4D CT for Hepatobiliary Malignancies:

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4D CT for Hepatobiliary Malignancies Organs at
Risk and Implications for Gating
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Purpose/Methods
Radiation therapy to hepatobiliary tumors is
limited by dose to critical organs such as the
liver and kidneys. The intrafractional
respiratory motion of abdominal organs may
adversely affect the radiation dose received by
these organs at risk. The purpose of this
dosimetric study was to determine the variations
in normal tissue dose as a function of breathing
phase. This may guided gated radiation therapy
protocols in the future.
Each patient underwent quantitative spirometry
during a multislice- computed tomography (CT)
following standard helical-CT simulation (Philips
Brilliance 16-slice). 4D CT images were
reconstructed by tidal volume, capturing
end-expiration, mid-inspiration, end inspiration,
and mid-expiration respiratory phases. Each CT
reconstruction was fused with the standard
helical-CT simulation, and the liver and kidneys
in each phase of respiration were contoured for
each patient (Figure 1.) The gross-tumor or
resection bed (CTV1) and regional lymphatics
(CTV2) were first contoured on the helical CT
scan, and then adjusted for target motion based
on fused 4D CT scan reconstructions to make an
ITV1 and ITV2. The ITVs were rarely more than a
few mm larger than the CTV. Margins to consider
expected treatment setup error were included to
generate the planning target volumes (PTV1 and
PTV2.) Most of these patients was planned using
a traditional four field technique (AP/PA R/L
lateral), though occasionally alternative beam
arrangements were used. 45 Gy was prescribed to
PTV1 and 50.4 Gy to PTV2. The V18 for right and
left kidneys and the V 30 and V 20 for the liver
were examined at tidal inspiration and expiration.
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Eleven patients, 9 male and 2 female, ranging
from 43 to 74 years old were examined. Of the
eleven patients, 1 patient had a biliary duct
carcinoma, one had an ampullary carcinoma, and
nine had pancreatic adenocarcinomas. Six were
status-post pancreatic resection.
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L Kidney
R Kidney
The liver consistently received more dose during
inspiration (Table 1). The mean increase of the
Liver V30 and Liver V20 was 4.82 and 5.91 with
maximum increases of 9 and 10 respectively.
Some patients had opposite changes in their
kidney dose during respiration (Figure 2), while
other patients had consistent changes in their
kidney dose during respiration (Figure 3). The
Right Kidney V18 changed by over 10 for 3/11
patients. Alternative beam arrangements, Iike an
anterior/superior posterior/inferior, increased
the difference (Figure 4.) The Left Kidney V18
did not show as dramatic of a change, with a
maximum difference of 9 in one patient.
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R Kidney
L Kidney
100
100
80
80
60
60
40
40
Expiration
Expiration
20
20
Inspiration
Inspiration
0
20
40
0
20
40
DOSE (Gy)
DOSE (Gy)
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Conclusion
Respiratory motion consistently irradiated more
of the liver during tidal inspiration. The
left kidney dose was minimally changed over the
respiratory cycle. No class generalization could
characterize the influence of breathing on the
right kidney dose, but there were significant
changes for certain patients. Individualized
patient protocols using gating treatment during
inspiration or expiration could potentially
reduce dose to liver and kidneys, allowing dose
escalation or increased chemotherapy during
radiation.