CLEMSON, S.C. (AP) - For more than half a century, doctors and scientists have been intrigued with the idea that light might be a better way to detect breast tumors.

Clemson University researchers are working on a prototype machine that shines invisible laser beams through a woman's breast without ever touching it.

This and other projects worldwide have raised hope for an alternative to the squeezing, pressing X-ray mammogram, derided by some women as the ``slammogram.''

``We are encouraged by our early findings, but the research is still in the preliminary stages,'' said assistant physics professor Huabei Jiang, who is part of the Clemson team.

The problem is that light scatters wildly inside breast tissue and the tissue's density poses some unique viewing challenges.

``The holy grail is to find the wavelength to isolate the cancer,'' said Robert Alfano, a laser physicist at City College in New York and a pioneer who holds a patent for optical imaging of breast tissue.

Clemson's machine uses a laser diode that shines infrared light through a breast at 16 different points. A computer analyzes the pattern of photons - the quirky part-particle, part-wave building blocks of light - to reconstruct an image of the breast's interior.

Out of 10 women in the first clinical trials, the system found five malignant tumors and one that was benign. They were confirmed by biopsies and were as small as 5 millimeters, about the size of an average pinky fingernail. Mammograms usually cannot detect lumps any smaller than twice that.

Judy Link, 49, was one of the first to try the new system. Link, whose mother is a breast cancer survivor, tested negative for the disease and was confident of the diagnosis.

``My procedure was painless,'' Link said. ``I have friends who don't get mammograms because the procedure is uncomfortable for them.''

In 1929, Dr. Max Cutler was one of the first physicians to shine a light bulb through a woman's breast. But the process, known as translumination, produced faded images and was unsuitable for diagnosis.

Translumination was later reborn as diaphanography. Scientists used infrared light to try and see breast cancers, but succeeded only in detecting large tumors near the surface.

Today's scientists still use infrared light but now have the added advantage of computer programs that mathematically translate the photon pattern of the breast into an image.

X-ray mammography is the only breast-cancer screening system now approved by the federal Food and Drug Administration. Other methods - ultrasound and magnetic resonance imaging among them - are used to follow up if a suspicious lump is detected.

The attraction of a light-based system is to give women, especially younger women with dense breast tissue, a safer, cheaper, noninvasive method.

Imaging Diagnostics Systems Inc. of Fort Lauderdale, Fla., recently gained approval to market its Computed Tomography Laser Mammography system in Europe and is seeking FDA approval to sell the machines here.

During testing, a patient is face down on a scanning table as laser beams rotate horizontally around the breast and a computer program translates the light pattern into an image. Unlike the Clemson machine, Image Diagnostics can construct 3-D images of the breast.

But Jiang says the Clemson model has software that creates absorption as well as scattering images of breast tissue and the extra information will help doctors better diagnose tumors.

Britton Chance, a researcher in optical imaging at the University of Pennsylvania, plans a large-scale study this summer to test optical mammography against X-ray mammography, magnetic resonance imaging, ultrasound and positron emission tomography, which uses radiation to measure biochemical reactions.

Stavros Demos, a research scientist at the Lawrence Livermore National Laboratory, expects optical imaging eventually will rival mammography, but not until scientists overcome the scattering problem.

``It had been thought initially that this was going to be an easy problem. But as scientists learn more about light and its interaction with tissue, they realize that breast optical mammography is not going to be the first one to win,'' Demos said.

Jiang and his Clemson team plan a second clinical trial later this year involving more than 100 patients and an improved 3-D system with 64 laser beams, up from 16. It should cut scanning time from 10 minutes to 3 minutes per breast.

But radiologist Robert Schmidt at Bellevue Hospital in New York says any new technology must win public acceptance and find cancers current machines cannot.

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