What is the optimum leaf width of a multileaf collimator?

The following question is investigated: How narrow do the leaves of a multi leaf collimator have to be such that further reduction of the leaf width do es not lead to physical improvements of the dose distribution. Because of t he physical principles of interaction between radiation and matter, dose di stributions in radiotherapy are generally relatively smoothe According to t he theory of sampling, the dose distribution can therefore be represented b y a set of evenly spaced samples. The distance between the samples is ident ified with the distance between the leaf centers of a multileaf collimator. The optimum sampling distance is derived from the 20% to 80% field edge pe numbra through the concept of the dose deposition kernel, which is approxim ated by a Gaussian. The leaf width of the multileaf collimator is considere d to be independent from the sampling distance. Two cases are studied in de tail: (i) the leaf width equals the sampling distance, which is the regular case, and (ii) the leaf width is twice the sampling distance. The practica l delivery of the latter treatment geometry requires a couch movement or a collimator rotation. The optimum sampling distance equals the 20%-80% penum bra divided by 1.7 and is on the order of 1.5-2 mm for a typical 6 MV beam in soft tissue. The optimum leaf width equals this sampling distance in the regular case. A relatively small deterioration results if the leaf width i s doubled, while the sampling distance remains the same. The deterioration can be corrected for by deconvolving the fluence profile with an inverse ti lter. Conclusions: With the help of the sampling theory and, more generally , the theory of linear systems, one can find a general answer to the questi on about the optimum leaf width of a multileaf collimator from a physical p oint of view. It is important to distinguish between the sampling distance and the leaf width. The sampling distance is more critical than the leaf wi dth. The leaf width can be up to twice as large as the sampling width. Furt hermore, the derived sampling distance can be used to select the optimum re solution of both the fluence and the dose grid in dose calculation and inve rse planning algorithms. (C) 2000 American Association of Physicists in Med icine. [S0094-2405 (00)02011-3].