THE FIELD-MATCHING PROBLEM AS IT APPLIES TO THE PEACOCK 3-DIMENSIONALCONFORMAL SYSTEM FOR INTENSITY MODULATION

Purpose: Intensity modulated beam systems have been developed as a mea ns of creating a high-dose region that closely conforms to the prescri bed target volume while also providing specific sparing of organs at r isk within complex treatment geometries. The slice-by-slice treatment paradigm used by one such system for delivering intensity modulated fi elds introduces regions of dose nonuniformity where each pair of treat ment slices abut, A study was designed to evaluate whether or not the magnitude of the nonuniformity that results from this segmental delive ry paradigm is significant relative to the overall dose nonuniformity present in the intensity modulation technique itself, An assessment wa s also made as to the increase in nonuniformity that would result if e rrors were made in indexing during treatment delivery. Methods and Mat erials: Treatment plans were generated to simulate correctly indexed a nd 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 th at did not require indexing of the treatment couch. Results: The nonun iformity 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 cou ch indexing error of as little as 1 mm can increase the total nonunifo rmity 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 s mall 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 treatm ent plans.