On her blog, Christie Hoos says she was sitting at her daughter's bedside in the pediatric oncology clinic at BC Children's Hospital when she got a message on her phone from a friend who recognized her child in the ad.
The image had been featured on a massive banner on the side of a building in Madrid by the Swiss-based company Genoma to promote its Tranquility test kit for Down syndrome and other chromosomal disorders.
A Spanish parent who spotted the ad snapped a photo of it and posted it online. It then spread around the world on social media before reaching Hoos at the clinic in British Columbia, where her 10-year-old daughter is being treated for leukemia.
"When I saw with my own eyes her sweet face on that ugly banner, it broke my heart. While my girl courageously fights for her life, this company questions whether she has a life worth living. How dare they?!" Hoos wrote in her blog post.
"They insulted and abused my innocent child in their pursuit of profit. They broke faith with common human decency. And the world is watching."
Images removed by company
Hoos says she contacted the company, and it eventually removed the banner and the image from its website.
The company apologized and told Hoos that it believed the image was obtained legitimately from a stock image website, according to reports.
Hoos alleges the stock image website, which is based in Germany, took the original portrait of her child from her blog after she posted it in March.
"Her photo was stolen. A beautiful shot of her face — one of my favourites, posted on a stock photo website and distributed for free."
While Hoos says at first she felt guilty about putting her daughter's picture online, she now realizes she did nothing wrong.
"Where initially I considered taking all our photos offline, deleting my social media accounts and hiding in my house for the next 10 years, now I'm determined to weather the storm.
"We will not flinch. We will not hide. My daughter is beautiful and her life is worth celebrating."
She says she and her husband are consulting a lawyer about legal action.
Christie says her daughter is in her eighth month of chemotheraphy, with 19 more months to go, and the recent events "have been particularly brutal."
"Every small setback takes a toll, but she doesn't let it keep her down for long. She's tough. Tough and sweet and feisty, and a thousand unique qualities all her own. She is the joy of our life."