Two Japanese boys are thought to have developed lung cancer after “breathing in” malignant cells from their mothers’ cervical tumours in the birth canal.
An unnamed 23-month-old boy, whose mother was diagnosed with cervical cancer three months after his delivery, was taken to hospital when he endured a cough for two weeks.
Doctors later diagnosed the toddler with lung cancer.
An unrelated case resulted in a six-year-old boy being taken to hospital with chest pain, only for a scan to reveal a 6cm (2.3inches) mass in his left lung.
His mother had a cervical tumour that was thought to be benign at the time of his birth, however, she died of cancer two years later.
Read more: Firefighter believes job triggered cancer
Genetic analyses revealed the boys’ cancers matched the tumours of their mothers, a phenomenon one expert had never heard of.
Both boys are alive following treatment.
“In our cases, we think tumours arose from mother-to-infant vaginal transmission through aspiration of tumour-contaminated vaginal fluids during birth,” said Dr Ayumu Arakawa, the lead author of a case report into the incidents.
The transmission of cancer from a mother to her child is said to be a very rare event, affecting just one newborn for every 500,000 mothers with the disease, the scientists reported in The New England Journal of Medicine.
For context, a mother has cancer in around one in every 1,000 live births, they added.
Previous known cases have involved malignant cells travelling across the placenta into the still-developing foetus, triggering leukaemia, lymphoma or melanoma.
Read more: Vitamin D may ward off advanced cancer
The two boys are thought to be the first incidents where newborns appear to have developed lung cancer after breathing in malignant cells from cervical tumours.
“I found it fascinating, personally,” said Dr Debbie Saslow from the American Cancer Society.
“I didn’t know this was possible.”
The 23-month-old had “spontaneous regression” of some of his cancerous tissue. He was later treated with the immunotherapy drug nivolumab, leading to “strong regression of all remaining tumours”.
It is unclear how the six-year-old was treated, however, his tumour is said to have been slow growing.
Watch: HPV vaccines curb cervical cancer risk
The sexually-transmitted infection human papillomavirus (HPV) is behind 99.7% of all cervical cancer cases in the UK.
Women aged 25 to 64 who are registered with a GP are automatically invited for a cervical screening, or smear test.
This involves a small sample of cervical cells being collected to test for “high risk” HPV variants.
If found, these variants can be treated before HPV triggers cervical cancer.
Girls and boys aged 12 and 13 in the UK are also offered the HPV vaccine.
“I think it's interesting this study was from Japan, where they’ve had a lot of backlash against the HPV vaccine and they saw vaccination rates plummet because of unfounded concerns,” said Dr Saslow.
“I also know Japan has had particularly low cervical screening rates.”
At the time of the boys’ birth, neither mother was known to have cancer.
“The first patient had a negative pap smear [cervical screening] and the second had a cervical mass but it was thought to be benign,” said Dr Shannon Neville Westin from the University of Texas MD Anderson Cancer Center.
“I don’t know the obstetricians would have done anything differently based on the information they had.”
Genetic testing later linked the boys’ tumours to those of their mothers.
“If we hadn’t been able to test the tumours from the mother and the infant, you never would have known those were truly related,” said Dr Westin.
“Because they determined that they were, they were able to direct therapy in a way that was very successful for the infants.”
Going forward, the scientists recommend pregnant women with cancer seriously consider having a C-section to reduce the risk of passing the disease to their baby in the birth canal.
“Mother-to-infant transmission of tumour may be a risk of vaginal delivery among women with cervical cancers,” said Dr Arakawa.
“Cesarean section should be recommended for mothers with uterine cervical cancer.”
Not everyone agrees, however, with Dr Westin arguing the evidence is too inconclusive to change birthing practices.
“If we are diligent about testing more of these infants with cancer, we may be able to move forward and change practice, and say every patient with cervical cancer should have a Cesarean section,” he said.
“We just need to gather up that data to be able to change the practice.”
Dr Westin accepts, however, a lack of cases makes the research difficult to carry out.
“Only about 1% to 3% of all women with cervical cancer are pregnant or postpartum at the time of diagnosis,” he said.
“The incidence of cervical cancer ranges, but it’s around 12,000 a year [in the US].
“You’re really selecting out such a tiny group of patients to even begin with.”
Watch: Future could be cervical cancer free