AbstractID: 7564 Title: A Comparison of Two Different Beam Modulation Approaches for Intensity
Modulated Radiation Therapy, IMRT
Beam modulation is a key component in IMRT treatment delivery. The critical parameters of the modulation device include spatial
resolution, modulation resolution, and minimum level of transmission. The time to deliver the treatment is significant since one may
have to compromise the various levels of resolution to obtain reasonable treatment times.
This work compares two different modulation approaches: one uses a conventional multileaf collimator, MLC, and the other a
physical modulator (brass compensator). The spatial resolution at isocenter is 10mm for the MLC and 5mm for the compensator. The
modulation resolution for the MLC is user defined and generally set between 10% to 33%. For compensators with a 6 MV x-ray
beam, a 1 mm machining increment in brass, results in modulation resolution of approximately 4%. The minimum level of beam
transmission of the MLC is 0.1%, except for leakage between the leaves, where it increases to approximately 1%. The minimum level
of transmission for a 6 MV x-ray beam through 5 cm of brass is approximately 16%.
IMRT treatments for the same patients have been planned on the same commercial planning system with the two different delivery
approaches described above. The results of these plans will be presented. The compensator produces beam modulation patterns with
higher spatial resolution, greater dose homogeneity, and shorter treatment times than those produced by a standard MLC. The MLC
provides a better overall level of attenuation of the beam, which may be important in clinical situations where critical organs required
very low dose.