AbstractID: 8220 Title: A method of adaptive filtering and retro-mapping of intensitymodulated beams to improve delivery efficiency
Intensity modulated beams generated with inverse planning are inherently susceptible to noise and numerical artifacts. The resulting
beam intensities are often of high degree of fluctuations, which in turn can lead to high monitor units (MUs) for the leaf sequences
generated with “sliding window” technique. Filtering (or smoothing) can reduce beam MUs significantly but the process of filtering
also instigates deformation to the desired intensities. An adaptive filtering scheme is proposed to minimize the amount of deformation
due to the filtering. Instead of filtering the whole intensities uniformly, the proposed scheme adopts a “filtering only as much as
needed” strategy to reduce MUs needed to deliver the beam. A retro mapping procedure is then applied to the filtered intensities to
minimize the root mean square (RMS) of the differences between the retro mapped and the desired intensities, while beam MU
reduction from filtering is preserved. Compared with the uniform filtering, this scheme reduces the differences between the retro
mapped and the desired intensities by a factor of 2~3. For a similar RMS differences as in uniform filtering, a heavier filtration can be
applied to further reduce the beam MUs. Studies on 15 clinical head-and-neck cases showed that this scheme could provide a better
target coverage for the same amount of total MU reduction as if uniform filtering is used. For the similar target coverage to uniform
filtering, a better sparing of the organs at risk can be achieved.