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Angela Kempen
February 16, 2012
Calculation Transmission Factor
Table Extension Transmission Factor Calculation
Objective: To determine the transmission factor for an Orfit carbon fiber table extension
used during treatments with an Orfit mask.
Purpose: When calculating dose to a patient, it is assumed that there is nothing in
between the radiation source and the patient. When an object is introduced into the path
of the beam, a certain amount of attenuation may occur. In order to deliver an accurate
dose to the treatment volume, this attenuation must be taken into account. The data from
this project was evaluated to measure and compare dosimetric changes produced when
the Orfit table extension board is used. A patient under treatment for an area in head and
neck region will commonly have a thermoplastic mask to aid in immobilization. In order
to utilize the mask, a table extension is mounted onto the treatment couch. When treating
with posterior fields, the beam will go through the extension. The attenuation of the table
extension can be accounted for with the use of a transmission factor, included in the
monitor unit dose calculation.
Methods and Materials: To determine the transmission factor for the Orfit table
extension, the data was measured on a Varian 21EX linear accelerator using 6
megavoltage (MV), 10 MV and 15 MV photon energies with a 10x10 field size at a 131.5
cm source-to-skin (SSD) distance. 200 monitor units (MU) were delivered at a dose rate
of 600 MU per minute to a depth of 5 cm in water equivalent epoxy blocks. An Exradin
A12 Farmer ionization chamber, with a 0.6cc collection volume, was placed at a 5 cm
depth into a pre-cut hole within the epoxy blocks. The Farmer chamber was connected to
a Keithley 616 electrometer to measure the charge. Two trials were done for each of the
energies, including 6, 10 and 15MV to obtain reference beam data. Once reference data
was recorded, the Orfit table extension was placed into the path of beam, and another two
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trials were done for each photon beam energy. The Orfit table extension has a nominal
thickness of 2.5cm.
Image 1: Set-up of epoxy blocks with Farmer chamber and head extension
Image 2: Data taken with a 616 electrometer for photon energies of 6MV, 10MV
and 15MV
Table extension attenuation
2/8/2012
Exradin Farmer Chamber --> 616 electrometer
5 deep in epoxy, phantom at 131.5SSD
Table at 100 cm
10x10 field
-8
6X
open
x10 C
table in
10x
open
table IN
15x
open
Table In
2.065
2.07
2.015
2.015
2.19
2.19
2.15
2.145
2.265
2.265
2.23
2.23
Avg
2.0675
Attenuation amount of table
2.015
2.19
2.1475
2.265
2.23
5cm dp
6cm dp
0.962
0.937
5cm dp
6cm dp
0.978
0.958
Readings
TMR
5cm dp
0.918
values
6cm dp
0.892
Attenuation of 1cm
of tissue
0.026
Tissue equivalent thickness of table
TF
0.975
0.025
TF
0.981
0.019
0.025
0.977
TF
0.985
0.015
0.02
0.776
0.773
Discussion: The transmission factor was found by taking a ratio of the reading with the
table in / reading without the table. The transmission factor was 0.975 for the 6 MV
photon energy, 0.981 for the 10 MV energy, and 0.985 for 15 MV energy. This data
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represents attenuation of the beam by the table extension. It attenuates 2.5%, 1.9% and
1.5% for 6 MV, 10 MV and 15 MV photon beam energies respectively. A similar study
was done at the Cleveland Clinic in Cleveland, OH, where it was found that the Orfit
table extension did not significantly alter the dose distribution.1 They found 1.2% was
the maximum attenuation of the Orfit board with a 6 MV photon beam energy.1
Clinical Application: The physicist was doing this project with me for his own
application as well. He is incorporating the data into the Pinnacle treatment planning
system. He evaluated the data, and calculated the tissue equivalent thickness of the table,
in order to create a structure to include in dose calculations in the planning system. This
information is not currently entered; therefore, I was unable to do a calculation with the
treatment planning system. This data can be applied to an AP/PA treatment of the upper
thoracic spine. The prescription is 300 centigray (cGy) per fraction for 10 daily
treatments. The beams are weighted equally. The hand MU calculation for this treatment
with and without the table extension is shown below. For the AP field, the transmission
factor was not used since the table extension will not be in the path of the beam. For the
PA field, the transmission factor indicates an increase in MU. This is reasonable since
the beam is being attenuated; more monitor units are needed to give the same dose to the
treatment volume. If the transmission factor were omitted from the dose calculation,
there is risk of the treatment volume being under-dosed.
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Image 3: Monitor Unit Calculations with and without the transmission factor
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Image 4: Percentage of attenuation
Conclusion: The transmission factor of the table extension demonstrates that attenuation
occurs and will have an impact on treatment delivery and the dose received at the target
volume. The table extension does not however have a large clinical impact on the
dosimetry because it usually impacts multiple field intensity modulated radiation therapy
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(IMRT) head and neck treatments. IMRT will likely only utilize one or two beams at
most that will be delivered through the table extension; therefore, the 2.5% attenuation of
the extension would be less by a factor corresponding to the number of beams used. A
patient dose distribution takes into account several different corrections, and the
magnitude of the correction is different for each type, although beam modifying
corrections are the most important.2 Only a small amount of the beam dose may be
penetrating through the extension, but with the precision in today’s world of radiation
oncology, this needs to be accounted for.
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REFERENCE:
1. Gajdos S, Muhieddine L, Chan G, Weinhous M. A Dosimetric Comparison of
Three Commercially Available Carbon Fiber Table Extension Boards. Med Phys.
2005;32(6):1. doi:10.1118/1.1997731.
2. Khan F. Treatment Planning in Radiation Oncology. 2nd ed. Philadelphia, PA:
Lippincott Williams & Wilkins; 2007.