Evaluation of dose calculation algorithm of the peacock system for multileaf intensity modulation collimator

Purpose: To evaluate the dose calculation algorithm used in the inverse treatment planning computer system for the intensity modulation multileaf collimator. Methods and Materials: The inverse treatment-planning computer system calculates the intensities of multiple pencil beams to achieve an optimal distribution sand modulates the beam intensity through the special multileaf collimator. The system's dose calculation algorithm made the two basic assumptions: (a) The tissue-maximum ratios (TMRs) of a single pencil beam have the same values as TMRs for raylines through each pencil beam that are determined from percentage depth dose isodose curves along the long axis of the 2 x 20 cm(2) field with all leaves open; and (b) the relative output factors (ROF) of each pencil beam also have the same values as the rayline TMR at d(max) of the 2 x 20 cm(2) field. To verify these two assumptions, a special multileaf collimator was installed to our linear accelerator which produces 4 MV x-rays. The TMRs and ROFs for the single leaves 1 through 10 were measured using an ion chamber and TLD dosimeter in either a water or a polystyrene phantom. The values of rayline TMRs were calculated from the measured crossplane Isodose curves of the 2 x 20 cm(2) field. Comparisons were made between these two sets of data. Results: Based on our measurements, we found that the ROFs of a pencil beam obtained from the rayline TMRs at d(max) are as much as 7.6% greater than that of single pencil beams. The ROF of the 1 x 1 cm(2) pencil beam is 4 and 6.5% less than that of a cluster of four neighboring pencil beams forming a 2 X 2 cm(2), and a 2 x 20 cm(2) field respectively. However, the rayline TMRs are generally larger than the TMRs of a single pencil beam. At a depth of 8 cm, the average depth in the middle of intracranial space, the rayline TMRs of the pencil beams of leaves 1 and 10 are 5.4 and 9% higher than a single pencil beam TMR at the same depth, respectively. Also interesting is to note that the TMRs of each of the single pencil beams were found to be equal. Conclusions: In our article, evaluations and comparisons of TMRs and ROFs were made for two extreme conditions. The measured values of TMRs and ROFs of a single beam have been shown to be significantly different from those used in the calculations. Because both the TMR and ROF are influenced by the scattering radiation in the same direction, the deviations for these two factors would be expected to be magnified. Thus, for the two extreme situations we have investigated, dose deviations would be on the order of 15%. In real patient treatment; of course, these deviations may be somewhat less, but still significant. Our results, however, show that further investigations are warranted. Copyright (C) 1996 Elsevier Science Inc.