Sen. Hubert H. Humphrey (D-Minn.), who Monday got the first in a series of radiation treatments to check the spread of cancer in his pelvis, has canceled plans to give a speech next Monday to the annual Minnesota AFL-CIO convention in St. Paul.

While Humphrey has not canceled other speaking dates in September and October, the outpatient radiation therapy at a University of Minnesota hospital has delayed his returns to Washington by a least two weeks.

Humphrey, whose inoperable cancer was found by University surgeons Aug. 18, expected then that he'd be able to resume his duties in Washington by mid-September.

His office said Monday there was now no specific date for the 66-year-old senator's return to Washington. There or for weekends of radiation treatments are now on his schedule, to be followed by anti-cancer drugs and related therapy.

When Humphrey, pale but smiling, was released Sept. 2 from the hospital, Dr. John Najarian, the hospital's chief of surgery who operated on the senator, said he hoped Humphrey would be able to return to Washington in one or two weeks.

On Monday Najarian described Humphrey as feeling fine after the therapy but added that nausea and sometimes other side effects could occur a day or more after the treatment.

The number of radiation treatments will depend, in part, on side effects. They will be followed by chemotherapy and immunotherapy in collaboration with experts at the National Cancer Institute in Bethesda.

When Humphrey walked out of the hospital Sept. 2 he weighed 156 pounds, three more than he weighed when he entered it two weeks before. In the past his optimum weight was considered to be 173 pounds.

Najarian has said that while the senator's cancer could be considered terminal because there is no known cure for it at this point, the treatment he is receiving is designed to control the cancer for as long as possible. As Najarian said after the operation on Aug. 18, "It could be months - it could be years."