AbstractID: 9337 Title: Determination of dose in small radiation fields using plane parallel ionization
chambers with varying diameter
Introduction: Many modern radiotherapy techniques employ small fields in order to provide highly conformal and intensity
modulated dose delivery. It was the aim of the present study to develop an accurate technique for small field dosimetry using ion
chambers with different dimensions. Methods: Four custom-made plane-parallel ionization chambers, that vary in active volume only
(0.62cc to 0.007cc), were used in the study. External and internal dimensions of the chambers are identical as only the ratio between
the area of the collecting and guard electrode was varied. The four chambers were used in a solid water slab phantom to measure the
output in square (10x10 to 0.5x0.5cm2) and rectangular (10x10 to 10x0.5cm2) fields of a 10MV X-ray beam (Varian 2100). For the
rectangular fields, the measurements were completed using both the upper and lower jaws to define the width of the field. Results: It
was found that the output signal for each chamber was a constant ratio until the field width was less than the chamber’s active volume
diameter. Output factors could be measured down to field sizes of 5 mm in width for the rectangular fields. Results were similar for
both the upper and lower jaws. Conclusions: It is possible to reliably determine output of small X-ray fields down to fields of less
than 1 cm2 using a set of ionization chambers with different internal diameters. We are working on a mathematical model, which will
allow us to extrapolate the reading to even smaller field sizes.