Effective penumbra and scalloping effect: a dosimetric study in multifield radiotherapy with multileaf collimator for...

The aim of the study was to test whether the multileaf collimator can be used to replace divergent alloy blocks for field shaping and to evaluate the dosimetric impact of the scalloping effect and the effective penumbra in multifield isocentric configurations routinely used at our department to treat early stage prostate tumor in supine patients. Deviations between measurements and treatment planning system calculations are also discussed in order to estimate the accuracy of effective penumbra calculations carried out by the software in the presence of blocks or a multileaf collimator.

The multileaf collimator installed on a dual energy (6 and 18 MV) linear accelerator Varian CLINAC 2100 C/D is an add-on component positioned below the standard jaws, with 40 computer-controlled opposed pairs of tungsten leaves. Transmission, effective penumbra and scalloping measurements were performed with films placed at different depths in a (30 x 30 x 20 cm3) acrylic phantom. A laser scanning photodensitometer was used to obtain the optical density and the relative dose profile. Effective penumbra and scalloping effect measured data were tested on a software phantom; the phantom, automatically performed with the treatment planning system, was a regular parallelepiped measuring 30 x 30 x 20 cm3 and the acrylic electronic density value.

For one multileaf collimator-shaped field, the width of effective penumbra was about 2 mm wider than penumbra for cerrobend blocks, at a 45 degrees angle between leaf motion direction and the field edge. Collimator rotation, automatically performed by the treatment planning system, by minimizing the sum of over- and underblocked areas, reduces the differences between the multileaf collimator and blocks. The differences between measured and treatment planning system calculated data were within the treatment planning system dose calculation accuracy limits, as recommended in ICRU Report No. 42.

Penumbra and, for the multileaf collimator, scalloping effect values seem to depend on the gantry angle, i.e., on the fields path of entry. The values for even fields are higher than the those for odd fields, because the dose gradient at the target edges is steeper for a single field than for two opposite fields, and the interplay of doses from the individual beams increases the distance between the isodose levels. Therefore, in order to reduce scalloping and effective penumbra values, it would be better to rotate the collimators, above all in even-number field techniques. In particular, the six-field technique, used mostly in prostate treatment, shows the same effective penumbra values with the multileaf collimator, with proper collimator rotation, and in blocked fields.