Koirala, 75, flew to the US 10 days ago for medical treatment after his doctors advised him to have a thorough medical check-up in a well equipped hospital after black spots were found in his lung, a couple of weeks ago.
According to a medical statement by Nepal's permanent mission to the UN quoting his personal physician Kabir N Yogi, who accompanied Koirala to the Memorial Sloan Kettering Cancer Center in New York, the Prime Minister will receive radiotherapy for treatment of the localised lesion of the lung since his chronic bronchitis does not permit a major surgery of the lung.
Koirala, who underwent operation for his tongue cancer seven years ago, also had medical follow up in the hospital.
He is expected to return to Kathmandu in 8-10 days after completing the necessary medical tests, said sources close to the prime minister.