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

Frederik Crop UGent, N Reynaert, GEERT PITTOMVILS UGent, LEEN PAELINCK UGent, Carlos De Wagter UGent, Luc Vakaet UGent and Hubert Thierens UGent (2009) PHYSICS IN MEDICINE AND BIOLOGY. 54(9). p.2951-2969
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.
Please use this url to cite or link to this publication:
author
organization
year
type
journalArticle (original)
publication status
published
journal title
PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol.
volume
54
issue
9
pages
2951 - 2969
Web of Science type
Article
Web of Science id
000265350300024
JCR category
RADIOLOGY, NUCLEAR MEDICINE & MEDICAL IMAGING
JCR impact factor
2.781 (2009)
JCR rank
25/104 (2009)
JCR quartile
1 (2009)
ISSN
0031-9155
DOI
10.1088/0031-9155/54/9/024
language
English
UGent publication?
yes
classification
A1
id
693447
handle
http://hdl.handle.net/1854/LU-693447
date created
2009-06-12 13:29:18
date last changed
2009-06-22 15:17:32
@article{693447,
  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.},
  author       = {Crop, Frederik and Reynaert, N and PITTOMVILS, GEERT and PAELINCK, LEEN and De Wagter, Carlos and Vakaet, Luc and Thierens, Hubert},
  issn         = {0031-9155},
  journal      = {PHYSICS IN MEDICINE AND BIOLOGY},
  language     = {eng},
  number       = {9},
  pages        = {2951--2969},
  title        = {The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers},
  url          = {http://dx.doi.org/10.1088/0031-9155/54/9/024},
  volume       = {54},
  year         = {2009},
}

Chicago
Crop, Frederik, N Reynaert, GEERT PITTOMVILS, LEEN PAELINCK, Carlos De Wagter, Luc Vakaet, and Hubert Thierens. 2009. “The Influence of Small Field Sizes, Penumbra, Spot Size and Measurement Depth on Perturbation Factors for Microionization Chambers.” Physics in Medicine and Biology 54 (9): 2951–2969.
APA
Crop, F., Reynaert, N., PITTOMVILS, G., PAELINCK, L., De Wagter, C., Vakaet, L., & Thierens, H. (2009). The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers. PHYSICS IN MEDICINE AND BIOLOGY, 54(9), 2951–2969.
Vancouver
1.
Crop F, Reynaert N, PITTOMVILS G, PAELINCK L, De Wagter C, Vakaet L, et al. The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers. PHYSICS IN MEDICINE AND BIOLOGY. 2009;54(9):2951–69.
MLA
Crop, Frederik, N Reynaert, GEERT PITTOMVILS, et al. “The Influence of Small Field Sizes, Penumbra, Spot Size and Measurement Depth on Perturbation Factors for Microionization Chambers.” PHYSICS IN MEDICINE AND BIOLOGY 54.9 (2009): 2951–2969. Print.