National medical hot lines, cancer societies and many doctors and clinics are being swamped with calls about colon and rectal cancer after the news that President Reagan has contracted the nation's second leading cancer killer.

"All of us in colorectal surgery around the country are finding our phones are ringing off the hook," said Dr. Norman Nigro of Detroit, secretary of the American Board of Colon and Rectal Surgery.

"There is good coming out of misfortune, because this is a cancer we should be curing more than we do."

At the District of Columbia Division of the American Cancer Society, for example, 65 self-test kits were requested yesterday. "Usually, we get one or two [requests] a week, and those are from people who have used them before," said Elena Fletcher, a spokeswoman for the society. "The reaction has been tremendous."

The massive attention focused on the disease is long overdue and is helping to dispel myths and embarrassment associated with it, according to Dr. LaSalle D. Leffall Jr., a surgeon at Howard University Hospital and the first president of the American Cancer Society's national task force for colorectal cancer, which has been trying with minimal success to focus attention on the disease.

"We haven't been able to get people to talk about this cancer," said Leffall. "They think 'stool, rectum, that's dirty. I'll ignore it.'

"As a result, we're only curing two out of five people with it and we should be curing four out of five people."

One reason people do not seek out screening tests is the unwarranted fear that colorectal surgery means they will need a colostomy, a procedure in which wastes are collected in a bag attached to the body.

"In a Gallup Poll we had done, it was the fear most often asked about colorectal surgery," said Leffall. "The truth is only one out of 10 people need a colostomy. Earlier detection and newer techniques for resections mean patients don't have to wear that bag."

The reticence of people to deal with this cancer is such a problem that a 1976 film about the ailment is titled, "The Cancer Nobody Talks About." Leffall noted, "Before this, we've had trouble getting television to run our public service announcements. They tell us, 'You can't cloud my nice program by talking about stool and rectum.' "

But Reagan has changed all that. As with Betty Ford and Happy Rockefeller's widely publicized experiences with breast cancer, the attention focused on the president's illness is causing more people to seek information and treatment, doctors said.

Leffall has received at least 150 calls in two days from people seeking information on colorectal cancer or appointments for exams.

"The myths are fading," he said. "Women think it's a disease of men. Men think it's a disease of women. The truth is this is a truly equal opportunity disease." Blacks and whites also are affected by the disease in almost equal percentages, he said.

"The ripple effect is taking place," said Dr. Barry Smith, of Greater Southeast Hospital, who helped run a special screening for colorectal cancer for six days in April at his hospital, Howard University and George Washington University Medical Center in which 106 people were checked.

"If we ran that screening again, the response would be phenomenal."

Irving Rimer, director of public relations for the American Cancer Society's headquarters in New York, noted, "A year ago, we gave a story board to one of the networks. It had the words "stool blood test" and was turned down for being in bad taste. That isn't happening today. This tremendous interest focused on colorectal cancer has been a godsend."

Many of the nation's 17 cancer hot lines report a big surge in calls concerning colorectal cancer in the last two days. "We had 115 calls yesterday with people asking about it," said Betty Barbour, a spokeswoman for the American Medical Center in Lakewood, Colo., which answers calls throughout the state.

The Cancer Information Service, which covers the state of Minnesota, logged 80 calls regarding colorectal cancer yesterday, according to Michael O'Hara, a spokesman for the Mayo Clinic. Usually, two of the 40 calls the hot line receives daily deal with that type of cancer.

The National Cancer Institute in Bethesda received 75 calls from the Washington area yesterday concerning colorectal cancer. Prior to the news of the president's illness, an average of seven calls a day were about colorectal cancer, said Pat Newman, an NCI press officer, who added: "We're watching the stock on several publications we have on colon cancer."

Harriette Gibson, executive director of the American Board of Colon and Rectal Surgery in Detroit, said several of its 1,300 surgeons reported that they are scheduling more checkups and that patients were requesting the names of manufacturers of the over-the-counter self-tests.

"We scheduled 50 people for appointments yesterday," said Carmen Morrison, appointments secretary for the Cleveland Clinic's five colorectal surgeons. "Thirty is normal. A lot of former patients are calling who have missed their yearly checks. Since they've had polyps in the past, they want to ease their mind.

"One man said his case was like President Reagan's, but since he now knew it didn't have to be all bad, he'd be crazy to let it go," Morrison said.

Rimer, of the American Cancer Society, said that future publicity on the president's checkups will help ensure that the awareness of the disease "will not fade in a month." He added, "Citizens have been sensitized that they should ask for the exams yearly if they're over 40. That's going to boost detection for a good long time." Information on colorectal cancer can be obtained from the American Cancer Society, 1825 Connecticut Ave. NW, Suite 3l5, Washington, D.C. 20009. Telephone is 232-3309 from 9 a.m. to 5 p.m., Monday through Friday. Since 1976, the society has been mailing free self-test kits to metropolitan Washington residents to check for stool blood.