Effect of CT Contrast Materials on Dose Calculations in
Tomotherapy Planning System
Yang Xu, Christopher Watchman, Shane Krafft and Si Young Jang
Radiation Oncology Department, School of Medicine, University of Arizona, Tucson AZ
85724
When head and neck cancer patients take a CT scan, contrast agents (CA) are usually
applied to enhance the visibility of the cardiovascular system, which helps physicians to
delineate the fine structures in this confined region. Depending on the type of CA,
among other factors, the physical density of certain organs (such as carotid artery) would
increase by up to 12-13%. To better represent the real density of human tissue for the
treatment planning process, in our Department, another set of CT scan is always acquired
right before the CA is administrated. Then the two sets of images are fused for contouring
purpose.
This study takes a closer look at the effect of the contrast agents on dose calculations
in the Tomotherapy planning system. Nine patients with various types of head and neck
cancer were randomly selected. Besides the regular PTV and ORA contours, the most
obvious contrast enhanced vessels were also carefully drawn. First a treatment plan was
done based on the CT images with CA. The optimization parameters were saved and
applied to a second plan, in which the same set of images was used with the density of
contrast region overwritten to 1.0. Then the optimization process of both plans ran for a
same number of iterations (up to 150). Finally the change of average doses of PTV, cord,
mandible and contrast areas were compared between the two plans.
Dose Change (%)
Patient
Site
PTV
Cord
Mandible
Contrast area
Male
Retro molar area
-0.1
0
-0.2
-0.6
Male
Tonsil
-0.1
0.1
0
-0.65
Tongue
-1.1
-0.6
-0.8
-1.8
Male
Scalp & neck skin
-0.35
1.2
-1.3
-0.4
Male
Right tonsil
-0.07
n/a
-0.1
-0.13
Female
Female
Pyriform sinus
0.2
0.5
-0.9
0.1
Male
Tonsil
0
0.1
0.1
-0.2
Male
Lymph nodes
0
-0.1
0
0.2
Tongue
0.6
0.2
0.3
0.75
Female
It is clear from the table above that the doses calculated from CA filled CT are not
significantly different from the doses from non-contrast CT, which suggests CT scan
without CA is not absolutely necessary for Tomotherapy treatment planning purpose. By
only scanning patients with CA, it would not only reduce the workload of the therapists
and dosimetrists, but also increase the overall efficiency of the patient care.
Patient
Disease Site
Average Density
of CA Filled Area
Male
Retro molar area
1.14
Male
Tonsil
1.13
Female
Tongue
1.20
Male
Scalp & neck skin
1.20
Male
Right tonsil
1.14
Female
Pyriform sinus
1.18
Male
Tonsil
1.13
Male
Lymph nodes
1.09
Female
Tongue
1.11
Dose Change (%)
Patient
Site
PTV
Cord
Mandible
Contrast area
Male
Retro molar area
-0.1
0
-0.2
-0.6
Male
Tonsil
-0.1
0.1
0
-0.65
Tongue
-1.1
-0.6
-0.8
-1.8
Male
Scalp & neck skin
-0.35
1.2
-1.3
-0.4
Male
Right tonsil
-0.07
n/a
-0.1
-0.13
Pyriform sinus
0.2
0.5
-0.9
0.1
Male
Tonsil
0
0.1
0.1
-0.2
Male
Lymph nodes
0
-0.1
0
0.2
Tongue
0.6
0.2
0.3
0.75
Female
Female
Female