The field-matching problem as it applies to the peacock three dimensional conformal system for intensity modulation

Purpose: Intensity modulated beam systems have been developed as a means of creating a high-dose region that closely conforms to the prescribed target volume while also providing specific sparing of organs at risk within complex treatment geometries. The slice-by-slice treatment paradigm used by one such system for delivering intensity modulated fields introduces regions of dose nonuniformity where each pair of treatment slices abut, A study was designed to evaluate whether or not the magnitude of the nonuniformity that results from this segmental delivery paradigm is significant relative to the overall dose nonuniformity present in the intensity modulation technique itself, An assessment was also made as to the increase in nonuniformity that would result if errors were made in indexing during treatment delivery. Methods and Materials: Treatment plans were generated to simulate correctly indexed and incorrectly indexed treatments of 4, 10, and 18 cm diameter targets, Indexing errors of from 0.1 to 2.0 mm were studied, Treatment plans were also generated for targets of the same diameter but of lengths that did not require indexing of the treatment couch. Results: The nonuniformity that results from the intensity modulation delivery paradigm is 11-16% for targets where indexing is not required, Correct indexing of the couch adds an additional 1-2% in nonuniformity, However, a couch indexing error of as little as 1 mm can increase the total nonuniformity to as much as 25%, All increases in nonuniformity from indexing are essentially independent of target diameter. Conclusions: The dose nonuniformity introduced by the segmental strip delivery paradigm is small relative to the nonuniformity present in the intensity modulation paradigm itself. A positioning accuracy of better than 0.5 mm appears to be required when implementing segmental intensity modulated treatment plans.