Koirala left for the US last week for treatment of his lung condition.
"It is unfortunate to say that Prime Minister Koirala has been diagnosed with lung cancer. Upon the recommendation of doctors at Memorial Sloan Kettering Cancer Center in New York, he will undergo radiotherapy," doctor Kabir Nath Yogi was quoted as telling Kantipur online.
Koirala, 74, had admitted before leaving for the US that he used to be a chain smoker, and chewed pan and gutka in his younger days.
Koirala used to be a chain smoker and chewed pan and gutka in his younger days.
"The doctors at Sloan Kettering told us that after receiving radiotherapy, it is expected that the prime minister's health condition will improve," Yogi said.
According to a press statement released Thursday by Nepal's Permanent Mission to the UN, doctors involved in Koirala's treatment have advised him to go for radiotherapy for treatment of the localised lesion of the lung since his chronic bronchitis does not permit a major surgery of the lung.
The radiotherapy will be done Friday.
Koirala is in the US since June 16 for treatment after a suspicious black spot was seen in one of his lungs in a CT-scan test conducted in Kathmandu.