A Quantitative Comparison of Fixed Conformal Beams vs. Arcs for Idealized Regular- and Irregular-Shaped Lesions

Fixed-shaped fields are compared to arcs for moderate-sized spherical- and ellipsoidal-shaped lesions, based on four measures: dose-volume histograms (DVHs), normal tissue complication probabilities (NTCPs), target dose heterogeneity (maximum dose in treatment volume in relation to prescription dose), and conformity index (prescription isodose volume in relation to target volume). To quantify lesion shape eccentricity distribution in a series of patients treated with conformal radiation. Dose calculations were performed with idealized irregular targets placed centrally and peripherally in the brain of a real patient data set. The three-dimensional (3-D) targets were defined as ellipsoids, characterized by two equal axes and one unequal axis (eccentricity is the ratio of unequal axis to equal axis). Target eccentricities were chosen based on an assessment of lesion eccentricity for a series of 30 consecutive patients treated with focused radiation. For centrally and peripherally located targets, dose calculations were performed with three, five, and seven fixed conformal beams (maximally distributed in feasible space), 480° collimated arcs (circular, elliptic, and continuously conformal collimation), and circularly collimated customized arcs (of total arc angle from 450°–1000°). For peripheral targets, coplanar and conical beam arrangements were also investigated. Wedges were used in all fixed beam cases to reduce target dose heterogeneity. The resulting DVHs, NTCPs, dose heterogeneity, and conformity indices for brain were compared for targets of variable eccentricity. In all beam arrangements tested, increasing the number of fixed conformal fields from three to five to seven decreases the normal brain volume at high dose and increases the normal brain volume at low dose, with diminishing differences as the number of fields increase. Among beam arrangements, seven fixed conformal beams maximally distributed in usable space (for peripheral or central targets) and seven fixed fields arranged in a cone (for peripheral targets) irradiated the least volume of normal tissue and had the lowest NTCPs. Within the class of beam arrangements that had the lowest normal brain volumes and NTCPS, fixed conformal beams maximally separated in space, dose heterogeneity, and conformity index remained relatively constant in going from three to five to seven beams. Among arc arrangements, in general, the 480° continuously conformal arc had the least normal tissue irradiation at high and low doses, and lowest NTCPs. Dose heterogeneity was relatively constant with increasingly tighter collimation among the 480° arc arrangements, i.e., circular to elliptic to conformal, while the conformity index improved significantly. Further improvements in DVH, NTCP, conformity index, and dose heterogeneity were provided by certain customized arc arrangements. In general, for all eccentricities and target locations, seven fixed fields maximally distributed in space were similar to the 480° continuously conformal arc in respect to normal brain volumes, NTCPs, dose heterogeneity, and conformity index. For both peripheral and central targets of variable eccentricity, the 480° continuously conformal arcs and seven fixed conformal fields (maximally separated in feasible space for central and peripheral lesions/arranged in a cone for peripheral lesions) appear to be very similar in respect to normal brain volumes, NTCPs, dose heterogeneity, and conformity index. They are overall superior to the other options tested.