of the arterial supply and venous drainage is important. CPAMs have an arterial supply from the pulmonary arteries and venous drainage to the pulmonary veins.
BPS is also usually hyperechoic and by gray scale appearance alone can be difficult to distinguish from a CPAM. The important aspect that can differentiate them is the blood supply. BPS always has a nonpulmonary arterial supply, although the source is variable, and may be intralobar (the most common type, contained within the pleura of a lobe) or extralobar (with its own pleural investment).
Differentiation between these is aided by the venous drainage. Intralobar BPS drains to the pulmonary circulation, typically to the pulmonary veins of the lobe in which they are located, whereas extralobar BPS drains to the systemic veins. Both the arterial supply and venous drainage of BPS may be variable. Arterial supply is often from the thoracic aorta but may be from subdiaphragmatic aortic branches. Venous drainage is also variable and depends on location. Extralobar BPS most often occur adjacent to the left hemidiaphragm and have a male predominance. Approximately 10% of extralobar BPS are extrathoracic. Intra-abdominal BPS venous drainage can be to the azygos system, phrenic veins, or other systemic veins. Unlike intralobar BPS, the extralobar type has a higher incidence of associated abnormalities, including CPAM, CDH, and cardiac and spinal anomalies. In rare circumstances, an extralobar BPS that is close to the esophagus or stomach may develop a fistula to these structures or be associated with an esophageal or gastric bronchus.
In some instances, lung masses may have features of both a CPAM and BPS, typically with both a pulmonary and systemic arterial supply. These are termed hybrid lesions and are evaluated much like a distinct CPAM or BPS.
For lung lesions, the CPAM or BPS is initially measured by US (and MRI), and a mass volume to HC ratio is calculated. This value, often termed the CVR (for CPAM volume ratio), is used for follow-up studies because some of these masses decrease in size as the pregnancy progresses. For those that grow, serial assessment of the CVR is helpful to determine the growth rate. The CVR is also predictive of outcome for CPAM, particularly the development of hydrops, a major complication of CPAM. If the CVR is ≤ 1.6, the risk for development of hydrops is low compared with those fetuses with a CVR >1.6.