A week passed from the time Michael Markwitz told his doctor about a pain in his lower back to the day this month he was lifted into a 44-by-16-foot mobile treatment center behind an office park in Southwest Baltimore. As he faded into unconsciousness, high- pressure shock waves blasted away at the stone in his kidney until it began dissolving.

A decade ago, Mr. Markwitz, a pastry chef at Innovative Gourmet in Owings Mills, might have undergone surgery to remove the stone and been out of work for up to four weeks. Two years ago he might have traveled to Virginia or Pennsylvania to be submerged in a bath to receive shock treatment or entered a hospital for less invasive methods such as ultrasonic probes -- a higher risk for Mr. Markwitz, since he has only one kidney. Or he might have suffered at home until the stone passed naturally out of the body.

For a growing number of people, however, stone removal by electrically generated shock wave is getting easier with the advent of mobile units that travel to patients.

Mr. Markwitz was treated by a Dornier MFL 5000 lithotripter, the latest in stone shock wave equipment by the German medical equipment manufacturer that first introduced the therapy via a bath in 1985. It is contained in a mobile medical unit assembled over three months' time in Chicago.

The bath-less shock wave treatment is easier for the patient because it requires less sedation and the patient can return home that day. It is also easier for the doctor, who directs the procedure in front of a bank of computer screens.

"There's minimum risk and minimum complication," said Dr. Sanford J. Siegel, a urologist at Northwest Hospital Center who treated Mr. Markwitz on the unit, which was hooked up to an independent outpatient surgery center.

The $1.5 million mobile medical unit is owned by Chesapeake Lithotripsy in a limited partnership between Heritage Group of Nashville, Tenn., the manager and majority owner, and about 40 urologists in Baltimore.

The 18-wheeler arrived in Baltimore in February 1992, hooked up to outpatient surgery centers, and began treating patients in Maryland, which has one of the highest rates of kidney stone occurrences in the country.

It is booked more than 10 hours a day and spends three days in Baltimore and two days in Rockville.

Since August a second unit purchased by the company spends one day a week in Frederick and travels Interstate 95 the rest of the week to outpatient centers in Northern Virginia and Wilmington, Del.

There are only a dozen or so of the mobile units in the country, including two others owned by Heritage and based in Dallas. In Maryland, a few hospitals have the older, bathtub version of the machine.

"The advantage to being mobile is that it can serve several hospitals that are too small to support one by themselves," said Amy Pollokoff, administrator of the mobile unit. Because the technology is too expensive for hospitals or surgery centers to justify, more mobile units are likely to pop up, she said.

In this latest model, treatment takes an hour and a half and is delivered through a circular water cushion about 7 inches in diameter that is moved beneath the patient. During his treatment, Mr. Markwitz lay on his back on a table while a #F computer pinpointed his stone and moved the table and the patient so that the part of his left lower back where the stone was located was exposed. Then, it moved the water cushion directly under the exposed skin. A buzz signaled that the machine had begun pelting Mr. Markwitz' stone with 3,000 electrically charged shock waves.

Had he been awake, Mr. Markwitz would have heard popping noises like the sound of rubber bands snapping and felt pings against his skin. Six feet away, Mr. Markwitz' doctor watched the stone crumble into sand-like grains on a set of computer screens. A registered nurse, a radiation technician and ananesthetist assist in the procedure.

Dr. Siegel said the procedure has a success rate of 85 percent to 90 percent.

The sticker price for the procedure is $4,400, including charges by the outpatient surgery center that is host to the mobile unit. XTC Often, however, insurance companies negotiate a lower rate in exchange for sending a volume of patients. Doctor's fees are extra.

The doctor's fee for lithotripsy is the same as it would be for invasive surgery performed in the hospital. The total bill is often less, however, than some treatments that require hospitalization. Blue Cross and Blue Shield of Maryland estimates that it saves about $1,000 a case by using lithotripsy methods. And it's safer, easier and more comfortable for the patient, who can return to work in a few days rather than a few weeks.

Another advantage, Dr. Siegel said, is that it is easier to treat stones that are lodged in the ureter, the tube that links the kidney and the bladder. The MFL 5000 is the only equipment approved by the Food and Drug Administration for the treatment of ureteral stones, and reduces the need for surgery. Chesapeake Lithotripsy also trains doctors to use the machine. In Baltimore, 50 doctors are certified to use it.

Late last month Johns Hopkins became the first hospital in the region to install the Dornier MFL 5000 in its outpatient center. It treats three patients a week, but doctors expect demand to rise with referrals.

Dr. Louis R. Kavoussi, chief of urology at Hopkins' Francis Scott Key Medical Center, said the machine is needed in an academic setting because of the waiting list that can exist for commercial machines. The Bayview campus hired three urologists this summer, he said.

Nevertheless, he cautioned that the treatment has the potential of being overused because it is so easy compared with other techniques. While more than half of all cases are appropriately treated, he said, 10 percent to 30 percent of patients should have been treated with other procedures.

"The machine is a double-edged sword in urology," Dr. Kavoussi said. In some instances, such as when a stone is too large or too dense, it doesn't work. He said he has seen patients who have undergone the treatment three, four or five times with no success before being referred for surgery.

"The key in this is not using the machine, but judgment: who needs to be treated and how many shocks to give to the stone," he said.

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