Response without shrinkage in bilateral Wilms tumor: Significance of rhabdomyomatous histology

Purpose: To test the hypothesis that poor response to chemotherapy in patients with bilateral Wilms tumor may be associated with the appearance of rhabdomyomatous histology, suggesting a differentiation response. Methods: Twenty-six patients with bilateral Wilms tumor were treated at the authors' hospital between 1985 and 1995. Radiologic response to presurgical chemotherapy was assessed, and postsurgery histology was reviewed. Results: There was a significant association between rhabdomyomatous differentiation in postchemotherapy surgical specimens and poor radiologic response. Poor response did not, however, necessarily mean poor outcome: of 11 patients with rhabdomyomatous differentiation, 7 are alive and disease-free, 2 died of complications, and only 2 died of uncontrolled Wilms tumor. Conclusions: Rhabdomyomatous differentiation in postchemotherapy bilateral Wilms tumor is associated with poor radiologic response. This observation may indicate a differentiation response rather than an absolute failure of response to chemotherapy. Clinical measures other than tumor volume are needed to distinguish between tumors that respond to chemotherapy but do not shrink, and those that genuinely do not respond.