A Connecticut court is set to weigh whether a 17-year-old girl with life-threatening cancer can refuse a treatment she views as "poison." The case tests a core issue of civil rights and modern medicine: At what age do we have legal control of our bodies?

The adolescent — called only “Cassandra C.” in court papers — made the stunning demand that she receive no more chemotherapy to treat her cancer, Hodgkin lymphoma. Her survival odds are 80 to 85 percent with chemo but she will die without it, her doctors have said.

Diagnosed in September, Cassandra is being treated against her will at Connecticut Children’s Medical Center (CCMC) in Hartford. She’s told her oncologist she does not want to be exposed to the “toxic poisons” of chemotherapy, court filings show.

Her mother, Jackie Fortin, fully supports Cassandra’s choice.

"My daughter does not want poison in her body. This is her constitutional right as a human being," Fortin told NBC News on Wednesday. "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. She knows what the poison can do. She knows what the effects can be long term for her body."

Cassandra also knows, her mother said, what happens if she doesn't receive the chemotherapy.

"Does she know she will die? Yes. And do I know that? Yes ... In my heart, I feel when the timing is right, when Cassandra feels right, or if she starts getting sick, she will come to me and say, 'Mom, I am ready to do chemo.' Right now, she is backed up against a wall and forced to do something (after) she said, 'I don't want poison in my body.' "

But Fortin’s behavior — including a string of skipped, refused or prematurely stopped medical appointments and tests involving her ill daughter — ultimately convinced child-protection investigators to temporarily take custody of Cassandra in December, removing her from Fortin’s home. Agents also seized the teen’s cell phone. She is now staying in a constantly monitored room at CCMC, the hospital where she’s receiving chemotherapy, court records show.

On Thursday, before the Connecticut Supreme Court, lawyers representing the state and the Fortin family are scheduled to offer their arguments for and against Cassandra’s continued treatment.

Each side is waging its legal battle along a single front — the girl’s competency. In short: Is Cassandra mentally fit to make a life-and-death decision now, nine months shy of her 18th birthday, the day she legally becomes an adult?

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“If you look at any state’s laws, and certainly Connecticut’s, there’s a whole wealth of laws that give kids under 18 certain rights,” said Joshua Michtom, an assistant public defender in Hartford who is representing Cassandra. (The Connecticut Department of Children and Families, DCF, has legal custody of the teen and is not making her available for interviews, an agency spokesman said.)

“They can consent to sex with someone who’s near an age to them. They can get contraception. They can get addiction treatment. They can donate blood. They can be tried as adults for certain crimes. So there’s recognition overall that maturity doesn’t happen overnight. You don’t go to sleep a 17-year-old knucklehead and wake up an 18-year-old sage,” Michtom told NBC News. “The law as recognized that in a lot of ways. We’re just asking that it recognize it in this other way.”

But Connecticut Attorney General George Jepsen has raised questions about the competency of both Cassandra and her mother in a recently filed court brief.

Jackie Fortin, who brought up Cassandra on her own following a divorce, failed to bring her daughter to at least four doctor’s appointments last summer and fall —two months before a noticeable lump in the teen’s neck finally was diagnosed as cancer, court records show.

Ultimately, a team of doctors formed to treat Cassandra’s cancer became concerned with Fortin’s behavior, worrying “that Cassandra's mother was not coming in and getting the appropriate medical … care for this … malignant disease which is lethal if it's untreated,” according to the brief filed by Jepsen. “…Interestingly enough, Cassandra also expressed concerns about not wanting to anger her mother because her mother was very distrustful of physicians.”

Doctors eventually notified child-welfare agents at DCF about their growing concerns. And during at a court hearing on the matter late last year, a judge observed “how closely (Cassandra) followed her mother's testimony and hung on her every word. The DCF investigations worker (also) testified … that Cassandra's mother did not appear to be in support of the chemotherapy and that Cassandra is concerned about going against what her mother would like to see happen,” Jepsen’s brief states.

Meanwhile, Jackie Fortin seethes at the state's removal of her daughter, calling it "a slap in the face." For 17 years, she said, they have never been apart, until now. In addition to the removal of Cassandra's cell phone, the hospital staff also took out the room phone. Fortin visits her daughter - she sat with her at the hospital on Monday - but a child-welfare worker remains in the room, monitoring those moments.

"I am not coercing her at all and that is what this is about, what they think I am doing," Fortin said. "I can't call her. I can't talk to her, (no) Facebook, nothing, We are totally separated."

During oral arguments to be made Thursday, Cassandra’s public defender will request that the case be returned to a trial court for a hearing on the teen’s legal competency to decide against further treatment.

“The goal is not to force chemo but to figure out a way to get her to agree to take it,” said Arthur Caplan, founding head of the division of bioethics at NYU Langone Medical Center and a frequent NBC News contributor. “Giving chemotherapy to a 17-year-old who doesn't want it is no picnic. It would be very hard to do it physically."

Doctors and social workers at the hospital should be working to open lines of communication with Cassandra by negotiating with her, and ultimately building her trust, Caplan said. One option, might be to tell her the chemo treatments will spaced apart more widely than usual so they’re not as hard on her body – although such a schedule wouldn’t be as good for her recovery.

"The aim of the thing is not to have her run away, to lie there peacefully when she gets the stuff,” Caplan said. "And you want her to come back later, too. The whole point of this is: No wars.”

Among oncologists, Hodgkin lymphoma is widely considered "one of the great success stories for cancer treatment because we can cure a high percentage of the cases," said Dr. Mitchell Smith, director of the lymphoid malignancy program at the Cleveland Clinic. (He is not involved in Cassandra's case).

Those outcomes depend mainly on the stage of the disease when it's detected. The stage also helps doctors map out the most effective treatment course, which could consist of chemotherapy alone or chemo plus radiation. The standard course of chemo for Hodgkin lymphoma spans four to six months.

"I would say, overall, 80 percent is probably a reasonable estimate of how many patients are cured," Smith said.

"It would be higher if you're early stage. ... The treatment's very good. It's all outpatient, usually well tolerated. Yes, patients lose their hair. Yes, there are risks to it. But in young, healthy patients, the vast majority of them get through it and go ahead and live essentially normal lives."