AbstractID: 7170 Title: Delivery Optimization with Combined Rotational and fixed-gantry
Both rotational and fixed-gantry IMRT methods have been implemented clinically using
dynamic multileaf collimation (DMLC). We found that neither method alone is truly optimal in
terms of dosimetry and efficiency of delivery. The goal of this study is to expend the scope of
IMRT optimization to include the mode of delivery, the number and orientation of beams. An
optimization algorithm is used to generate beam intensities at all beam angles. The final value of
the cost function is recorded as the baseline cost with all beams present. Then, a beam is deleted
from the initial set and the intensities for the remaining beams are re-optimized. The difference
between the new and the baseline cost represents the importance of the deleted beam in
minimizing the cost function. The importance factors, G, for all beam angles, θ, are evaluated.
This approach can be used to deduce the optimal delivery technique. If G(θ) peaks at certain
angles, fixed fields at these angles will be used. If G(θ) is flat, indicating no advantage of any
angle, rotational delivery will be chosen. In most cases G(θ) fluctuates over a smooth
background. The optimal dose distribution and efficient delivery can be achieved using a
combination of rotational and fixed-gantry IMRT. Compared to pure rotational or fixed-gantry
IMRT, the hybrid approach provides improved dosimetry with fewer beams (an arc is counted as
a beam). Since both delivery methods are possible on a linear accelerator with DMLC
capability, the optimized delivery scheme can have a significant clinical impact.