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By Daniella Gray
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When a routine test led to a diagnosis of rare thyroid cancer, a father from Brazil thought he was facing a personal health battle.
Instead, it became a life-altering family decision.
Engineer Marcos Vinicius Sampaio Vieira was first alerted to a problem during a cardiology appointment at age 35, when doctors discovered three nodules on his thyroid.
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At first, doctors thought the nodules were part of a goiter, or enlarged thyroid gland, but Vieira pushed for a biopsy, which revealed cancer.
After undergoing surgery to remove his thyroid and a subsequent procedure to examine nearby lymph nodes, he received encouraging news that there was no evidence the disease had spread.
But the story did not end there. Years later, follow-up testing revealed metastatic lesions in his liver, followed by tumors in his spine.
Meanwhile, a molecular test identified the cause of his cancer: an inherited mutation in the RET gene, which is associated with hereditary medullary thyroid carcinoma, a rare but aggressive form of thyroid cancer.
People who inherit certain RET mutations face an extremely high lifetime risk of developing the disease.
The discovery transformed Vieira's diagnosis from an individual health challenge into a family-wide concern.
"When I discovered that my cancer was linked to an inherited genetic mutation, it was an extremely difficult situation," Vieira, now 56, told Newsweek.
The shock of the diagnosis was compounded by the realization that his children—Lara and Henrique, who were 10 and 7 at the time—each had a one-in-two chance of carrying the same genetic mutation.
"Learning that there was a 50 percent chance I had passed this mutation on to my children completely pulled the ground out from under my feet," Vieira said.
The family's fears became reality when testing showed that both of his children carried the mutation.
At first, the father of two felt overwhelmed when trying to determine the best course of action.
"I went through numerous medical consultations and spent countless hours researching online," he explained. "Deep down, I knew that the right decision would eventually be to have their thyroids removed, but the question that constantly troubled me was when."
The turning point came during a conversation with his doctor, who told him she would proceed with surgery if she were making the decision for her own children.
That advice helped Vieira decide to move forward with preventive thyroid removal surgeries for his two young children, whom he shares with his wife, 58-year-old Jaqueliny Maria Fernandes Silveira Vieira.
Today, due to genetic testing, both Lara and Henrique, now 26 and 23, are "completely healthy" and lead "normal lives," their father said.
"Genetic testing gave us the opportunity to act early and make informed decisions that changed the course of their lives for the better," he added.
Vieira later learned that the mutation likely originated with his own father, whose preserved tissue samples helped researchers trace the family's medical history.
The family's story illustrates why experts say genetic testing can be transformative.
According to a recent American Cancer Society report, genetic testing is playing an increasingly important role in identifying people at elevated risk of hereditary cancers and helping families make informed decisions about prevention and treatment.
Ana Hoff, the head of endocrinology at ICESP and the University of São Paulo Medical School, said hereditary cancer syndromes are often identified only after one family member is diagnosed and found to carry a cancer-linked mutation.
"Detection of a cancer-predisposing gene mutation has significant benefits because it will lead to an earlier detection of tumors, making cure more possible," Hoff told Newsweek.
For Vieira, that meant learning that his cancer was caused by an inherited RET mutation and taking steps to protect his children before they developed the disease.
Hoff added that a negative genetic test does not always rule out a hereditary cancer syndrome. Families with a strong history of cancer should seek advice from specialists in cancer genetics, as new cancer-linked genes continue to be discovered, and updated testing may uncover mutations that were previously missed.
"I would strongly recommend genetic testing to any family facing a hereditary cancer diagnosis," Vieira said. "It is a fundamental tool for identifying who is free from the mutation and who carries it and should follow the appropriate medical recommendations and preventive measures."
Reference
Kallas-Silva L, dos Santos Valsecchi VA, Hoff AO, et al. Medullary thyroid carcinoma and RET molecular testing: A virtual tumor board on clinical decision-making. CA Cancer J Clin. 2026;e70086. doi:10.3322/caac.70086
Contact Newsweek editors on this story: Kara Dolman and Gray R. Thomas
Update 8/24/26, 11:43 a.m. ET: This article was updated with comments from Ana Hoff.
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