Stereotactic body radiation therapy (SBRT) is an effective treatment for medically
inoperable Stage I non-small-cell lung cancer. However, changes in the patient's breathing
patterns during the course of SBRT may result in a geographic miss or an overexposure
of healthy tissues to radiation. However, the precise extent of these changes in breathing
pattern is not well known. We evaluated the inter- and intrafractional changes in
tumor motion amplitude (DeltaM) over an SBRT course.
Eighteen patients received image-guided SBRT delivered in three fractions; this therapy
was done with abdominal compression in four patients. For each fraction, cone beam
computed tomography (CBCT) was performed for tumor localization (+/- 3-mm tolerance)
and then repeated to confirm geometric accuracy. Additional CBCT images were acquired
at the midpoint and end of each SBRT fraction. Respiration-correlated CBCT (rcCBCT)
reconstructions allowed retrospective assessment of inter- and intrafractional DeltaM
by a comparison of tumor displacements in all four-dimensional CT and rcCBCT scans.
The DeltaM was measured in mediolateral, superior-inferior, and anterior-posterior
directions.
A total of 201 rcCBCT images were analyzed. The mean time from localization of the
tumor to the end-fraction CBCT was 35 +/- 7 min. Compared with the motion recorded
on four-dimensional CT, the mean DeltaM was 0.4, 1.0, and 0.4 mm, respectively, in
the mediolateral, superior-inferior, and anterior-posterior directions. On treatment,
the observed DeltaM was, on average, <1 mm; no DeltaM was statistically different
with respect to the initial rcCBCT. However, patients in whom abdominal compression
was used showed a statistically significant difference (p < 0.05) in the variance
of DeltaM with respect to the initial rcCBCT in the superior-inferior direction.
The inter- and intrafractional DeltaM that occur during a course of lung SBRT are
small. However, abdominal compression causes larger variations in the time spent on
the treatment couch and in the inter- and intrafractional DeltaM values.