Use of a multileaf collimator as a dynamic missing-tissue compensator

A dynamic multileaf collimator (D-MLC) was used to investigate the feasibility of producing missing-tissue compensators. The modulation of the x-ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D-MLC-shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moire camera. The tissue deficit data were used to generate a series of D-MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D-MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters. (C) 1996 American Association of Physicists in Medicine.