Dependence of linac output on the switch rate of an intensity-modulated tomotherapy collimator

The electro-mechanical, multivane intensity modulated collimator (''MIMiC'' ) slit collimator with 40 vanes has been applied in the delivery of inverse ly planned sequential tomotherapy to over 4000 patients. The collimator is binary in that each vane switches between fully open or closed status. Resu lting beamlet patterns provide the intensity distributions imparting dose t o the patient. The bouncing and damping of vanes at the two ends of their t ravel. cause transient dose perturbations near and at the borders of the tr eatment field. These perturbations are not explicitly modeled by the planni ng system. Clinical beamlet profiles and output factors may then differ fro m those in the planning system and as a function of the vane switch period. A mechanical model of vane switching was developed to describe this depend ency. Dose output and distribution of seven simple vane patterns with diffe rent switch times were measured with ionization chambers and radiographic f ilms in polystyrene and anthropomorphic phantoms. Linac output dependence o n switch time relative to vane open time was determined for four intensity modulated radiotherapy (IMRT) patients from measurements of an ionization c hamber embedded in a cylindrical polystyrene phantom. Results demonstrate o utput dependence on switch time and, accordingly, on the servo mechanism fo r monitor units, are length, dose rate, and gantry speed. In conclusion, th e output dependence borders on clinical significance-improvements to collim ator, dose calculation, commissioning, and quality assurance (QA) are sugge sted. (C) 2000 American Association. of Physicists ia Medicine. [S0094-2405 (00)00409-0].