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Systematic in vivo dosimetry for quality assurance using diodes. Part 1: Experiences and results of the implementation of entrance dose measurements

Published online by Cambridge University Press:  31 January 2005

R. Appleyard ,
K. Ball ,
F.E. Hughes ,
W. Kilby ,
S. Lassen ,
R. Nicholls ,
V. Rabett ,
J. Sage ,
M. Smith  and
E. Thomson
R. Appleyard
Affiliation:
Sheffield Hallam University
K. Ball
Affiliation:
Addenbrookes Hospital, Cambridge
F.E. Hughes
Affiliation:
Norfolk and Norwich University Hospital, UK
W. Kilby
Affiliation:
Norfolk and Norwich University Hospital, UK
S. Lassen
Affiliation:
Finsen Centre, Copenhagen, Denmark
R. Nicholls
Affiliation:
Norfolk and Norwich University Hospital, UK
V. Rabett
Affiliation:
Norfolk and Norwich University Hospital, UK
J. Sage
Affiliation:
Clatterbridge Centre for Oncology, Wirral, UK
M. Smith
Affiliation:
Norfolk and Norwich University Hospital, UK
E. Thomson
Affiliation:
Accuray Incorporated, Sunnyvale, USA
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Abstract

Purpose: This paper describes our experiences of implementing systematic in vivo dosimetry at the Norfolk and Norwich Hospital and reviews the results of 2,254 entrance dose measurements made over a 17-month period.

Methods and materials: Entrance dose measurements using p-type diodes were performed on all new planned patients. The calibration procedure and correction factors applied are described. A 4% tolerance was applied.

Results: The results of all measurements indicated a small mean deviation from expected entrance dose of 10.77% and a standard deviation of 2.85%. 16.7% of all measurements exceeded the 4% tolerance with 9.2% exceeding a 5% level. The estimated overall errors for 578 treatments were calculated using the weighted averages of all beams. A narrower SD of 1.96% combined with only 4.8% of all treatments exceeding a 4% tolerance show that large deviations from a single field do not always translate into significant overall errors.

Conclusions: Global dosimetric accuracy was within clinically acceptable limits and variations between measured and expected doses were mainly attributable to factors affecting diode reading. A number of errors in calculating deviations and the inconsistent application of the protocol suggest the need for interfacing the diode system with software control.

Keywords

In vivo dosimetrydiodesquality assurance
Type
Original Article
Information
Journal of Radiotherapy in Practice , Volume 3 , Issue 4 , March 2003 , pp. 185 - 196
Copyright
© 2004 Cambridge University Press

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