ST. LOUIS (KTVI) — Jenna Fischer, the beloved St. Louis-raised actress well-known for her role as Pam on “The Office” television sitcom, shared a deeply personal health update on Tuesday.

“I never thought I’d be making an announcement like this but here we are,” Fischer wrote in an emotional Instagram post explaining that she was diagnosed with Stage 1 triple-positive breast cancer last December.

About 10% of breast cancer diagnoses are triple-positive, a subtype of HER2 positive, according to The University of Texas MD Anderson Cancer Center. Dr. Jason Mouabbi, a breast medical oncologist with the center, explains that when a patient has breast cancer cells that contain estrogen receptors and progesterone receptors (which can be used to fuel cancer growth) as well as an above-normal level of the HER2 protein, they are considered as having triple-positive breast cancer.

According to Fischer, a routine mammogram led to inconclusive results, which led to a breast ultrasound, and ultimately, her diagnosis. Fischer bravely described her journey, explaining that she had a lumpectomy in January to remove a tumor, followed by 12 rounds of weekly chemotherapy and three weeks of radiation.

“I’m happy to say that I was recently re-screened, and the treatments worked. I am cancer free. I will continue to be treated and monitored to help me stay that way,” said Fischer.

Fischer described the treatment as “aggressive,” though she credits her early diagnosis for saving her life. She explained that had the cancer gone undetected any longer, it could have spread to her lymph nodes and the rest of her body.

While triple-positive breast cancer and other HER2 positive breast cancers are aggressive, they are also considered the “most responsive to treatment,” according to Mouabbi.

Fischer also opened up about the physical and emotional toll of chemotherapy, explaining that she lost her hair and wore wigs and hats for a period of time. She says she is now ready to leave wigs behind.

Fischer went on to share that she hopes her story inspires others to take their health seriously and encourages women to get an annual mammogram, even if they don’t suspect a cancer diagnosis.

“I’m serious, call your doctor right now,” said Fischer. “My tumor was so small it could not be felt on a physical exam. If I had waited six months longer, things could have been much worse.”

Fischer expressed deep gratitude for her husband, children, and close friend and “The Office” co-star Angela Kinsey for their strong support throughout her battle. She also said she “will continue to be treated and monitored” to help remain cancer-free.

October is Breast Cancer Awareness Month.