Improving Health Through Medical Physics


David Followill, PhD | Houston, TX

AAPM Newsletter — Volume 44 No. 3 — May | June 2019

A Change in OSLD Electron Depth Dose Criterion

A review of the IROC Houston QA Center OSLD electron depth dose results was recently performed. We had noticed that our ±5 mm criterion was never exceeded unless the OSLD were irradiated with the wrong energy. We researched the origin of the 5 mm criterion and could not find any data, reference or person who could identify its origin. In the 1980s when electron beam checks were initiated, it was probably a consensus value of the RPC physicists at the time, and the value continued up until today.

An analysis of the electron depth dose results since 2010 was performed. Just as we have done in the past for the output checks, we established what the three sigma level was for our depth dose results to establish a more meaningful acceptance criterion. These data are shown in the table below.

OSLD Results (2010-2019)
Nominal Energy average (mm) N std Dev (mm) 3 x (std dev) (mm) Proposed new criteria (±mm) # of e- PDD audits outside prop. Criteria*
6 0.4 3138 0.85 2.6 3 4
9 0 2889 0.89 2.7 3 7
12 0.2 3055 0.97 2.9 3 15
15/16 -0.2 2785 1.04 3.1 3 10
18 -0.4 1131 1.28 3.8 4 16
20-22 -0.7 1799 1.46 4.4 4 15
* based on results between 2010-2019

Instead of the ±5 mm criterion used in the past, the new IROC Houston OSLD electron depth dose criterion will be ±3 mm for nominal electron energies between 6 and 16 MeV and ±4 mm for nominal electron energies greater than 16 MeV. Our new criterion would have earmarked a total of 67 results that needed correction since 2010, as compared to 23 results using the 5 mm criterion. To be outside of the new criterion, is to be a true outlier compared to the nearly 15,000 other electron beams monitored by IROC Houston during the past nine years. Implementation of this new OSLD electron depth dose criterion will begin on May 1, 2019.

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