An investigation of beam parameters for Co-60 tomotherapy.

The effect of various physical beam parameters for Co-60 tomotherapy is examined, including: Cylindrical source size, source-to-collimator distance (SCD) and collimator leaf width. In general, beam profile effects are seen with larger Co-60 sources that are not seen with conventional linacs and multileaf collimators that are used for IMRT, including a broadening of the profile width as the source width increases. A treatment planning study was conducted to evaluate the effect of various beam parameter combinations with planning regions of interest typical of a simultaneous boost head and neck treatment. Combinations of SCD and source FWHM that produce an 80%-20% profile distance less than approximately 0.85 cm at 0.5 cm depth produced a Co-60 tomotherapy plan with better DVH results than a conventional 7-field linac plan for a 1 cm leaf width. Further improvement may be achieved by reducing the leaf width. In general it may be concluded that commercially available cylindrical Co-60 sources of 1.5-2.0 cm diameter may be appropriate for tomotherapy if the proper combination of beam parameters is chosen.