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Title: TH-C-19A-08: PDD Discrepancies at Opposite Biases From Very Small Volume Ion Chambers When Using Water Scanners

Purpose: As more so-called micro ion chambers become commercially available, medical physicists may be inclined to use them during the linear accelerator commissioning process, in an attempt to better characterize the beam in steep dose gradient areas. The purpose of this work is to inform the medical physics community of a non-trivial, anomalous behavior observed when very small chambers are used in certain beam scanning configurations. Methods: A total of six ion chambers (0.007cc to 0.6cc) were used to scan PDDs from a 10×10cm2 field at both +300V and −300V biases. PDDs were scanned using three different water tank scanning systems to determine whether different scanners exhibit the same abnormality. Finally, PDDs were sampled using an external electrometer to bypass the internal electrometer of the scanner to determine the potential contributions of the scanner electronics to the abnormality observed. Results: We observed a reproducible, significant difference (over-response with depth) in PDDs acquired when using very small ion chambers with certain bias and watertank combinations, on the order of 3–5% at a depth of 25 cm in water. This difference was not observed when the PDDs were sampled using the ion chambers in conjunction with an external electrometer. This suggests amore » contribution of interference produced by the controller box and scanning system, which becomes significant for the very small signals collected by very small ion chambers, especially at depth, as the signal level is reduced even further. Conclusion: Based on the results observed here, if currently available very small ion chambers are used with specific bias and scanning water-tank combinations, erroneous PDD data may be collected. If this data is used as input to the Treatment Planning System, systematic errors on the order of 3%–5% may be introduced into the treatment planning process.« less
Authors:
; ; ; ; ; ;  [1] ; ; ;  [2] ;  [3]
  1. University of Utah Hospitals, Salt Lake City, UT (United States)
  2. University Louisville, Louisville, KY (United States)
  3. Center for Cancer Care, Huntsville, AL (United States)
Publication Date:
OSTI Identifier:
22409832
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 41; Journal Issue: 6; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; COMMISSIONING; DEPTH DOSE DISTRIBUTIONS; DOSIMETRY; IONIZATION CHAMBERS; LINEAR ACCELERATORS; QUALITY ASSURANCE; RADIOTHERAPY; TANKS; WATER