BOCHENIEC, Poland (AP) — Twenty-two-month-old Yeva Vakulenko had been through four rounds of chemotherapy for leukemia at a hospital in Ukraine, and then suffered a relapse. As she began returning once again for more treatment, Russia invaded, disrupting doctors' efforts to cure her.

Air raids forced the toddler to shelter in a basement for hours at a time, making her feel even worse. She cried a lot and sought comfort from her grandmother, who is caring for her after her parents were in an accident that left her mother disabled with brain and leg injuries.

So when doctors told Yeva's grandmother that they could evacuate to Poland, she seized the chance.

“It is very difficult for children to go somewhere in the middle of the night and sit in the basement for a long time," said Nadia Kryminec as she held her granddaughter, whose sweet-natured smiles gave no hint of the ordeal she has endured.

“We were told that she was in stable condition and we should try to go. Otherwise, she is simply doomed to death,” the grandmother said.

The little girl, who her grandmother says is intelligent and understands everything, is one of more than 500 Ukrainian children with cancer who have been evacuated to a clinic in Poland. Doctors then place them in one of some 200 hospitals in 28 countries.

“We triage the patients when they arrive at our center," said Dr. Marcin Włodarski, a pediatric hematologist at St. Jude Children's Research Hospital in Memphis, Tennessee, who is volunteering at the Unicorn Clinic of Marian Wilemski in Bocheniec, in central Poland.

Stable patients are transferred quickly from there to hospitals in other countries while those in worse condition are first stabilized in Polish hospitals, he said.

“Then they return to us and can be sent for further travel,” Włodarski said.

Decisions have to be made fast because time is critical for the young oncology patients.

The evacuations began immediately after Russia attacked Ukraine on Feb. 24, and is a joint effort of St. Jude, the Polish Society of Pediatric Oncology and Hematology, Poland's Fundacja Herosi (Heroes Foundation), and Tabletochki, a Ukrainian charity that advocates for children with cancer.

Dr. Marta Salek, another pediatric hematologist with St. Jude who is volunteering at the Polish clinic, said the center receives large numbers of patients and convoys that arrive from the western Ukrainian city of Lviv through humanitarian corridors.

"At times we can have convoys with only 20-something patients but we can have up to 70 patients at a time and even more,” she said.

At the clinic, a large bin of white unicorn stuffed animals sits in a room, along with a wooden train set, brightly colored balloons and other toys that the children happily play with.

More than 3 million people — more than half of them children — have fled Ukraine as the country faces a brutal military onslaught by Russian forces that has targeted civilians. Of those, more than 2 million people have arrived in Poland, the largest of Ukraine's neighbors to its west. A Polish health ministry official said Friday that the country is treating 1,500 refugees in hospitals, many of whom are suffering hypothermia after their journey, and 840 of whom are children.

The World Health Organization said Friday that cancer is one of the major health challenges resulting from the war. It said it was supporting the effort by the organizations who “are working against the clock to reconnect pediatric cancer patients with their treatments.”

"Cancer itself is a problem, but treatment interruptions, stress and risk of infection mean that hundreds of children might die prematurely,” said Dr. Roman Kizym, head of the Western Ukrainian Specialized Children’s Medical Centre in Lviv, where Ukrainian pediatric oncology patients are first stabilized before they are sent across the border into Poland.

“We believe that these are the indirect victims of this war,” Kizym said in a WHO statement.

Among those at the clinic this week were mother Anna Riabiko, from Poltava, Ukraine, who was seeking treatment for her daughter Lubov, who has neuroblastoma.

“Treatment is currently impossible in Ukraine. Fighting is taking place, there are no doctors, it is impossible to have surgery or chemotherapy. And even maintenance therapy is also impossible to obtain,” she said. “So we had to look for salvation somewhere.”

It's not a step that all parents were able to take for their sick children, she said.

“A lot of sick children stayed there,” she said. “Because parents were worried and did not want to go into the unknown.”

Gera reported from Warsaw.

Follow the AP’s coverage of the war at https://apnews.com/hub/russia-ukraine