[en] Purpose/Objective: Extensive volumetric dosimetry is conducted on humanoid and selected patients in order to extract physical factors which contribute to the dosimetric advantage of multiple-field treatment. Using non-coplanar beams shaped by MLC, the advantage is resolved into three gains: integral dose gain factor, multi-beam arrangement, and multileaf conformity factors. Materials and Methods: Fine resolution CT volumetric data were axially/hellically acquired for full chest on Rondo and selected lung cancer (Stage II,III) patients. Many field conformal plans (MLC 2100CD, 6 and 18MV) were generated for beams arbitrarily arranged in quatrad, quintad, hexad, etc ... treatment geometry. Dose distributions were computed by fast kernel pencil beam algorithm. Multad arrangement was optimized by algorithm determination of (n) incident beams, optimal spatial arrangement, volume of intercepts around tumors and transit dose to heterogenous normal tissues. Integral doses (ID) were derived by combination of cumulative dose voxels histograms (DVH) with organ distributed DVH. Geometric gain factor was extracted from multad irradiation of spherical/humanoid phantoms imbedded with ellipsoidal tumors. Conformity factors were derived for conformal dose sparing of contralateral lungs, spinal cord and heart (Fig. 2-4). Results: The ID gain factor was stated as quotient of ID to transit tissues divided by ID to tumor; the factor decreased rapidly with n-trad conformal fields (Fig. 6). The beam geometry factor reflected the undesirable volume of beam fans overlapping outside the tumor and was lowest for specific spatial beam arrangement obtainable by a maximization of angular hinges in between conformal beam. Once a multad beam set was oriented at tumor centroid, each conformal beam's-eye-view (BEV) outlined by MLC produced a degree of conformity indexed as the quotient of the dose cumulative in volume subtended in BEV to that in gross tumor (Fig. 5). Conclusion: When introducing a technical modality (i.e. 3-D conformal multifield), it is clinically preferable that the new modality is evaluated for its dosimetric advantage in terms of physical gains which exclude effects of photon radiobiological effectiveness. We have presented a method whereby dosimetric efficacy of 3-D can be evaluated in terms of an advantage; it is resolved distinctly in three gains in the integral dose, multad spatial arrangement and photon beam conformity