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Medical Physics / Volume 37 / Issue 6 / RADIATION THERAPY PHYSICS

Med. Phys. 37, 2404 (2010); http://dx.doi.org/10.1118/1.3392247 (10 pages)

Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields

Eunah Chung and Jan Seuntjens

Medical Physics Unit, McGill University, Montreal General Hospital (L5-113), 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada

Hugo Bouchard

Département de Physique, Université de Montréal, Pavillon Roger-Gaudry (D-428), 2900 Boulevard Édouard-Montpetit, Montréal, Québec H3T 1J4, Canada and Département de Radio-Oncologie, Centre Hospitalier de l’Université de Montréal (CHUM), 1560 Sherbrooke Est, Montréal, Québec H2L 4M1, Canada

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(Received 16 November 2009; accepted 17 March 2010; revised 17 March 2010; published online 5 May 2010)

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Purpose: To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields.
Methods: A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [ Alfonso et al., Med. Phys. 35, 5179–5186 (2008) ], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10×10 cm2 field was measured using three radiation detectors: Gafchromic® EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors kQpcsr,Qfpcsr,fref were determined for five different types of air-filled ionization chambers (Exradin A12, NE2571, Exradin A1SL, Exradin A14, and PinPoint® 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery).
Results: The combined standard uncertainty in measuring the correction factor kQpcsr,Qfpcsr,fref using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than ±0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955–0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922–1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence.
Conclusions: The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors kQpcsr,Qfpcsr,fref for air-filled ionization chambers at the 0.3% 1σ uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.

© 2010 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

We acknowledge the contributions of Justin Sutherland (Carleton University). Dr. S. Vatnitsky is thanked for his guidance on the diamond detector. This paper benefited from the reviews by the anonymous associate editor and referees. This work was supported in part by NSERC Grant No. RGPIN 298181. E.C. is a recipient of a P. G. Fellowship of the Dept. Physics at McGill University. H.B. is a recipient of a NSERC doctoral scholarship.

Article Outline

  1. INTRODUCTION
  2. FORMALISM
  3. MATERIALS AND METHODS
    1. Experimental apparatus
      1. Dynamic head and neck IMRT delivery
      2. Reference detectors
      3. Air-filled ionization chambers and electrometer
    2. Experimental techniques
      1. Gafchromic ® EBT film calibration
      2. Dose rate dependence test of the diamond detector
      3. Response stability and ion recombination effect tests of the GLIC-03
      4. RDF measurements and calculations
      5. RCD measurements
  4. RESULTS
    1. RDF measurements and calculations
    2. RCD measurements
    3. Correction factors kQpcsr,Qfpcsr,fref
  5. DISCUSSION
  6. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3392247

KEYWORDS and PACS

Keywords

computerised tomography, diamond, dosimetry, liquid ionisation chambers, particle detectors, phantoms, radiation therapy, nonstandard field, plan-class specific reference field, correction factor, radiation dosimetry

PACS

  • 87.50.wj

    Dosimetry/exposure assessment

  • 29.40.Cs

    Gas-filled counters: ionization chambers, proportional, and avalanche counters

  • 87.50.wp

    Therapeutic applications

  • 87.55.dk

    Dose-volume analysis

  • 87.57.Q-

    Computed tomography

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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