BBC's Mark Lowen how patients are being taken to a hotel to be examined, with the most urgent cases being sent to hospitals in Poland and across Europe.
"They too are the fighters of a debilitating disease made deadlier by the war they fled," Lowen said. "Ukraine's young cancer patients evacuated to Poland, victims of a battle on two fronts."
He also spoke to Alla, a mother concerned about her sick son: "Our hearts are tearing from pain. My son always says he's fine but we need to continue his treatment."
Another young boy Olexiy has a brain tumour is "weak from the journey," his older sister Lilia tells Lowen he's had three operations and chemotherapy "but he didn't complete chemotherapy because the war started."
It's the next day where children are waiting for triage, as Lowen explained: "This is just a desperately sad situation, children who are safe from the war have now plunged back into danger with their illness, rushing to resume treatment that's been broken off. This is a conflict that is killing, uprooting and now threatening the most vulnerable."
This one broke our hearts: Ukrainian children cancer patients fleeing their hospitals and evacuated to Poland. Vital chemotherapy broken off by Russia's bombs. It is unspeakably cruel. Our latest for #BBCNewsTen with @FrancescoTosto6 @MichaSteininger and Dominika Zurawska.pic.twitter.com/EAXqNU3wx0
There is just one "overwhelmed" doctor checking patients, Dr Anna Zelwianska described this as a "very serious situation."
When asked if there are patients who she thinks could die as a result of the war interrupting their treatment, Dr Zelwianka replied: "Yes, most of these kids are in a life-threatening situation and any kind of delay in treatment is very bad for them.."
One of the patients being checked over is 12-year-old Andrii Kuz whose soft tissue cancer has spread to his lungs. He and his family managed to get out of Ukraine, in what is Andrii's first time abroad.
"I miss my home, and my grandma and grandpa," he tells Lowen. "At least here we don't have to run into shelters because that was scary and hard."
The report ends showing volunteers taking some of the children to a hospital in Germany, and as those children left more arrived.
"The cruelty of one man's war forcing them to battle," Lowden concluded.
This upsetting report has evoked people on Twitter, who expressed their sympathies for the child patient's plight.
— Gary Lineker \ud83d\udc99\ud83d\udc9b (@Gary Lineker \ud83d\udc99\ud83d\udc9b)
This really is heartbreaking. Imagine being so evil that you bomb a hospital that is treating children with cancer. There are no words.https://twitter.com/marklowen/status/1500744308512673793\u00a0\u2026
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