The former BBC Breakfast host, 51, told Woman & Home magazine of how she was diagnosed with the illness a week after her 50th birthday.
'I thought I was healthy. I did all the right things – I was a green tea drinker, a salmon eater, a runner,' Sian explained.
'So being told I had cancer seemed so improbable that when I went for the results of a biopsy, following a mammogram, I didn't even ask Paul to come because I just assumed it was routine.'
Sian, now the main presenter of Channel 5's 5 News at 5, said that her biggest fear throughout her cancer journey was 'not being there as a mum', adding, 'for some unfathomable reason I couldn't stop thinking that I want to be here for my daughter Evie to watch her get married'.
Sian pictured with her BBC Breakfast co-host Bill Turnbull
She kept her diagnosis a secret, and admits that initially, she even struggled to open up to her husband Paul about how she was feeling.
'I was horrible to Paul at times because I was so intent on being strong that I didn't feel I could share my fears with him,' she said. 'He never knew if I was 'strong capable Sian' or if I needed help, which was very confusing for him. And we had some really explosive moments and furious rows as a result.'
'Paul is an extraordinary man. I've learned I need to let him know if I need support, or an outstretched hand to help me up.'
'I discovered there's a strength in letting myself be vulnerable. I allowed myself to say, 'Today I feel less like a TV presenter and more like a cancer patient – and that's OK'.'
Sian, who also has three sons from a previous marriage, underwent treatment, and had a double mastectomy followed by reconstructive surgery, although 'nobody knew I'd had the operation apart from the children's teacher'.
Now, after her operation, Sian says she's more 'forgiving of her body'.
'I have a caesarean scar, and these scars are just like that; scars you carry from previous experiences,' she says. 'In the same way you have to acknowledge previous traumas, you have to acknowledge that scars are a part of your body.'
'I look at my body and I think, 'You're bloody lucky you're here, Sian'.'