Construction of a $185 million proton beam treatment center at Mayo Clinic passed a milestone Monday when iron workers installed the final steel support structure in the building that will house the high-tech cancer killer.

The Rochester Post-Bulletin reports that many Mayo doctors and executives came outside to sign the steel I-beam, which was painted white and in accordance with iron worker tradition had a Christmas tree attached, before it was hoisted into place.

Mayo predicts that about 1,240 patients per year will come for treatment at the facility, which is projected to open by mid-2015. The clinic is building a second $185 million proton treatment center on its Arizona campus They and five others under construction will join 10 such facilities currently operating nationwide..

Compared to conventional radiation treatment, a proton beam allows oncologists to focus a higher dose of particles directly at diseased tissue, killing more cancer cells while reducing collateral damage to healthy tissue and vital organs. Advocates say cancer can be controlled with fewer treatments, reducing costs and side-effects.

Plans call for the Rochester facility to have four "pencil-beam" treatment rooms. Each will be supported by machinery two stories high to enable technicians to rotate and aim the beams with precision.

The Post-Bulletin's full story is here.