R. Mohan et al., The impact of fluctuations in intensity patterns on the number of monitor units and the quality and accuracy of intensity modulated radiotherapy, MED PHYS, 27(6), 2000, pp. 1226-1237
Citations number
16
Categorie Soggetti
Radiology ,Nuclear Medicine & Imaging","Medical Research Diagnosis & Treatment
The purpose of this work is to examine the potential impact of the frequenc
y and amplitude of fluctuations ("complexity") in intensity distributions o
n intensity-modulated radiotherapy (IMRT) dose distributions. The intensity
-modulated beams are efficiently delivered using a multileaf collimator (ML
C). Radiation may be delivered through a continuous (dynamic mode) or discr
ete (step-and-shoot) sequence of windows formed by the leaves. Algorithms a
nd software that convert optimized intensity distributions into leaf trajec
tories apply approximate empirical corrections to account for the various e
ffects associated with MLC characteristics, such as the rounded leaf tips,
tongue-and-groove leaf design, leaf transmission, leaf scatter, and collima
tor scatter upstream from the MLC. Typically, the difference between inter-
and intraleaf transmissions is ignored. In this paper, using a schematic e
xample of IMRT for head and neck carcinomas, we demonstrate that complex an
atomy and severe optimization constraints produce complex intensity pattern
s. Using idealized intensity patterns we also demonstrate that, for complex
intensity patterns, the average window width tends to be smaller and, for
the same dose received by the tumor, the number of MUs is larger. We found
that as the complexity increases, so does the contribution of radiation tra
nsmitted through and scattered from the leaves ("indirect radiation") to th
e total delivered dose. As a consequence, the lowest deliverable intensity
in complex intensity patterns may be significantly greater than that requir
ed to provide adequate protection for some normal tissues. Furthermore, sin
ce corrections for leaf transmission and scatter effects are approximate an
d the difference between inter- and intraleaf transmission is ignored, the
accuracy of the delivered dose may be affected. Using the results of a simp
le experiment and a typical intensity-modulated beam for a head and neck ca
se as examples, we show the effect of window width and complexity on the ac
curacy and deliverability of intensity patterns. Some possible strategies f
or improving the accuracy and for relaxing the lower limit on deliverable i
ntensity are discussed. (C) 2000 American Association of Physicists in Medi
cine. [S0094-2405(00)00306-0].