Botryoid Wilms tumor: a non-existent "entity" causing diagnostic and staging difficulties.

Wilms tumors growing in a botryoid fashion into the renal pelvis have been reported since the 1960s as a rare tumor type usually associated with stromal histology and a good prognosis. However, the true frequency, association with Wilms tumor subtypes, and stage have never been comprehensively studied. We analyzed all Wilms tumors enrolled into the International Society of Paediatric Oncology (SIOP) United Kingdom 2001 Trial (2001-2011), which showed botryoid growth. In addition, we reviewed published series reporting papers on botryoid Wilms tumors. 77/739 patients (10.4%) showed at least one Wilms tumor with a botryoid pattern, and they were sub-classified according to the SIOP criteria as follows: 28 stromal, 21 mixed, 7 regressive, 3 completely necrotic, 4 blastemal, 2 epithelial, 3 diffuse anaplasia, 1 focal anaplasia, and 10 non-anaplastic type (treated with primary surgery). Stage was as follows: 25 stage I, 21 stage II, 12 stage III, 11 stage IV, and 8 stage V. In six cases, local pathologists incorrectly upstaged the tumor from stage I to stage II based on botryoid growth. The event-free and overall survivals were 90 and 96%, respectively. We concluded that botryoid growth pattern is a common finding in Wilms tumor and that all histological types and stages can share this feature. The botryoid growth itself is not a criterion for stage II. Botryoid Wilms tumor is not an entity but merely represents a pattern of tumor growth; such tumors should be sub-classified according to their overall histological features, which will determine treatment and prognosis.