assessment of build-up with different bolus methods

Assessment of build-up with

different bolus methods Authors: Navneeth Hariharan1, Nicholas Hopkins1, Per H. Halvorsen1

1Radiation Oncology Department, Lahey Hospital & Medical Center, Burlington, MA

Purpose:

To experimentally quantify the buildup characteristic of six different bolus materials and compare them to equal

thickness of Superflab, in order to determine the equivalent thickness of bolus material to provide a buildup effect

comparable to a standard thickness of Superflab bolus.

Methods:

All measurements were performed on a Varian 21EX linac using an Advanced Markus chamber and Keithley

electrometer. 10 cm solid water was used for uniform backscatter. The SDD was set to 100cm to front surface of

the detector. An isocentric setup was chosen to eliminate the distance effect and characterize dose buildup effect.

A 6MV beam with 10x10 field setup and 100MU was delivered for each material and thickness. The bolus

materials of varying thicknesses were placed on the detector to measure charge corresponding to buildup dose.

Physical thickness of all the bolus material was measured using a ruler.

Results:

A thickness factor was established for chamois cloth, medium wet towel and wrung out towel compared to 0.5cm

Superflab. The factors for wet gauze, Aquaphor gauze and Super Stuff bolus are within 5% of Superflab buildup

dose.

Equivalent thicknesses to 0.5 cm Superflab:

• Chamois cloth, 3 layers (0.6 cm)

• Medium wet towel, 3 layers (0.8 cm)

• Wrung out towel, 6 layers (1.5 cm)

• Wet Gauze, Aquaphor Gauze and Super Stuff are within 5% buildup dose of Superflab

Conclusions:

Alternatives to wet towel bolus should be explored first, as the wet towel bolus method produces highly variable

water-equivalent thicknesses. If wet towel bolus is deemed necessary, carefully note the configuration (wet vs.

wrung out, number of layers) for the desired bolus effect.

0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.3 1.5

Superflab 100.0% 100.0% 100.0% 100.0%

Wet Towel - Wrung Out 83.4% 92.7%

Wet Towel - Medium Wet 92.8% 98.5%

Chamois Cloth 99.1% 97.3% 97.4% 98.5% 98.9%

Wet Gauze 97.8% 97.9%

Aquaphor Gauze 95.6%

Super Stuff 103.6% 99.0% 99.1%

65.0%

70.0%

75.0%

80.0%

85.0%

90.0%

95.0%

100.0%

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

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Thickness of Bolus Material (cm)

Superflab Wet Towel - Wrung Out Wet Towel - Medium Wet

Chamois Cloth Wet Gauze Aquaphor Gauze

Super Stuff

Equipment:

• Varian 21 EX

• Advanced Markus chamber (TN34045)

and Keithley electrometer (35614EBS)

• Solid water slabs for back scatter

Setup:

• Energy 6 MV

• Field Size 10 x 10

• SAD (to chamber) 100

• Depth varies

• MU 100

• Dose Rate 600 MU/min

• Backscatter 10 cm

Photos of experimental setup

Data normalized to 1.5 cm Superflab

Data normalized to same thickness of Superflab

Bolus Material Creation:

Towels were saturated and then lightly squeezed to not be dripping for the “Medium Wet” condition, and were

folded and twisted, removing most of the water, for the “Wrung Out” condition. Chamois Cloth was saturated and

held vertically with edges parallel to the floor until no water was dripping from the bottom edge. Super Stuff is

highly malleable and applying pressure can change the thickness easily; the material was allowed to settle on clear

wrap to the desired thickness and stopped by folding over the clear wrap edges.