  Lung tumor motion due to respiration poses a challenge in the application of
modern three-dimensional conformal radiotherapy. Direct tracking of the lung
tumor during radiation therapy is very difficult without implanted fiducial
markers. Indirect tracking relies on the correlation of the tumor's motion and
the surrogate's motion. The present paper presents an analysis of the
correlation between the tumor motion and the diaphragm motion in order to
evaluate the potential use of diaphragm as a surrogate for tumor motion. We
have analyzed the correlation between diaphragm motion and superior-inferior
lung tumor motion in 32 fluoroscopic image sequences from 10 lung cancer
patients. A simple linear model and a more complex linear model that accounts
for phase delays between the two motions have been used. Results show that the
diaphragm is a good surrogate for tumor motion prediction for most patients,
resulting in an average correlation factor of 0.94 and 0.98 with each model
respectively. The model that accounts for delays leads to an average
localization prediction error of 0.8mm and an error at the 95% confidence level
of 2.1mm. However, for one patient studied, the correlation is much weaker
compared to other patients. This indicates that, before using diaphragm for
lung tumor prediction, the correlation should be examined on a
patient-by-patient basis.
