Type I, II and III PPB
The PPB/DICER1 Registry believes that Type I, Type II, and Type III PPB are on a biologic continuum:
Type I PPB is a purely cystic lesion typically occurring in children under 2 years of age, median 10 months of age.
Type II PPB is a cystic and solid lesion occurring at median age 35 months.
Type III PPB is a completely solid lesion occurring at median age 41 months.
Progression of Type: In a given patient, the aggressiveness of PPB appears to progress over time. Types I, II, and III occur, on average, in progressively older cohorts of children (see Basic Facts about PPB, Age at Diagnosis page in Health Professionals). Among literature and Registry cases are several well-documented examples of Type I disease recurring as Type II and as Type III disease.
Type Ir (regressed) PPB
Since 2006 PPB/DICER1 Registry pathologists have recognized a fourth “Type” of PPB, Type Ir for “Type I-regressed PPB”. This is a cystic lesion similar to Type I PPB. It has a micro-architecture similar to Type I PPB with very delicate septa without malignant cells. There may be small spindle cells which are not primitive and foci of dystrophic calcification. It appears to be 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 a dysplastic path as to become malignant. The cysts of Type Ir PPB are different from other lung cyst pathologies included in “congenital pulmonary airway malformation” (CPAM) or congenital cystic adenomatoid malformation (CCAM) categories. [see Registry Publications Hill 2008; Messinger abstract 2012 Type I and Type Ir.]
Pathologists encountering unusual lung cysts are encouraged to obtain consultation from PPB Registry pathologists at no cost.