Página 285 - irpa

285 ST 3.4 –Protección radiológica en radioterapia
SMALL FIELDS IN RADIOTHERAPY: SOURCE OCCLUSION EFFECTS
Fernández, María Luz1,2*; Minsky, Daniel M.1 ; Sánchez, Gustavo3
1 Comisión Nacional de Energía Atómica (CNEA). Argentina.
2 Universidad Nacional de La Plata. Argentina.
3 Facultad de Ciencias Exactas, Universidad Nacional de La Plata. Argentina.
* Responsible author, email: fernandezmarialuz@gmail.com
The need of small radiation fields is increasing in modern radiotherapy. The use of these fields,
typically smaller than 4cmx4cm, involves several dosimetric challenges due to the lack of
electronic equilibrium, the availability of detectors with dimensions comparable to the field size
and the effect of the source size. This last effect contributes tothe difficulties by widening the
profiles resulting in that the methods to determinate field sizes such as Full Width at a Half a
Maximum (FWHM) fail. In this work, source occlusion was determined by profile measures for
field sizes of 2cmx2cm, 1.5cmx1.5cm and 1cmx1cm in a Varian 6EX at 10cm depth in a water
phantom, and compared to MonteCarlo calculation and geometrical data. The numeric model
was validated with a 10cmx10cm field at 10cm depth. Results show that for a 10cmx10cm field
the occlusion was 1%, both for experimental and simulated data. In the case of 2cmx2cm field
values, FWHM for experimental and simulated data were lower than geometrical data in 3% and
8% respectively. For 1.5cmx1.5cm and 1cmx1cm fields the occlusion for experimental data was
10% and 18% and for simulated data was 15% and 10% respectively. This demonstrates the
influence of source size in the determination of FWHM when collimators jaws set fields of small
dimensions. Finally we discuss the strong influence of the measure voxel dimensions in the
accuracy of calculation.