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Letter to the Editor: “Evaluation of pressure-controlled mammography compression paddles with respect to force-controlled compression paddles in clinical practice”

by G.J. den Heeten, C.A. Grimbergen

Evaluation of pressure-controlled mammography compression paddles with respect to force-controlled compression paddles in clinical practice

Dear Editor,

With great interest we read the article by Jeukens et al. “Evaluation of pressure-controlled mammography compression paddles with respect to force-controlled compression paddles in clinical practice” [1]. We would like to thank the authors for their attention for pressure-controlled compression as compared to force-controlled compression in mammography.

We would like to emphasize that the two compression approaches are fundamentally different. In the force-controlled protocol, a target (range) expressed in force is being used. In the pressure-controlled protocol a fixed pressure is being pursued. Pressure is related to force and is calculated as force divided by contact area.

In the force-controlled protocol, the authors used a conventional rigid paddle. In this protocol (generally used in hospitals around the world) the information of the compression force is displayed. However, in practice local guidelines, technician’s experience and the assertiveness of the patient determine the degree of compression.

In the pressure-controlled protocol the authors used the first edition of a commercially available pressure-controlled rigid compression paddle (Sensitive Sigma Paddle, Sigmascreening BV). Our group has been involved in developing pressure-guided compression in mammography, using a dedicated paddle with a transparent conducting foil which enables a real-time measurement of the pressure, communicated to the technicians by LED lights [2, 3]. Since our work was cited 6 times, we felt the obligation to give our response.

In the Materials and Methods section two compression conditions are defined:
In the force-controlled compression “the paddle is lowered until a force of 17 – 18 daN was reached or until the patient indicated intolerable pain”.
“With the pressure-controlled paddle, the breast was compressed until the target pressure of 10 kPa (75 mmHg) was reached”.

In Figure 3 of the manuscript, scatterplots show the compression force and pressure of individual exams of both compression conditions; the median and interquartile range (IQR) can be found in the legends. It becomes clear that the force-controlled protocol shows very high forces (median force 17 daN, IQR: 15-18 daN) and uncommonly high pressures. Zooming in on the pressures calculated in the force-controlled protocol, it appears that the median of the calculated pressures is 25.3 kPa (187 mmHg). This means that 50% of the patients were submitted to pressures between 25.3 kPa and 80 kPa (600 mmHg). We are aware of one study from 2013 where in a 18 daN target force protocol the median pressure (in the CC-view) was not more than 20 kPa [3]. Median pressures of more than 25 kPa can not be found in literature [4–7].

Since the authors found no differences between the force- and pressure-controlled protocol, the same forces and pressures apply for the pressure-controlled protocol. The pressure-controlled compression paddle is meant to achieve median pressures of 10 kPa with a range of 8–14 kPa (60–105 mmHg) and as they described in the methods “The target pressure is reached when six LED’s light up”. However, as can be seen in Figure 3d, only a small number (less than 10%) of pressures are centered around 10 kPa. On top of this, the pressures revealed in this paper are on average more than 2.5 times higher than the intended use of the device and, just as important, as intended by the authors themselves. In our opinion, this shows that the intended pressure-controlled protocol was not realized. An explanation of this surprising result would be expected, but is missing.

The fact that the extremely high averages and variations are not mentioned at all in the discussion of the manuscript, suggests that this is not considered as a key outcome by the authors. In addition, the conclusion of the authors is: “…we found no basis for preferring the pressure-controlled paddle over the force-controlled paddle in our clinical practice”.
In our view, the conclusion should have been that the authors in this paper showed in both protocols rather extreme forces and pressures, and in any case failed to realize the proposed pressure-controlled protocol. To our opinion, a comparison between two protocols can only be made if the proposed protocols are executed as intended.

To come back to the title of this paper, we obviously disagree with the term “evaluation of pressure-controlled compression”, because there were no data presented to substantiate this.