Less than two months before the FDA is set to make a politically charged decision on the use of Genentech Inc.’s best-selling cancer drug, Avastin,

 in breast cancer patients, a potentially influential voice has weighed in in favor of the drug.

To a point.

The National Comprehensive Cancer Network — a not-for-profit alliance of 21 top cancer centers, including the Stanford Comprehensive Cancer Center and the University of California, San Francisco’s Helen Diller Family Comprehensive Cancer Center — affirmed its guidelines for using Avastin in combination with the Bristol-Myers Squibb Co. chemotherapy agent Taxol to treat metastatic breast cancer.

NCCN’s guidelines are widely accepted by doctors, so its affirmation is largely a vote for Avastin.

That is especially important since the Food and Drug Administration last month delayed a decision on enlarging Avastin’s use by women with previously untreated advanced HER2-negative breast cancer. The agency now will decide by Dec. 17 whether to approve Avastin in combination with the chemo agents anthracyclines or capecitabines, and it could decide at any time whether to rescind its conditional approval of Avastin with Taxol, also known as paclitaxel.

But the NCCN also said Monday that its panel revised the footnote to its guidelines. Although it is unclear what exactly was changed — an NCCN spokeswoman wasn’t available late Monday and a Genentech spokeswoman referred questions to the network — the guidelines now say that Avastin in combination with some chemotherapy agents “modestly improves time to progession and response rates but does not improve overall survival.”

The NCCN said the impact of Avastin on progression-free survival — the time during which the cancer does not worsen — may vary but appears greatest in combination with weekly paclitaxel.

That doesn’t sound like a glowing recommendation to widen Avastin’s use.

An FDA advisory panel voted in July to recommend that Avastin’s use with paclitaxel be rescinded and that it not be approved in combination with anthracyclines or capecitabines. It said data, including that from two trials after Avastin was conditionally approved in February 2008, hadn’t shown that the drug improved progression-free survival.

The FDA’s conditional approval came after an earlier advisory panel narrowly voted to recommend that Avastin not be approved for breast cancer patients.

The July advisory panel’s decision put Avastin square in the middle of the health-care debate, as some conservatives claimed the advisory group’s recommendation was an example of a “death panel” picking who should live and who should die. Some breast cancer patients say Avastin has saved their lives and have led grassroots efforts to keep or extend Avastin’s approved use.

Some other breast cancer researchers, however, said the panel’s recommendation was scientifically solid because Avastin hadn’t shown a significant-enough impact in metastatic breast cancer patients, or those who have seen the cancer spread to other parts of their body.

Avastin costs about $88,000 wholesale. It is approved to treat types of colon, lung, breast, kidney and brain cancers.