Shannen Doherty's Breast Cancer Had Spread Before Treatment

Shannen Doherty’s breast cancer had spread before she began treatment, the former "Beverly Hills, 90210" star says.

The actress, 45, spoke to “Entertainment Tonight,” saying, "I had breast cancer that spread to the lymph nodes, and from one of my surgeries we discovered that some of the cancer cells might have actually gone out of the lymph nodes. So for that reason, we are doing chemo, and then after chemo, I'll do radiation."

She had a single mastectomy in May, she said, also sharing a story of how her mother was there to support her when she got fitted for a new bra after the surgery.

"I didn't think anything of it at the time. Then my mom went with me and I broke down crying in the dressing room and ran out and then sat in the car crying," she said.

Her mother, Rosa, has been with her through every step since the early-2015 diagnosis. There's a touching photo of the two women embracing while the actress is cutting off her hair.

"The unknown is always the scariest part. Is the chemo going to work? Is the radiation going to work?" Doherty said. "Everything else is manageable. Pain is manageable, you know living without a breast is manageable. It's the worry of your future and how your future is going to affect the people that you love."

Doherty had been documenting the effects of treatment, including losing her hair. She posted a series of pictures showing her shaving her head to inspire others battling cancer, she told ET.

"I wanted to put it out there the way that it felt the best for me to put it out there," she said. "If I could help one person, it makes me go, 'OK, it's easier to live with cancer if I know I'm helping one person.'"

She continued, "I'm being so personal right now. But I think a lot of women can probably go, 'Yeah, I've been through that.'"

Doherty also said, in this new interview, that she lost 10 pounds after her first treatment.

"It's, you know, you're throwing up every single second. After my first treatment I lost 10 pounds, instantly. You're throwing up and the last thing you want to do is be in a car. You don't want to be moved, you can't eat," she said.

At the time of Doherty's cancer reveal last year, she also filed a lawsuit against her former business managers for allegedly failing to keep up on her insurance payments, according to the suit obtained by ABC News. The management firm has denied the claim and the suit is still ongoing.