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Abstract
This study presents a computational method applied to determine the dose at a point in medium (equivelant soft tissue) when it is irradiated by an external team of gamma—rays (Caesium - 137) source.
The measurements of depth dose were carried out experimentally in a water phantom using farmer dosemeter. A geometrical set up was specially designed permitting a horizontal I-Y motion at different depth Z. The dose could he measured at points spaced 1 cm. in the horizon¬tal X - Y plane with a maximum range of + 15 cm. on eit¬her side from the centeral point. These could be repea¬ted for horizontal planes at depths spaced 1 cm. to a maximum depth of 25 cm.
The percentage depth dose was calculated using an empirical mathematical model based on mainly two assumpt¬ions introduced by Sterling et al (196$). The first one assumed that there should be a simple relation-ship between the percentage depth dose at points away from the central axis and those on the central axis at the same depth. The second assumption was that the decriment Of dose along the major axis could be expressed as a function of field configuration and depth below the sur¬face .These two assumptions are combined to develop the mathematical expressions for the percentage depth
dose (% DD) on the major axis as a function of field
size and depth below the surface for depths Z less than
3 cm. Z more than 3 cm. and the percentage depth dose
on the central plane (% DD) •
The experimentally obtained (%DD) values on the
major axis were then compared to the 2 mathematically derived expressions enabling the determination of the fitting par¬ameters.
Using these equations for the central axis percent¬age depth dose at any depth, the percentage depth dose
(%DD) _ at any point In the central plane (x,o,z) could cp
be obtained if the off-axis ratio F(X) is determined, being related according to Allewart (1972) by the equat¬ion (%DD) = (%DD) • P (X) cp c
In order to determine P (X), the experimentally measured doses in the central plane were used to determine the normal dose distribution (t) and consequently P(X).
The percentage depth dose at any position (x,o,z) was easily determined by adopting the central axis depth dose value at the particular depth Z to calculate the off - axis depth dose at the position (x,z). This value was then considered 45 the central axis depth dose of a new central plane (yfz) perpendicular to the (x,z) plane. O.E. Badawy scatter with field size. It was hoped that this method could provide the radiotherapist with an easy applicable tool for treat¬ment planning: in the standard cases involving _x — and
The results obtained from the experimental work carried out helped in obtaining precise values for the primary and scatters dose calculations. Then a suitable mathematical formula was derived to calculate the per-centage depth dose at any point in the irradiated homo-geneous medium . The variations of the dose in non -homogenous phantom was also discussed.