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PPB is considered to occur in three 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 consists virtually entirely of cystic tissue without any tumor lumps (“nodules”) 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 PPB 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 later in this section of the website).
Type II PPB
Type II tumors consist of both cystic spaces and solid tumor lumps (“nodules”) as seen by the naked eye after the tumor is removed. The nodules will be cancerous under the microscope. Children with Type II PPB average about 34 months of age at diagnosis. Compared to Type I PPB, the treatment required for Type II must be more powerful, and the prognosis is less favorable (see paragraphs on Treatment later in this section of the website).
Type III PPB
Type III tumors are entirely solid. The average age of children with Type III PPB at diagnosis is about 44 months. The general recommendation would be that Type III tumors require powerful therapies, as do Type II tumors.
Some children have had the PPB Type change if they have had a recurrence of their disease [Bibliography; Focus: type progression]; this would be a change from Type I to II or III or from Type II to 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 understood.
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