NEW UCSF PROTON OCULAR BEAM FACILITY AT THE CROCKER NUCLEAR LABORATORY CYCLOTRON (UC DAVIS)

A new facility has been constructed at the Crocker Nuclear Laboratory at University of California Davis for the purpose of treating ocular t umors using the 67.5 MeV protons from the 76-in. isochronous cyclotron . Beam line design, commissioning, control system, beam characteristic s, dosimetry, patient positioner and system performance are discussed. The unmodulated Bragg peak has a penetration of 29 mm in tissue at th e isocenter with a peak to plateau ratio of 3.8:1 and a width of 5 mm (FWHM measured in water). The delivered dose is monitored by two trans mission ionization chambers which are calibrated against a thimble ion ization chamber with an NIST-traceable Co-60 calibration factor. The B ragg peak is spread across the target volume by the use of range modul ators. The residual range is varied by means of a variable water colum n. Daily variation in patient dosimetry is within +/-3%. The beam penu mbra (defined here as the distance between the 90% and 10% isodose lev els) is 1.5 mm for a range-modulated beam at a collimator-to-isocenter distance of 50 mm. The beam flatness in a 25 mm diameter beam is with in +/-2% and the beam symmetry is +/-1%. In the first 18 months, 50 pa tients have been treated with an average field size of 16.8 x 16.6 mm( 2). The residual range varied between 13.0 mm to 29.2 mm with an avera ge value of 22.4 mm, and the range modulation varied between 16 mm to 24 mm with an average value of 20 mm. The tumor thickness (height) ran ged between 1.2 to 11.5 mm with mean of 5.2 mm. The age of the patient s ranged from 25 to 88 yr.