Intensity modulated radiation therapy with irregular multileaf collimated field: a dosimetric study on the penumbra region with different leaf stepping patterns.

Using a Varian 21 EX linear accelerator with a multileaf collimator (MLC) of 120 leaves, the penumbra regions of beam profiles within an irregular multileaf collimated fields were studied. MLC fields with different leaf stepping angles from 21.8 degrees to 68.2 degrees were used. Beam profiles in different directions: (1) along the cross-line and in-line axis, (2) along the leaf stepping edges of the field, and (3) parallel to the stepping edges but in the middle of the field, were measured and calculated using Kodak XV radiographic film and Pinnacle3 treatment planning system version 7.4f. These beam profiles were measured and calculated at source to axis distance= 100 cm with 5 cm of solid water slab on top. On the one hand, for both cross-line and in-line beam profiles, the penumbra widths of 20%-80% did not vary with the leaf stepping angles and were about 0.4 cm. On the other hand, the penumbra widths of 10%-90% of the above two profiles varied with the stepping angles and had maximum widths of about 1.9 cm (cross-line) and 1.65 cm (in-line) for stepping angles of 38.7 degrees and 51.3 degrees , respectively. For profiles crossing the "rippled" stepping edges of the field, the penumbra widths (10%-90%) at the regions between two opposite leaves (i.e., profile end at the Y1/Y2 jaw position) decreased with the stepping angles. At the penumbra regions between two leaf edges with the tongue-and-groove structure of the same bank (i.e., profile end at the X1/X2 jaw position), the penumbra widths increased with the stepping angles. When the penumbra widths were measured between two opposite leaf edges and at corners between two leaves, the widths first decreased with the stepping angles and then increased beyond the minimum width point at stepping angle of 45 degrees. The penumbra width (10%-90%) measured at the leaf edge was larger than that at the corner. For the beam profiles calculated using Pinnacle3, it is found that the results agreed well with the measurements along the cross-line and in-line axis, while there was a deviation for the profiles along the leaf stepping edge of the field compared to the film measurements. The measured results in this study can help us to understand the dosimetric effect of the leaf stepping (due to finite leaf width), tongue-and-groove and rounded leaf end structure in the penumbra region of an irregular MLC field. A more dedicated penumbra model can be developed for the treatment planning system.