DOSIMETRIC EVALUATION OF THE CONFORMATION OF THE MULTILEAF COLLIMATORTO IRREGULARLY SHAPED FIELDS

Purpose: The goal of this study was to evaluate the dosimetric charact eristics of geometric MLC prescription strategies and compare them to those of conventional shielding block. Methods and Materials: Circular fields, square fields, and 12 irregular fields for patients with canc er of the head and neck,lung, and pelvis were included in this study. All fields were shaped using the MLC and conventional blocks. A geomet ric criterion was defined as the amount of area discrepancy between th e MLC and the prescription outline. The ''least area discrepancy'' (LA D) of the MLC conformation was searched by selecting the collimator an gle, meanwhile keeping a preselected position along the width of the l eaf into the prescribed field. Five LAD conventions were studied These included the LAD-0, LAD-1/3, LAD-1/2, and LAD-2/3 that inserted the l eaves at the 0, 1/3, 1/2, and 2/3 of the leaf end into tbe prescriptio n field, respectively. In addition, the LAD optimization was applied t o the transecting (TRN) approach for leaf conformation that prescribed an equal area of overblocking and underblocking under each leaf. Film dosimetry was performed in a 20 cm polystyrene phantom at 10 cm depth 100 cm from source to axis distance (SAD) for both 6 and 18 MV photon s with each of the above MLC conformations and conventional blocks. Th e field penumbra width, defined as the mean of the separation between the 20% and 80% isodose lines along the normal of the prescription fie ld edge, was calculated using both the MLC and conventional block film dosimetry and compared. In a similar way, the d20 is defined as the m ean separation between the 20% isodose Line and the prescription field edge, and the d80 is defined as the mean separation between the 80% i sodose line and the prescription held edge. Results: The field penumbr a width for all MLC conventions was approximately 2 mm larger than tha t of the was conventional block. However, there was a larger variation of the separation distribution in the penumbra region of the irregula r held for the MLC, which had a standard deviation of 1 mm (a factor o f 5 larger than the conventional block). The dosimetry for the circula r held showed that the LAD-TRN, LAD-1/2, and LAD-1/3 approximated the conventional blocking well in terms of d20 and d80; however, no single convention produced the best conformation for both measures. The dosi metric result of the patient treatment fields was similar for all site s. The LAD-1/3, LAD-1/2, and LAD-TRN strategies conformed to within I to 15 mm of the d80 of the conventional block for bath 6 MV and 18 MV, respectively. The LAD-1/2 and LAD-TRN conformations were virtually id entical, although it is proven analytically that the LAD-1/2 conventio n has the least overall area discrepancy of all conventions. Conclusio ns: The five MLC conformation conventions resulted in similar dosimetr ic penumbrae for all field shapes studied The LAD-1/3, LAD-TRN, and LA D-1/2 produced the more favorable approximation to conventional block The field penumbra width, although useful for evaluating irregular fie ld shapes, could not describe the large local variations in the penumb ra along the field edge for the MLC. These local variations could be o f clinical concern when they appear near vital organs. However, the va riation in a local region can potentially be reduced by minimizing the jaggedness of the leaf steps in that local region. The dosimetric res ults were useful as guidelines for the clinicians in the evaluation an d adjustment of MLC leaf positions.