Metro

Red Sox prospect had a very rare, aggressive cancer, according to death certificate

Daniel Flores in February.
Bill Mitchell
Daniel Flores in February.

Daniel Flores, the 17-year-old Red Sox prospect who died Nov. 8 of testicular cancer, had an extremely rare and aggressive form of the illness, according to his death certificate.

The document states that Flores had metastatic choriocarcinoma , a type of cancer that represents less than 1 percent of all testicular cancer cases.

A switch-hitting catcher from Venezuela, Flores was recruited by the Red Sox with a $3.1 million signing bonus and labeled “a future superstar.” His death was shocking because he had been practicing just two weeks earlier and because testicular cancer is curable 95 percent of the time.

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But Flores had a form of the illness with a much worse prognosis. The American Cancer Society describes choriocarcinoma as “likely to spread rapidly to distant organs of the body, including the lungs, bones, and brain.”

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Most forms of testicular cancer are detected when a lump is found. Typically removing the testicle is the only treatment that’s needed. If it has spread beyond the testicle, the cancer can usually be destroyed with chemotherapy or, sometimes, radiation.

But testicular choriocarcinoma is different. It can start as a tumor too small to be felt. If that’s what happened with Flores, it would explain why no problems were detected when he underwent a full physical in Boston before he signed in July.

As the cancer roared through his body, Flores might have blamed any symptoms on his training. His trainer and close family friend, Jose Salas Jr., said Flores complained of lower back pain but attributed it to a pulled muscle or just “a bad night.”

Salas said Flores had come to Boston Oct. 28 and was diagnosed with testicular cancer that had spread undetected to his lungs and elsewhere.

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The death certificate states that the immediate cause of death was loss of oxygen to the brain as a result of respiratory failure, and the underlying cause was metastatic choriocarcinoma.

The death certificate was signed by Dr. David M. Shahian, a surgeon at Massachusetts General Hospital, where Flores was treated and died.

The death certificate states that Flores’s remains were cremated.

Felice J. Freyer can be reached at felice.freyer@globe.com.