Effects of High Energy Electron Beams on Small Fields by Using TPS

 
ENERGIES of electron beams studied were 6, 8, 10, and 15 MeV. ……Small fields from 2x2 to 4x4 cm2 were obtained from Cerrobend shields (cutout) attached to the bottom face of a 6 x 6 cm2 electron cone applicator. Measurements were carried out in a solid phantom with a Roos ionization chamber and Kodak X-OmatV films.
For small field this extra shielding will affect the percentage depth dose (PDD) and the output factors due to little of lateral scatter. The change in dose at maximum depth (d
max) as well as changes in the PDDs with small field must be accounted for when measuring output factors.
The aim of the present work was to achieve an accurate calculation of dose for small field dimensions and performing this by evaluating the accuracy of planning system calculation. This will be compared with real measurement of dose for same small field dimensions using different detectors.
Practical work was performed in two steps: (1) Determination of same physical factors required for dose estimation measured by Ross ionization chamber and calculated by treatment planning system (TPS) based on the latest technical report series (IAEA TRS-398). (2) Comparison of calculated and measured data was done.
For all values energies of used where the field area of the electron beam becomes smaller this lead to the following: (i) The PDD of maximum dose shifts toward the surface, (ii) The depth of 90% and 80%, which are common dose for electron energy, becomes smaller, (iii) The surface dose increases due to scattering and (iv) the dose fall-off region becomes more gradual.
The output of the electron beam decreases significantly with reduction in electron beam field area as a consequence of mild lateral equilibrium.
It can be concluded by considering the milde difference between the calculated data and measured values by Roos IC from 3x3 to 10x10 cm
2 field dimenions, where as for 2x2 cm2 field dimenion this difference exceeded 40% for all energy. Radiographic film showed a variation in results according the following: 10% for 6 MeV, less than