After a 3,700-mile trans-Atlantic journey and a police escort, the first cyclotron to be located in the mid-Atlantic region arrived at Penn's Roberts Proton Therapy Center yesterday morning.

The 220-ton particle accelerator was constructed in Belgium and escorted through Philadelphia yesterday by the Port Authority of Pennsylvania on a specially built 19-axle, 200-foot long truck to Penn.

The cyclotron is the most important piece of equipment used in proton therapy, a kind of external beam radiotherapy in which protons, rather than X-rays, are used to target tumor sites. The cyclotron directs the proton beam to the tumors.

The arrival of the cyclotron - a device that costs between $7 and $8 million - marks an important milestone for the $144-million Roberts Proton Therapy Center, which is scheduled to treat its first patients in the summer of 2009.

Due to its enormous weight, the cyclotron had to be moved by crane into the building before the structure's walls can be completed.

Philadelphia's Mummer All-Stars and members of the Roberts family, which donated $15 million to help build the cancer treatment center, were among those who greeted the cyclotron.

"Today is all about a celebration," Mummers saxophonist Bobby Mansure said, adding that the day was especially meaningful for fellow band member Bruce Mulford, who is a prostate cancer survivor.

Proton therapy is favored for treating certain types of tumors, such as prostate, brain and pediatric cancers, because it effectively targets and kills tumors without severely damaging surrounding healthy tissues.

Through ionization, the cyclotron separates a hydrogen atom's protons from its electrons. Then, using electric and magnetic fields, the cyclotron accelerates the protons and directs them to a beamline.

A carbon-graphite wheel, called a degrader, slows the beam, and electromagnets shape and steer the beam through electromagnetic fields.

The beam is then routed to one of four gantries-the rotational machines that can deliver the beam at any angle-- or to a fixed-beam room.

A 21,000-pound magnet guides the beam to the patient through a nozzle.

Despite its massive weight, the cyclotron is only 18-feet in diameter and eight feet high.

Leading figures present at yesterday's ceremony included Ralph and Suzanne Roberts, Penn Health System CEO Ralph Muller, Penn Health System Executive Vice President and Dean of the School of Medicine Arthur Rubenstein and Radiation Oncology Chairman Stephen Hahn.

Once completed, the Roberts Proton Therapy Center will be the largest proton-treatment facility in the world. The center is expected to treat 3,000 patients a year, including several hundred children.