In the emotional Instagram posts, the 45-year-old is pictured with her husband, photographer Kurt Iswarienko, whom she married in 2011, and a nurse named Dawn, who she says 'takes excellent care of me'.
Describing the process as having the 'red devil injected into me', Shannen reminds us that there are 'many faces of cancer'.
The actress, who is best known for her roles in Charmed and Beverly Hills 90210, was diagnosed in February 2015, and since revealed to news site Entertainment Tonight that the disease has spread.
'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,' she said during a recent interview. 'So for that reason, we are doing chemo, and then after chemo, I'll do radiation.'
Shannen added that she'd had a single mastectomy in May as part of her ongoing battle with the illness, but the hardest aspect so far had been not having any answers as to what comes next.
'The unknown is always the scariest part. Is the chemo going to work? Is the radiation going to work? You know, am I going to have to go through this again, or am I going to get secondary cancer? Everything else is manageable,' she explained.
'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.'
Shannen also recently made the choice to shave her hair, posting a photograph showing a picture of chocolates and a Remington hair trimmer, writing to her 322k Instagram followers, 'Cupcake pan, chocolates and a razor.... Stay tuned. @annemkortright has got this. #cancersucks #thankgodforfriends'
She then bravely uploaded a selection of pictures of herself, her mother Rosa, and friend, model Anne Marie Korkright, during the various stages of the process, from cutting the long strands to the final short crop.
Fans were quick to reach out to Shannen in the comments, with one saying, 'Take charge! I did the same thing after chemo round 2. Breast cancer survivor for 7 mos.', and another agreeing, 'Been there, done that. Sending positive thoughts and prayers. We survivors have a special kind of bond. Glad you're so open about all this.'
Speaking to Entertainment Tonight about the decision, she said, 'After my second treatment, my hair was really matted, like in dreadlocks. And I went to try and brush it out, and it just fell out.'
'I just remember holding onto huge clumps of my hair in my hands, and just running to my mom crying, like, "My hair, my hair, my hair, my hair." It was just shedding and it was driving me crazy.'
'We did stages,' she continued. 'We did a pixie. And then we did a mohawk, which was my favourite look. And then finally, we had to get the shaver thing and just buzz it off.'
During an appearance on The Dr Oz Show earlier this year, Shannen added that her treatment had impeded her starting a family, a goal which she had previously hoped to achieve.
'I'm 44 and my husband and I wanted children,' she said, explaining that she's on a medicine that prevents her from getting pregnant.
Referring to having a mastectomy in addition to drug treatment and chemotherapy, Shannen commented, 'They're just breasts. In the grand scheme of things, I would rather be alive and I would rather grow old with my husband.'
'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.'
She also urged other women to be aware of the symptoms of breast cancer and check themselves regularly to catch the illness early.
'You have no idea what that extra time might have afforded you,' Shannen said. 'Mine for instance spread to a lymph node or two. It might not have spread... The most important thing is catching it as early as humanly possible.'