AbstractID: 7810 Title: Use of a Mini-multileaf Collimator for Simultaneous IMRT
Boost in Prostate Cancer Treatment
It is common to designate a boost volume that is to receive a higher dose than the rest of the PTV. With IMRT the boost is most
efficiently realized by specifying to the optimizer that this volume is to receive a higher dose per fraction. The ability to separate the
dose levels between the boost volume and the rest of the PTV depends on the ability to modulate the field intensity, as well as on the
physics of secondary electron transport. Separation is more difficult for a small boost volume because of its width with respect to that
of the intensity map bixels and to the range of secondary electrons. A mini-multileaf collimator (MMLC) can reduce bixel width in
both directions and thus aid in achieving good dose separation for small boost volumes. Such a device, along with accurate target
delineation and positioning, thus has promise for prostate cancer treatment when dominant intra-prostatic lesions (DILs) are
designated. A comparison between such IMRT plans using 10x10 mm bixels (corresponding to a MLC with 10 mm leaf width) and
4x4 mm bixels (MMLC with 4 mm leaf width) shows that improved dose separation and uniformity is achieved with the higherresolution intensity maps. In particular, the volume of the PTV outside the DILs that receives a dose above that designated is reduced
and the dose inside the DILs is more uniform. Rectal sparing is also improved with the MMLC, despite better target coverage.