Use of a quantitative index of beam modulation to characterize dose conformality: illustration by a comparison of full beamlet IMRT, few-segment IMRT (fsIMRT) and conformal unmodulated radiotherapy.

A technique is presented for characterizing the degree of modulation in an intensity-modulated beam. It is shown that the modulation increases as dose conformality increases. Full intensity-modulated radiation therapy (IMRT) is compared with a two-weight-per-field technique and with simple geometrically conformal beams. It is suggested that each individual planning problem requires some comparative planning of this type because there is no simple answer to the question of the degree to which IMRT improves dose conformality. This depends on the problem geometry, the dose prescription, the cost function, the number of beams and other planning conditions. A methodology is presented for such comparative planning studies and this is illustrated with the solution of two planning problems.