1 answer

6a) A tumor located at the depth of 10cm is treated using a non-isocentric technique with...

Question:

6a) A tumor located at the depth of 10cm is treated using a non-isocentric technique with a Co60 machine, to a dose of 1.8 Gy, with a single 5cm x 5cm beam. Compute the maximum dose received anywhere in the patient's anatomy. Refer to the attached table for PDD data.
b) At what depth does the maximum dose occur? Explain.
c) Compute the dose at a depth of 5ccm along the beam axis.
d) Would this be a good treatment strategy? Explain.

Attached Table:

Field Size (cm2)

5 x 5

10 x 10

20 x 20

Depth (cm)

0.5

100

100

100

5.0

76.7

80.4

83.0

10.0

53.3

58.7

63.3


Answers

Answer:

For a),b)& c)

Relative dose measurements:
It is generally performed in a large water phantom and for this purpose a suitable ionization
chamber is used in the water phantom. In this way dose values are determined at many
points under varying treatment conditions, such as field size, source skin distance (SSD),
and the presence of beam modifiers[3].
When multiple fields are used for treatment of a particular tumor inside the patient,
isocentric, source axis distance (SAD) setups are often used. In contrast to source skin
distance SSD setups, which rely on percentage depth dose (PDD) distributions, SAD setups
rely on other functions, such as tissue-air ratios and tissue-phantom ratios, for dosimetric
calculations. So acquiring a group of measurements is divided to two groups the first group
is the central axis depth doses in water (SSD SET-UP) and the second group is the central
axis depth doses in water (SAD SET-UP).
Concerning the central axis depth doses in water (SSD SET-Up) the measuring sessions of
the PDD is to place the center of the ion chamber in the water phantom over the range of
depth of 0 to 25 cm in increments not larger than 1 cm and over the range of field sizes
rf .
from 4cm x 4 cm to 40cm x 40cm in increments not larger than 5 cm for the side. The
central axis dose distributions inside the phantom are usually normalized to depth of
maximum D max = 100%.
The measurements of the central axis depth doses in water for (SAD SET-UP) consist of
the tissue-air ratio (TAR), tissue-phantom ratio (TPR), tissue maximum ratio (TMR),

output factor, blocking tray factor, and beam profiles.
Concerning the measuring sessions of the (TAR) (suggested by Johns in 1953), the ion
chamber should be placed in water phantom over the range of same depths and field size as
PDD measurements, in both water and air.

The ratio between the absorbed dose in water at
the same point in free air is referred to as the (TAR). [7]
The measurements of the TPR at a point in a water phantom irradiated by a photon beam
can be done by divide the total absorbed dose at that point by the total absorbed dose at a
point or the beam axis at affixed reference depth, usually 5 cm or 10 cm.
i
TMR is special case of TPR and may be determined by dividing the dose at a given point in
water phantom to the dose at the same point at the reference depth of maximum dose[7].


Absolute dose measurements:

Absolute dose measurements are also performed in a water phantom. For this purpose a
calibrated ionization chamber is used. The generally applied procedure for specification of
beam quality using IAEA TRS-398 protocol is to specify the tissue-phantom ratio,
TPR20,10. This is the ratio of the absorbed doses at depths of 20 cm and 10 cm in a water
phantom, measured with a constant source-chamber distance of 100 cm and a field size of
10x10 cm2 at the plane of the chamber [2].
-• J& Determination of absorbed dose to water under the reference conditions is given' by
pf'-
multiplying the corrected reading of the dosimeter with the calibration factor arid KQ go
which is the chamber-specific factor which corrects for the difference between the
reference beam quality go and the actual quality being used Q [

Answer d) When the tumor size is within the above mentioned limit this is a good treatment to be followed.

.

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