Background: Many cancer patients who require external beam radiotherapy such as
breast cancer patients, present with irregular surface topographies and tissue
inhomogenieties in the treatment field. Such irregularities give rise to unacceptable
dose non-uniformity. Standard fields cannot be applied without compensation for
missing tissue. 1-D and 2-D missing tissue compensators can be used but they have
limitations. 3-D compensators are the most effective but they are normally fabricated
using very expensive automated systems.
Objectives: To study the variation of linear attenuation coefficients of different
materials in megavoltage photon beams, select a tissue equivalent compensating
material and commission a local 3-D manual missing tissue compensator cutter.
Methods and materials: Linear attenuation coefficients were measured for tin, River
sand mix, Lincolnshire bolus and dental modelling wax for different energy
megavoltage photon beams. Measurements were done in a water phantom using a
cylindrical ionisation chamber at varying depths. The CT numbers and densities of the
materials were also measured. Negative plaster of paris moulds of the breast and head
and neck areas were made using a RANDOTM Alderson anthropomorphic phantom
from typically simulated fields. 3-D missing tissue compensators were then fabricated
on the manual cutter and were tested for their effectiveness during treatment delivery. Results: Linear attenuation coefficients were dependent on photon beam energy, the
thickness and density of the attenuator, but independent of the depth of measurement
for compensator thickness of more than 2 cm. Lincolnshire bolus and dental
modelling wax with CT numbers of –78 ± 9 and -88 ± 18 and densities of 1.4 ± 0.0
g/cm3 and 0.9 ± 0.0 g/cm3 respectively can be regarded as tissue equivalent materials.
The fabricated 3-D missing tissue compensators were effective in correcting for dose
non-uniformities compared to fields with no beam-modifying devices or wedges (1-D
compensators).
Conclusions: The 3-D missing tissue compensators were effective in correcting for
dose non-uniformities in treatment fields involving very irregular surface
topographies compared to 1-D and 2-D methods. They can be fabricated cheaply
using a 3-D manual missing tissue compensator cutter. Quality control procedures
need to be followed during fabrication.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/7231 |
| Date | 10 September 2009 |
| Creators | Nakatudde, Rebecca |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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