The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers

Abstract

The purpose of this study was the investigation of perturbation factors for microionization chambers in small field dosimetry and the influence of penumbra for different spot sizes. To this purpose, correlated sampling was implemented in the EGSnrc Monte Carlo (MC) user code cavity: CScavity. CScavity was first benchmarked against results in the literature for an NE2571 chamber. An efficiency increase of 17 was attained for the calculation of a realistic chamber perturbation factor in a water phantom. Calculations have been performed for microionization chambers of type PinPoint 31006 and PinPoint 31016 in full BEAMnrc linac simulations. Investigating the physical backgrounds of the differences for these small field settings, perturbation factors have been split up into (1) central electrode perturbation, (2) wall perturbation, (3) air-to-water perturbation (chamber volume air-to-water) and (4) water volume perturbation (water chamber volume to 1 mm(3) voxel). The influence of different spot sizes, position in penumbra, measuring depth and detector geometry on these perturbation factors has been investigated, in a 0.8 x 0.8 cm(2) field setting. p(cel) for the PP31006 steel electrode shows a variation of up to 1% in the lateral position, but only 0.4% for the PP31016 with an Al electrode. The air-to-water perturbation in the optimal scanning direction for both profiles and depth is most influenced by the radiation field, and only to a small extent the chamber geometry. The PP31016 geometry (shorter, larger radius) requires less total perturbation within the central axis of the field, but results in slightly larger variations off axis in the optimal scanning direction. Smaller spot sizes (0.6 mm FWHM) and sharper penumbras, compared to larger spot sizes (2 mm FWHM), result in larger perturbation starting in the penumbra. The longer geometries of the PP31006/14/15 exhibit in the non-optimal scanning direction large variations in total perturbation (p(tot) 1.201(4) (0.6 mm spot, 3 mm off axis, type A MC uncertainty) to 0.803(4) (5 mm off axis)) mainly due to volume perturbation. Therefore in IMRT settings, when the detector is not always in the optimal scanning direction, the PP31016 geometry requires less extreme perturbation (max p(tot) 1.130(3)) and shows less variation. However, these results suggest that small variations in positioning, spot size or MLC result in large differences in perturbation factors. Therefore even these 0.016 cm(3) ionization chambers are limited in their use for a field setting of 0.8 x 0.8 cm(2), as used in this investigation.

Ionization chamber gradient effects in nonstandard beam configurations

“…In order to obtain a consistent conversion of absorbed dose in the chamber cavity to absorbed dose in water ͑i.e., the product of all perturbation factors must be equal to the ratio of absorbed dose in water to absorbed dose in the chamber͒, a series of scoring volumes is defined starting at the fully modeled chamber and ending at a small volume of water placed at the centroid of the chamber. Similar to previous studies, [23][24][25] a series of cavity doses is defined as follows: with liquid water ͑density of 1.000 g / cm 3 ͒, and ͑7͒ D w,point : absorbed dose in a 1 mm radius sphere of water placed at the centroid of the chamber and representing absorbed dose at a point in water at the location of measurement.…”

“…The convention adopted here corresponds to the definitions used in previous publications. [23][24][25] The Exradin A12 and A14 cylindrical ionization chambers ͑Standard Imaging, WI, USA͒ are modeled based on drawings kindly provided by the manufacturer. A series of scoring volumes is created, with the chamber model modified for different, correlated calculations as described above.…”

“…For ionization chambers, this perturbation is caused by several factors, which can be classified as follows: ͑1͒ The presence of structural and electronic components, ͑2͒ a detection material with a different atomic composition than water, ͑3͒ a detection material with different density than water, and ͑4͒ a detector volume that is finite. As formalized in reference dosimetry protocols, 11-14 and more recent publications, [23][24][25] ionization chamber perturbation factors represent each of these effects individually ͑although in practice, they are correlated͒ when converting absorbed dose to the chamber to absorbed dose to water. The presence of the chamber stem, central electrode, and wall will cause the charged-particle fluence to change in terms of quality and quantity due to attenuation and scatter in these components.…”

Decomposition of the dose conversion factor based on fluence spectra of secondary charged particles: Application to lateral dose profiles in photon fields

“…Over the past decades, Monte Carlo methods have been improved in accuracy such that they are more and more used for the calculation of the dose conversion factor, in particular for the dosimetry under reference conditions using air‐filled ionization chambers. Furthermore, the Monte Carlo‐based decomposition of the dose conversion factor into perturbation factors as described by Bouchard et al . and others turned out to be an effective method for the dosimetry also under nonreference conditions.…”

Dosimetry for Small Fields in Stereotactic Radiosurgery Using Gafchromic MD-V2-55 Film, TLD-100 and Alanine Dosimeters

“…Ionization chambers that are commonly used in CD fields are not suitable in small and nonstandard radiation fields because of a lack of spatial resolution and accuracy in the absorbed dose measurements caused by the fluence perturbation. Besides this, a recent study that quantified perturbation factors for small ionization chambers in small field dosimetry has revealed that even 0.016 cm 3 volume ionization chambers are not suitable to be used in a 0.8×0.8 cm 2 field [11]. Another disadvantage of the small ionization chamber is the amount of charge collected within the radiation field which can be comparable to the leakage of the dosimetry system itself.…”