The former 'Beverly Hills 90210' star was diagnosed with the disease in March and disclosed she will undergo a mastectomy in a new lawsuit against her former management and accounting firm, Tanner, Mainstain, Glynn & Johnson, who she is suing for allegedly failing to pay her SAG [Screen Actor's Guild] medical insurance without her knowledge.
In court documents, obtained by gossip website TMZ.com, the 44-year-old actress, who is receiving chemotherapy, accused the firm of ignoring an invoice to pay her insurance premium in early 2014.
The 'Charmed' star re-enrolled in the program this year shortly before doctors discovered she has "invasive breast cancer metastatic to at least one lymph node."
Her medical team believe the cancer spread last year and would have been easier to treat if it had been detected earlier, but she failed to get a check-up at the time due to her lack of insurance.
Shannen is seeking compensatory and restitutionary damages.
Tanner, Mainstain, Glynn & Johnson plan to "aggressively defend" themselves in court.
Responding to her allegations in a statement, the firm said: "Tanner Mainstain is saddened to learn that Ms. Doherty is suffering from cancer and wishes her a full recovery. However, the claim that Tanner Mainstain caused her to be uninsured, prevented her from seeking medical care, or somehow contributed to her cancer is patently false."
Despite her health issues, the brunette beauty told PEOPLE.com she is determined to remain positive.
She said: "Yes, I have breast cancer, and I am currently undergoing treatment. I am continuing to eat right, exercise and stay very positive about my life. I am thankful to my family, friends and doctors for their support and, of course, my fans who have stood by me."