For patient 1, number of MUs, MLD, CI, and V20 for plans with various cone diameters (each mark represents a plan). For two-cone plans, the collimator size of the large cone was varied while the small cone was fixed at 20 mm. The large cone was targeted to the PTV reduced by 8 mm. Beam directions for the one- and two-cone plans were identical. With a single cone, acceptable plans (within constraints, but with a relatively high MLD) could only be generated for diameters smaller than 30 mm.
Number of MUs, MLD, CI, and V20 as a function of inward targeting of the large cone in the two-cone technique for patient 1. A fixed cone combination of 25/35 mm was used.
Number of MUs, MLD, CI, and V20 as a function of the collimator size of the small cone in the two-cone technique for patient 1. The large cone was fixed at 35 mm and was targeted to the PTV reduced by 8 mm.
Delivered dose in a two-cone plan of patient 1. The thick line in the images represents the PTV. (a) Dose delivered by the small cone, primarily to the tumor edge. (b) Dose delivered by the large cone, with a maximum in the tumor center. (c) Cumulative dose delivered by the small and large cones. A dose of was prescribed to the isodose level.
MU and MLD for the one- and two-cone plans of patient 1. The curve consists of one-cone plans with cone diameters of 15 to 30 mm targeted to the PTV. For the two-cone plans, the small cone was always targeted to the PTV and the large cone was targeted to the PTV minus 2, 4, 6, 8, or 10 mm. The legend on the left side refers to (small cone collimator size, large cone collimator size, inward targeting of the large cone). For example, a treatment plan with a small cone of 25 mm, a large cone of 40 mm, and targeted to the PTV minus 6 mm is represented as (25, 40, ).
Upper graph, inward targeting as a function of the volume of the PTV for the two-cone plans with lowest number of MUs. The straight line is a linear fit to the data. Lower graph, size of the small and large collimators as a function of the volume of the PTV for the two-cone plans with lowest number of MU. The straight lines are linear fits to the data.
For the ten patients in this study, data from the two-cone plan with the lowest MU and data from the one-cone plans that approximate (i.e., are neighbors of) the MLD of the two-cone plans. The column labeled “one-cone sizes (mm)” lists the collimator sizes that yielded nearest MLDs to the two-cone plans. (For patient 8, 30 mm was the largest cone that the optimization process yielded a solution for, so only the low-MLD neighbor is listed.) The “one-cone MLD (Gy)” is the mean MLD of the neighboring one-cone plans.
Patient group means of the plan parameters for the two-cone plans with the lowest MU and for the averaged one-cone plans.
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