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 tumors using the 67.5 MeV protons from the 76-in. isochronous cyclotron. Beam line design, commissioning, control system, beam characteristics, dosimetry, patient positioner and system performance are discussed. The unmodulated Bragg peak has a penetration of 29 mm in tissue at the 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 transmission ionization chambers which are calibrated against a thimble ionization chamber with an NIST-traceable ⁶⁰Co calibration factor. The Bragg peak is spread across the target volume by the use of range modulators. The residual range is varied by means of a variable water column. Daily variation in patient dosimetry is within ±3%. The beam penumbra (defined here as the distance between the 90% and 10% isodose levels) 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 within ±2% and the beam symmetry is ±1%. In the first 18 months, 50 patients have been treated with an average field size of 16.8 × 16.6 mm ². The residual range varied between 13.0 mm to 29.2 mm with an average 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) ranged between 1.2 to 11.5 mm with mean of 5.2 mm. The age of the patients ranged from 25 to 88 yr.