As the Pentagon seeks to trim spending, there are some programs Congress believes the military can’t do without. Among them: cancer research.

For almost 20 years, the Defense Department has been the recipient of more than $3.6 billion for cancer research. The programs have never been requested in any presidential budget, and are outside the Pentagon’s traditional mission of battlefield medicine and research.

Nonetheless, lawmakers, prodded by grass-roots lobbyists, have annually added in the money as part of the appropriations process.

With Congress now debating a spending bill for the remainder of this fiscal year — and both parties calling for budgetary restraint — lawmakers have shown no inclination to curtail this spending. The Senate Appropriations Committee recently added $250 million to the fiscal 2011 Continuing Resolution for cancer research.

Defense Secretary Robert M. Gates has said that the Pentagon, faced with mounting fiscal pressures, plans to cut projected spending by $78 billion over the next five years. He has also acknowledged that “not every defense dollar is sacred or well-spent.”

But the inclusion of funds on programs that are outside of the Pentagon’s core mission highlights the persistence of grass-roots organizations that have come to depend on the defense budget as a sacrosanct source of funding.

Spending on cancer research represents a tiny drop in the bucket of the Pentagon’s budget. But at a hearing earlier this month, Rep. Mac Thornberry (R-Tex.), chairman of the Armed Services Committee’s subcommittee on emerging threats, questioned whether aspects of the Defense Department’s medical research on cancer should remain a priority at a time of tightened budgets.

He noted that much of the research has a “tenuous connection to the warfighter or even our service people” and that funding had been “foisted upon the department by Congress.”

Even though cancer research “may not have been entirely for the military, it has had a great benefit, as have many of those kinds of efforts,” Marilyn Freeman, the Army’s deputy assistant secretary for research and technology, told Thornberry, noting that breast cancer is a “huge issue” for women in the military.

Nevertheless, when it comes to priorities for the fiscal 2012 budget, “we’re going to have to look at that hard,” she said, referring to the cancer research.

The Pentagon’s cancer research programs were initiated with a $25 million congressional earmark in 1992 for research on breast cancer. Since then, annual appropriations have increased, and the Pentagon’s Congressionally Directed Medical Research Programs have expanded to focus on prostate, lung and other kinds of cancers. Most of the spending has gone to individual grants managed through contractors.

The National Breast Cancer Coalition says the development of the breast cancer research program alone — the largest cancer research program funded through the Pentagon — has attracted more than 40,000 research proposals over the years. In 2009, the group noted, 214 members of the U.S. House and the 57 senators signed letters supporting $150 million for the breast cancer research program.

By contrast, the National Institute of Health’s National Cancer Institute spent $600 million on breast cancer research in 2009.

The NBCC is only one of a host of grass-roots organizations that have helped sustain the research. Of the $250 million appropriated in this year’s continuing resolution, $80 million is to go to prostate cancer research. That program is backed by an advocacy group called ZERO — The Project to End Prostate Cancer, whose senior vice president for public policy is a former staff member of the Senate Appropriations Committee.

In a memo posted on the organization’s Web site, the vice president, Kevin S. Johnson, described his experience with the group, saying that his “first couple of years with ZERO was spent fortifying our position on Capitol Hill through existing relationships that I built through the years of working there.”

Last year, after Gates first said reductions would have to be made in Defense Department health programs, Johnson received an e-mail from an officer of one of his constituent organizations. The officer expressed concern that the funds could be at risk and cited a Washington Post article focusing attention on the issue.

Johnson replied: “There are far too many votes for Members of Congress who DO control the funding of this program to not continue this funding — this is one of the reasons that Members who like to talk about waste, fraud and abuse who want to cut programs have such a hard time doing so — the political scaffold that supports these programs is often too complicated to bring them down.”

In an e-mail this week, Johnson said his organization has not spent a great deal of time lobbying for cancer research funding through the Pentagon. Indeed, while ZERO reported spending $230,000 on lobbying Congress in 2009, that figure dropped to $50,000 in 2010.

“We are preparing for the fiscal year 2012 process, which I see as a larger threat to the program,” Johnson said.

Back in 2002, Sen. Tom Harkin (D-Iowa) wrote to his constituents about how 10 years earlier he had put an amendment on the Defense Department budget calling for spending $210 million for breast cancer research. “This funding was in addition to the funding for breast cancer research conducted at the National Institutes of Health,” he wrote, “and — overnight — it doubled federal funding for breast cancer.”

Annually, the National Breast Cancer Coalition presents various members of Congress with an award for their support. It also maintains a “Public Policy Hall of Fame,” which includes Harkin.

In 2008, ZERO created its own award: the Golden Glove Award for Prostate Cancer. Nominated as a recipient of the first award was the late Sen. Ted Stevens (R-Alaska), who died in August in a plane crash.

A survivor of prostate cancer, Stevens was cited as having advocated for the addition of $80 million for the creation of the prostate cancer research program.