New Delhi: Actor Irrfan Khan revealed on Friday that he has been diagnosed with a neuroendocrine tumour, a condition that doctors say can silently creep up on patients without warning and display a range of behaviours - slow-growing and indolent to fast and aggressive.

Khan announced the diagnosis through a post on the microblogging site Twitter, prefacing it with a quote from Margaret Mitchell's epic Gone with the Wind: "Life is under no obligation to give us what we expect."

"The unexpected makes us grow, which is what the past few days have been about. Learning that I have been diagnosed with NeuroEndocrine Tumour as of now has been admittedly difficult, but the love and strength of those around me and that I found within me have brought me to a place of hope," Khan wrote.

The post also indicated he is travelling abroad. "The journey of this is taking me out of the country, and I request everyone to continue sending their wishes," Khan wrote.

Neuroendocrine tumours originate in cells that resemble nerve cells but behave like endocrine, or hormone-producing, cells found in the colon, lungs, intestines, stomach, pancreas, as well as in the adrenal, thyroid and pituitary glands.

Symptoms, if any, are likely to depend on the site where they have emerged, doctors said. For example, neuroendocrine tumours in the gastrointestinal tract could lead to discomfort or pain in the abdomen, nausea, vomiting, diarrhoea among others. Pancreatic neuroendocrine tumours may lead to diabetes, low blood sugar, gallstones, stomach ulcers and diarrhoea.

Doctors consider neuroendocrine tumours as rare, though amid evidence for an increase in their incidence in the country over the past decade.

"The eyes don't see what the mind does not know - neuroendocrine tumours fit this line," said Shailesh Shrikhande, professor and head of gastrointestinal surgery at the Tata Memorial Hospital, Mumbai. "Most such tumours are picked up incidentally during other investigations."

Malignant neuroendocrine tumours are broadly classified into three grades - I, II and III. Patients diagnosed with grade I tumours have the best chance of surviving the longest, even 15 or 20 years after the diagnosis, specialists in the field say. Patients with grade II and grade III have progressively poorer prognosis, the doctors said.

"Surgery is the first option - if the tumours can be removed," said Sachin Daga, a gastrointestinal surgeon at the Krishna Institute of Medical Sciences, Hyderabad, who has seen about 30 patients with neuroendocrine tumours over the past decade. "If the tumours have spread, the options are chemotherapy and targeted therapy," Daga said.

Khan has not provided information about the site or the extent of his tumour. He has also not said if it is malignant.

"What is most important is a good biopsy from specialised pathologists who're able to grade the tumour, and later the use of modern nuclear imaging to assess the spread and stage the tumour," Shrikhande said. "This will determine the course of treatment."

Medical oncologists familiar with the management of neuroendocrine tumours say a centre in Uppsala, Sweden, and one in Berlin, Germany, are considered among the world's top centres with the greatest experience with these tumours.

But comprehensive treatment services are also available in India at centres such as the TMC, Mumbai.

"For some reason, we're seeing more patients with neuroendocrine tumours now than about a decade ago," said Shrikhande, a leading principal investigator of a study spanning six cancer centres in five cities across India that has detected an increase in numbers of patients.

The study found that in these hospitals, the numbers collectively rose from 37 patients between 2001 and 2005, to 136 patients between 2006 and 2010, to 235 patients between 2011 and 2016.

The pancreas was the most common primary site of the neuroendocrine tumours in each of the three groups of patients, the doctors said in their report published in the Indian Journal of Gastroenterology.