PPB is considered to occur in four different "types". There is no strict dividing line between these types, and they may be points along a biologic continuum or spectrum. (Physicians often categorize diseases in order to try to find useful differences for the purpose of treatment and prognosis.) The Type of PPB a child has may be suggested by the appearance on x-rays, but is formally determined after surgery, when the tumor can be examined in the laboratory and under a microscope:
Type I PPB
Type I PPB consists virtually entirely of cystic tissue without any tumor “lumps” visible to the naked eye. Under the microscope, a thin layer of malignant tissue can be seen in the wall of the cyst. Type I tends to occur in younger children (the typical age at diagnosis 10 months); it may have a better prognosis and may require less powerful treatment than the other types of PPB (see paragraphs on Treatment in this section of the website).
Type II PPB
Type II PPB tumors consist of both cystic spaces and solid tumor “lumps” as seen by the naked eye after the tumor is removed. The tumor will be cancerous under the microscope. Children with Type II average about 35 months of age at diagnosis. Compared to Type I, the treatment required for Type II must be more powerful, and the prognosis is less favorable (see paragraphs on Treatment in this section of the website).
Type III PPB
Type III PPB tumors are entirely solid. The average age of children with Type III at diagnosis is about 41 months. , the treatment required for Type III must be more powerful, and the prognosis is even less favorable than Type II (see paragraphs on Treatment in this section of the website).
Progression from cystic only Type I PPB to the more aggressive
Type II or III
Some children have had their PPB Type change if they have had a recurrence of their disease; this would be a change from Type I to II or III. In general this phenomenon is interpreted as meaning that in a given patient, if the PPB is not cured, it may become more aggressive over time; the scientific basis for such a change is not yet understood.
Type Ir (regressed) PPB
In 2006 PPB Registry pathologists recognized a fourth “Type” of PPB called “Type Ir PPB” for “Type I-regressed PPB”. This is a cystic lesion similar to Type I PPB, but it is a lesion which has “regressed’ from an earlier Type I PPB or, alternatively, it is a genetically-determined lung cyst which did not evolve so far along as to become malignant. Type Ir was initially recognized in several members of families in which one or more relatives had frank PPB. Subsequently, Type Ir has also been recognized in a few individuals with no known PPB relatives. Type Ir may present with pneumothorax. It may exhibit large or small lung cysts and may be an incidental finding in a relative of a PPB patient. Type Ir is recognized in individuals from infancy to adulthood. The cysts of Type Ir are different from other lung cyst pathologies included in “congenital pulmonary airway malformation” (CPAM) or congenital cystic adenomatoid malformation (CCAM) categories.
PPB Registry data in 2012 shows 25 cases T-Ir confirmed by central Registry pathology review. Patients with T-Ir diagnosed on average at 48 months. Progression to high-grade Type II or III occurred in 2/25 (8%). Diagnosis after age 6 years included 10 T-Ir patients.
A cyst in an older individual most likely will be Type Ir. Type Ir in a young child has a small risk of progression to the advanced Type II/III up to 6.5 years of age. The PPB/DICER1 Registry does not have sufficient data to comment on the role of chemotherapy for the prevention of progression in Type Ir PPB in children up to 6.5 years of age, but believes Type Ir in older children and adults does not require chemotherapy.