Abstract # 2119 Image-guided Radiation Therapy (RT) for Rectal Cancer Using Cone Beam CT (CBCT)

Presenter: Lee, Percy

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Nine patients (8 rectal cancers, 1 anal cancer) underwent > 1 CBCT during a
course of RT using Varian Trilogy. Each CBCT scan had a field of view of 17 cm
in the superior-inferior direction and was co-registered to its respective
simulation CT scan by matching bony anatomy. Rectal volume (RV) contours were
drawn on the simulation CT scan and subsequent CBCT scans. All two-dimensional
measurements were taken at the superior, middle and inferior aspects of the
CBCT scans. Parameters studied included: lateral (LAT) and anterior-posterior
(AP) dimensions, geometric center displacement, RV, and degree of RV overlap.
The paired RV from the simulation CT scan and a CBCT scan or two different CBCT
scans were matched and the percent RV overlap was calculated. IMRT plans were generated
for the 8 rectal cancer patients and forward calculations were applied to the
subsequent CBCT scans. IMRT constraints were 95% bowel < 45 Gy, mean bladder
dose < 30 Gy and Dmax femoral head < 45 Gy. GTV plus 1.5 cm and the
pelvic nodes were prescribed to 45 Gy. A cone down to GTV plus 1.5 cm was
prescribed to 50.4 Gy.

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