To the Editor,

Wimberly and colleagues (1) examined the impact of maternal substance use on the development of childhood cancer. The authors surveyed the parents of 3,145 children with cancer and found that prenatal exposure to illicit drugs was associated with an increased prevalence of retinoblastoma and intracranial embryonal tumors but was protective against acute lymphocytic leukemia.

The study was based on a case series of children with cancer, the most frequent types being acute lymphocytic leukemia (29%) and neuroblastoma (14%). A representative sample of children without cancer was not included. To account for the absence of a comparison group, the authors compared children with a given cancer against children with other cancers. In total, 15 subtypes of cancer were examined, with each subtype contrasted against the remaining 14. Because of the design, patients with a given cancer were moved from the numerator to the denominator in each...

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