The doctor who brought conjoined twins from Europe to the United States to be evaluated for surgery says there's no longer a chance the 4-year-old girls will one day lead separate lives.

Last summer, doctors at Rainbow Babies & Children's Hospital in Cleveland attempted to separate Anastasia and Tatiana Dogaru, who are joined at the head. But even after that surgery was deemed too dangerous and called off, the craniofacial surgeon whose foundation brought the girls to Dallas as babies was hopeful that separation might still be possible.

But no longer.

"We have finally decided that it's in these girls' best interest that they remain like they are and that's really hard for me to say because I've been optimistic about separation," Dr. Kenneth Salyer, chairman and founder of the Dallas-based nonprofit World Craniofacial Foundation, told The Associated Press.

He said attempts to find other medical centers to take the case were unsuccessful after the failed Ohio attempt. Other complications arose as the twins grew older. One girl's brain is growing into the other's, making surgery impossible. Also, their brains' ability to recover from a separation surgery diminished with time.

"As they've gotten older and they've grown and developed — it's now too dangerous to separate the children," Salyer said.

The girls, who will be 5 in January, were born in Rome to Romanian parents. The top of Tatiana's head is attached to the back of Anastasia's, meaning the girls have never been able to look each other in the eye. Anastasia has no kidney function and relies on Tatiana's kidneys.

Twins born joined at the head — known as craniopagus twins — occur about once every 2.5 million births.

After their 2004 birth, doctors in Italy told the girls' parents, Claudia and Alin Dogaru, that nothing could be done for them. But the parents heard about the successful separation in Dallas a year earlier of Egyptian twins joined at the head. Through his foundation, Salyer had brought Ahmed and Mohamed Ibrahim to Dallas and was part of the team of surgeons that performed the 34-hour surgery.

So the Dogarus contacted Salyer and the girls came to Dallas in October 2004. Not long after arriving, Tatiana underwent heart surgery to fix a constriction of the main vessel of her heart — an ailment that would have been deadly.

After more than a year, Salyer said a plan was developed for a separation surgery. "I had nothing that told me at that time that we couldn't do it," he said.

Next was finding a hospital for the surgery and the foundation hoped to find an institution that could donate the majority of the costs, said Sue Blackwood, foundation vice president.

Salyer said the foundation first approached Children's Medical Center Dallas, where the Ibrahim twins were successfully separated. But Children's could not do the procedure as a charity case. As a not-for-profit, the hospital said in a statement that it can't absorb the cost of every complex surgery and still serve its regional community.

Another Dallas hospital also passed on the case, he said.

Finally, Rainbow Babies agreed to do the surgery as a charity case, Salyer said. The plan was for Anastasia to get dialysis after the separation and then get a kidney transplant, likely from one of her parents.

Even after the surgery was called off at Rainbow Babies last summer, Salyer didn't give up. He consulted three more centers across the U.S., but all three eventually decided against taking on the surgery again.

He said he told the girls' parents a couple of months ago that separation would be impossible. The family now lives in the Chicago area, where Alin Dogaru, a Byzantine Rite Catholic priest, has accepted an assignment at a parish. The parents declined to comment for this story.

The girls' future is uncertain because of their complicated connection. Besides their joined brains, they also share blood vessels and don't have enough venous drainage, Salyer said. "They don't have normal systems," Salyer said. "All of the medical issues in total, you can't say how these children are going to do," he said.

He said that based on today's medical capabilities, the girls cannot be separated.

"Nobody is going to go in there unless we get some new magical methods," Salyer said.

But he points out that the girls have already overcome many odds to become the smart, active girls they are today.

"They're troopers and they may be with us a long time — God willing," Salyer said.