The head of the largest program tracking the health of World Trade Center site workers said several have developed rare blood cell cancers, raising fears that cancer will become a "third wave" of illnesses among those exposed to toxic dust after Sept. 11. Dr. Robin Herbert, co-director of the World Trade Center Medical Monitoring Program at Mount Sinai Medical Center, said researchers who have screened 20,000 of the estimated 40,000 ground zero workers are "most concerned" about lymphatic and blood cancer cases. "We're worried about a third wave, which is the possibility of cancer down the road," Herbert said in an audiotaped interview posted on the New England Journal of Medicine's Web site. "The kind of thing that worries us is that we know we have a handful of cases of multiple myeloma in very young individuals, and multiple myeloma is a condition that ... almost always presents later in life," she added. "That's the kind of odd, unusual and troubling finding that we're seeing already." The city's health commissioner said Thursday there was no evidence of a link to cancers and trade center dust exposure.