Small Fields in Radiotherapy: Source occlusion effects

M.L. FERNÁNDEZ; D.M. MINSKY; G. SÁNCHEZ

Buenos Aires

Congreso; X Congreso Regional Latinoamericano IRPA de Protección y Seguridad Radiológica; 2015

Sociedad Argentina de Radioprotección

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 to the 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 Monte Carlo 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.