Lewis was diagnosed with acute lymphoblastic leukaemia when he was just six years old, before fortunately making a full recovery in 2013.
"My darling son who had childhood leukemia now has long-term effects check-ups… (this may sound ominous to civilians but to those of us who have trod the unpaved cancer road, it is a fraught blessing),” Kate captioned her post.
"Anyway, today Lewis is getting his thyroid ultrasound to check that seeds from previous radiation have not enlarged… we hope they have not — but Lewis certainly has. My lanky boyo is now too long for the bed. Praise be."
The 56-year-old then shared “blessings and strength” to other people affected by cancer.
In 2020, Kate revealed Lewis nearly died during his heartbreaking cancer battle.
"Today is an important day. It is my eldest son's birthday; 17, which seems like a miracle because in 2009, when he was six, Lewis Lewis nearly died," she wrote on Instagram at the time.
"He had leukaemia and it was a gruelling, terrible, frightening and painful three and a half years before he returned to health. Today he is tall and strong and healthy and ALIVE and we give thanks; to doctors and nurses and friends — and to those who came before us."
The family moved to Italy in January 2019, with Kate saying her son's diagnosis made her realise just how short and precious life is.
"For so long we couldn't go anywhere, we couldn't travel anywhere — I mean we went to the hospital, we went home," she said in 2019.
"There seemed something really life-affirming about having an adventure together as a family."
Kate and her husband Peter Allen Lewis, along with Lewis and their three other kids, Sunday, 16, Artie, 13, and Jannie, 12, made the move from Melbourne to Bologna, Italy.
Sadly, when the COVID-19 pandemic swept across the globe in early 2020, Kate and her family were forced into lockdown before eventually returning to Australia.
"It came upon us like a tsunami, really. [Italy was] only the second country in the world to go into lockdown, after China, so it was frightening at first,” Kate said at the time.
“But, after a while, when it became apparent lockdown was going to be longer than two weeks, we kind of relaxed into it.”
Despite their new adventure being cut short, Kate said she still looks back fondly on her family's time in Italy.
“It was so beautiful to us, we love it - and it felt like the country loved us back. There is a lot to be learnt from Italians - their love of family, their love of food, the way they take time to rest. The way they make everything beautiful.”