Comments on “Clarification of proper ion chamber biasing”

For most basic radiobiological research applications involving irradiation of small animals, it is difficult to achieve the same high precision dose distribution realized with human radiotherapy. The precision for irradiations performed with standard radiotherapy equipment is ±2 mm in each dimension, and is adequate for most human treatment applications. For small animals such as rodents, whose organs and tissue structures may be an order of magnitude smaller than those of humans, the corresponding precision required is closer to ±0.2 mm, if comparisons or extrapolations are to be made to human data. The Leksell Gamma Knife is a high precision radiosurgery irradiator, with precision in each dimension not exceeding 0.5 mm, and overall precision of 0.7 mm. It has recently been utilized to treat ocular melanoma and induce targeted lesions in the brains of small animals. This paper describes the dosimetry and a technique for performing irradiation of a single rat eye and lens with the Gamma Knife while allowing the contralateral eye and lens of the same rat to serve as the “control”. The dosimetry was performed with a phantom in vitro utilizing a pinpoint ion chamber and thermoluminescent dosimeters, and verified by Monte Carlo simulations. We found that the contralateral eye received less than 5% of the administered dose for a 15 Gy exposure to the targeted eye. In addition, after 15 Gy irradiation 15 out of 16 animals developed cataracts in the irradiated target eyes, while 0 out of 16 contralateral eyes developed cataracts over a 6-month period of observation. Experiments at 5 and 10 Gy also confirmed the lack of cataractogenesis in the contralateral eye. Our results validate the use of the Gamma Knife for cataract studies in rodents, and confirmed the precision and utility of the instrument as a small animal irradiator for translational radiobiology experiments.

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Using an Ion-Chamber Matrixx Array to Access the Relative Contributions of Backscatter and Source Transit Time for 192I Surface Applications

Brachytherapy ◽

10.1016/j.brachy.2011.02.126 ◽

2011 ◽

Vol 10 ◽

pp. S68

Author(s):

Desmond A. O’Farrell ◽

Marianne Weiler ◽

Robert A. Cormack ◽

Mandar S. Bhagwat

Keyword(s):

Transit Time ◽

Ion Chamber

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Investigation of correction factors for non-reference conditions in ion chamber photon dosimetry with Monte-Carlo simulations

Zeitschrift für Medizinische Physik ◽

10.1016/j.zemedi.2009.09.003 ◽

2010 ◽

Vol 20 (1) ◽

pp. 25-33 ◽

Cited By ~ 5

Author(s):

Jörg Wulff ◽

Johannes T. Heverhagen ◽

Heiko Karle ◽

Klemens Zink

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Reference Conditions ◽

Correction Factors ◽

Ion Chamber ◽

Photon Dosimetry

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VMAT monthly QA using two techniques: 2D ion chamber array with an isocentric gantry mount and an in vivo dosimetric device attached to gantry

Journal of Radiotherapy in Practice ◽

10.1017/s1460396912000556 ◽

2013 ◽

Vol 13 (2) ◽

pp. 240-246 ◽

Cited By ~ 5

Author(s):

P. Myers ◽

S. Stathakis ◽

C. Buckey ◽

N. Papanikolaou

Keyword(s):

Dose Rate ◽

Volumetric Modulated Arc Therapy ◽

Field Measurements ◽

Rate Variation ◽

Speed Variation ◽

Ion Chamber ◽

Position Accuracy ◽

Picket Fence ◽

Isocentric Gantry

AbstractPurposeVarian RapidArc is a volumetric modulated arc therapy (VMAT) that obtains a conformal dose around the desired structure by employing variable gantry speed, dose rate and dynamic multileaf collimator (DMLC) speed as the gantry rotates about machine isocenter. This study is meant to build upon previous research by Ling et al. by completing the tests with an in vivo dosimetric device attached to the linac gantry and a 2D ionisation chamber array with an isocentric gantry mount.Materials and methodsTwo PTW detectors, seven29 array with gantry mount and DAVID, were attached to the linear accelerator gantry, allowing each device to remain perpendicular to the beam at all gantry angles. Three tests for RapidArc evaluation were performed on these devices including: dose rate and gantry speed variation, DMLC speed and dose rate variation and DMLC position accuracy. The reproducibility of the arc data was also reported.ResultsA picket fence plan varying dose rates (111 to 600 MU/minute) and gantry speeds (5·5 to 4·3°/second) was delivered consisting of seven sections of different combinations. These measurements were compared with static gantry, open field measurements and found to be within 2·39% for the DAVID device and 0·84% for the seven29. A four-section picket fence of varying DMLC speeds (0·46, 0·92, 1·84 and 2·76 cm/second) was similarly evaluated and found to be within 1·99% and 3·66% for the DAVID and seven29, respectively. For DMLC position accuracy, a picket fence arc plan was compared with a static picket fence and found to agree within 0.38% and 2.91%. Reproducibility for these three RapidArc plans was found to be within 0·30% and 2·70% for the DAVID and seven29.ConclusionThe DAVID and seven29 detectors were able to perform the RapidArc quality assurance tests efficiently and accurately and the results were reproducible. Periodic verification of DMLC movement, dose rate variation and gantry speed variation relating to RapidArc delivery can be completed in a timelier manner using this equipment.

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Comparison of film and ion chamber systems for depth-dose measurements for a 25 MV beam

Physics in Medicine and Biology ◽

10.1088/0031-9155/24/3/015 ◽

1979 ◽

Vol 24 (3) ◽

pp. 636-638 ◽

Cited By ~ 9

Author(s):

D W Anderson ◽

F St George

Keyword(s):

Ion Chamber ◽

Depth Dose ◽

Dose Measurements

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A study of enhanced dynamic wedge dosimetry using a 2D ion chamber array detector

Journal of Radiotherapy in Practice ◽

10.1017/s1460396915000370 ◽

2015 ◽

Vol 14 (4) ◽

pp. 394-402

Author(s):

S. A. Syam Kumar ◽

P. Aparna ◽

P. T. Anjana ◽

C. P. Aswathi ◽

G. P. Sitha

Keyword(s):

Array Detector ◽

Good Tool ◽

Dose Distributions ◽

Ion Chamber ◽

Dynamic Wedge ◽

Shoulder Region ◽

Different Depths ◽

Solid Water ◽

Scanned Images ◽

2D Array

AbstractThe purpose of this work was to study the dosimetric properties of the enhanced dynamic wedge using a Seven29 ion chamber array. The PTW Seven29 ion chamber array and solid water phantoms were used for the study. Primarily, the solid water phantoms with the two-dimensional (2D) array were scanned using a computed tomography scanner at different depths. Using these scanned images, planning was performed for different wedge angles at 6 and 15 MV. A dose of 100 CGy was delivered in each case. For each delivery, the required monitoring units (MUs) were calculated. Using the same setup with a Varian Clinac iX, the calculated MU was delivered for different wedge angles. Subsequently, the different wedged dose distributions that had been obtained were analysed using Verisoft software. A shoulder-like region was observed in the profile; this region reduced as depth increased. The percentage deviation between the planned and measured doses at the shoulder region fell within the range of 0·9–4·3%. The standard deviation between planned and measured doses at shoulder region in the profile fell within 0·08±0·02 at different depths. The standard deviations between planned and measured wedge factors for different depths (2·5, 5, 10 and 15 cm) were 0·0021, 0·0007, 0·0050 and 0·0001 for 6 MV and 0·0024, 0·0191, 0·0013 and 0·0005 for 15 MV, respectively. On the basis of the studies that we performed, it can be concluded that the 2D ion chamber array is a good tool for enhanced dynamic wedge dosimetry.

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