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A 17-year-old Connecticut girl with a highly curable cancer is not mentally competent to make her own medical decisions and will continue to receive the chemotherapy treatments she's battled to halt, the Connecticut Supreme Court ordered Thursday.

Chief Justice Chase T. Rogers ruled that the teen — listed only as Cassandra C. in legal records — is not mature by any standard.

That means Cassandra will remain at a Hartford hospital, in the temporary custody of child-welfare workers, and will receive her full course of chemotherapy to treat Hodgkin lymphoma. Doctors have said her odds at recovery are 80 to 85 percent with chemo, but that she will die without it.

Cassandra, diagnosed in September, had sought to avoid chemo treatments because she views the medicine as "poison." Her mother, Jackie Fortin, has said she support's her daughter's decision.

But recent behaviors and actions by Cassandra's mother — including several skipped oncology appointments and exams — were cited by state lawyers Thursday as they asserted the teen is too immature to make life-altering decisions.

"The mother took the front seat on this," John E. Tucker, assistant Connecticut attorney general, testified before the panel. "She (Fortin) didn’t bring her to the first medical appointment.

"The child was very quiet, did not engage in conversations during the medical appointments. And for a 17-year-old, as you can imagine, that’s a little bit unusual," Tucker added. "Really, the mother did all of the talking and sort of the fighting with the medical personnel. And so, really, the child stands in the shadow of her mother here. She’s not an independent decision maker.

"It was really the mother driving the bus."

In an interview Wednesday with NBC News, Fortin denied pressuring her daughter into her decision to forgo chemo.

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"I am not coercing her at all and that is what this is about, what they think I am doing," Fortin said.

Cassandra simply does not want to be infused with "toxic" chemicals, Fortin added.

"My daughter does not want poison in her body. This is her constitutional right as a human being," Fortin told NBC News. "She is almost 18. [Her birthday is nine months away]. If she was 18, I don't think this would be an issue. She is not 10. She is over 17. She is very bright, very smart."

To that point, Fortin's attorney, Michael Taylor, argued before the panel Thursday that Connecticut laws allow teens under age 18 to drive and to donate blood.

And when it comes to a minor's ability to make decisions about their own bodies, Taylor argued that both Connecticut courts and the U.S. Supreme Court have previously ruled that a person's right to "common-law bodily integrity is a fundamental right," and that right "exists in the minor the same as it could in an adult."

What's more, if Cassandra is found to have reached an adult level of maturity, that should preclude state child-welfare workers from retaining temporary custody of the 17-year-old and allow the girl to make her own medical decisions, Taylor argued.

"If a person is sufficiently mature, if Cassandra is sufficiently mature, then that state interest evaporates. Her fundamental right remains. … Then, the state has no interest in undermining her fundamental rights," Taylor said.

After Cassandra was removed from her mother's home in December by state child-welfare workers, she was admitted to Connecticut Children's Medical Center (CCMC) in Hartford. The teen's cell phone was taken away and hospital staff pulled the land line out of her room, blocking communication between daughter and mother, court records show.

Since early December, she's been undergoing chemotherapy. Assistant attorney general Tucker told the panel "the child is doing remarkably well" amid those treatments.

"And I would note that the child is mid-treatment here," Tucker said. "To interrupt that treatment would be devastating, even more devastating than delaying the treatment in the initial instance.”

Starting, stopping and then, later, re-starting chemotherapy for Hodgkin lymphoma can transform the disease from a highly curable illness to far more lethal disease, Dr. Mitchell Smith, director of the lymphoid malignancy program at the Cleveland Clinic, told NBC News.

"If she’s been in the middle of treatment and then stops and then the disease comes back, that’s harder because a lot of times what (cancer) cells that will then come back are now resistant to the chemotherapy," Smith said. (He is not involved in Cassandra's care).

"Then you’re talking about, if you want to cure the disease, you have to go through stem-cell transplant," Smith said. "You (would then) have to go through even more intense treatment with more risks to try to get rid of it."

Bioethicist Arthur Caplan believes Cassandra, at 17, should not have the right to make a life-and-death medical decision.

"The primary goal in this case is to save a young life," Caplan wrote in an essay for NBCNews.com. "This is a disease where medicine can do that. Admittedly, the treatment sucks, but it works ..." Caplan is founding head of the division of bioethics at NYU Langone Medical Center.

"Respecting choice is important," Caplan wrote. "Not burying a young teenage girl who would have lived is far more important.

-- Christiane Cordero and Stephanie Gosk contributed to this story.