Fwd: Mammography with two different screens

[Ccja@aol.com]

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Forwarded message:
From:	zamenhof@MIT.EDU (Robert G. Zamenhof)
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	MEDPHYS@LISTS.WAYNE.EDU (Medical Physics Mailing List)
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-03-11 16:22:23 EST

> I would like to get some feedback to the following occurrence.  A hospital
> that I serve started using a new cassette/screen (about two months ago)
>along with their old ones (film remained unchanged) without informing me.
The
>FDA inspector came to make an inspection and was given the choice of using
>either cassette, they did
> not conceal the fact that they were using both types of cassettes
>/screens, he judged their performance acceptable. I came in the following
>week to  perform the annual medical physicist inspection. I discovered what
>had happened and determined that the performance of the two cassette/screen
>systems both gave acceptable
> densities 1.35 vs. 1.55 under the same conditions (DMR using the CNT mode
of
>operation), I suggested however that I thought th
>at using the two systems was not desirable and that they should use only one
>of the two cassette/screen systems. Questions  - am I being to picky and
>should I be concerned that the FDA inspector did not make note of what I
>thought was an undesirable
> practice. Note - each system individually would meet the requirements as
>would both systems together however the average density of the two system
>would be different by 0.
>2. I don't know that it happened but I suppose that it is p


I am surprised that you passed the FDA component of the MQSA inspection!
However, I think that it is dangerous practice to do this for the following
reasons:

1)  the different densities of 1.35 vs 1.55 is a difference in film
darkness that is visible.  Unless the radiographers kept a very careful
watch over which system was being used and correspondingly adjusted the
density control, you would, in essence, end up with an ADDITIONAL film
density variability of +/-0.1 OD OVER AND AVOVE what is permitted.  Even if
the cassettes were wildly different colors, experience tells me that unless
you have just one or two tremendously dedicated full-time mammographic
radiographers, this approach of making separate density adjustments for
each system realistically would not work.

2)  Another serious consequence is that the different screens (depending on
what they actually are) might respond differently to changes in X-ray
spectrum; so, if your DMR decided to automatically raise the kVp during the
exposure when it encountered a specially thick or dense breast, the
differences in film density might become even further apart.

But the question really is -- why? Why did the department change to
different screens/cassettes? There does not appear to be significant gain
in speed which would warrant the new screen, and the fact that they
continue to use the old cassettes and that you passed the inspection
suggests that they were not replaced because they were in such bad shape??

Robert G. Zamenhof, Ph.D.
Director, Section of Radiological Physics
Department of Radiology
Beth Israel-Deaconess Medical Center West Campus, Harvard Medical School
1 Deaconess Road, Boston, MA 02215, U.S.A.
Tel: (617) 732-2003  Fax: (617) 732-2005  E-Mail: zamenhof@mit.edu
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