Technical needs can vary widely in stereotactic breast biopsy. This article addresses some of the more common special cases that may arise related to stereotactic breast biopsy QC.
ACR’s 1998 Stereotactic Breast Biopsy QC and Digital Mammography QC Manuals
Most of the tests that are required in the ACR Digital Mammography QC Manual are also required in the 1998 ACR Stereotactic Breast Biopsy QC Manual. However, some of the tests in the 1998 stereo QC manual (eg, annual kVp accuracy and reproducibility, annual collimation, etc.) are no longer required in the new ACR digital mammography manual. If a facility uses the ACR Digital Mammography QC Manual for a digital mammography unit, the stereotactic breast biopsy-specific tests may be added to the basic digital mammography annual survey report in order to comply with ACR requirements for the stereotactic breast biopsy QC, with one addition: You must also perform the phantom image quality using either the Small ACR Mammography Phantom or the Mini Digital Stereotactic Phantom (and not the larger, ACR Digital Mammography Phantom) for routine and annual phantom image quality evaluation.
In summary, if you use the ACR Digital Mammography Quality Control Manual to test a digital mammography with an add-on stereotactic device, you must also perform the following tests as outlined in the 1998 ACR Stereotactic Breast Biopsy QC Manual for annual QC of the stereotactic breast biopsy component of the system:
Localization Accuracy Test
Image Quality Evaluation (using the Small ACR Mammography Phantom or the Mini Digital Stereotactic Phantom)
The ACR plans to update the 1998 ACR Stereotactic Breast Biopsy QC Manual based on the ACR Digital Mammography Quality Control Manual and the ACR Digital Mammography Phantom.
QC for Large Field-of-View Units
If your stereotactic breast biopsy unit’s field-of-view is relatively large, you may use the following test procedure for phantom exposure with your digital image receptor:
Position the phantom (without the acrylic disk) so that the wax insert is centered in the field of view. Use the light localizer to facilitate the positioning. If the lateral approach is most commonly used for biopsies, apply compression to the phantom using a paddle without an aperture. If the vertical approach is most commonly used, do not install a compression paddle.
Take a scout view (0º) exposure using the site’s standard technique for a 4–4.5 cm thick compressed breast. Inspect the image to ensure that the entire insert area has been included. Record the technique factors on the data form.
As shown in Fig. 1, position the needle holder as far as possible from the chest wall and toward the right side to enable visualization of the fifth and sixth fibers.
Acquire a scout view image.
As shown in Fig. 2, move the needle holder as far as possible from the chest wall and toward the left side to enable visualization of the third and fourth fibers.
Acquire a scout view image.
View the images from steps 4 and 5 to determine the number of fibers, speck groups, and masses visible. Adjust window level and width settings to maximize detection of each object type. Use the scoring method for fibers, specks, and masses described below.
Examine the phantom images for artifacts. It may help to use several different window level and width settings to be sensitive to all artifacts.
Upright Add-On Biopsy Systems
Phantom selection: If a facility is using the 2016 ACR Digital Mammography Quality Control Manual (and new digital phantom) for their digital mammography unit and is also doing 2D stereo with an upright add on stereo attachment, the facility should continue using either the small ACR Mammography Phantom or the Mini Digital Stereotactic Phantom for routine quality control and submitting phantom images to the ACR for accreditation.
Separate exposures for unit and add-on device: In cases where the same phantom is used to perform routine phantom image quality testing for both the mammography unit and the add-on stereotactic breast biopsy system, the technologist may expose and evaluate the phantom once each week and have the results apply to both the mammography and stereotactic breast biopsy QC. (Be sure to document the reason why in your QC logs.) However, if techniques change for either mammography or stereotactic breast biopsy, your medical physicist should determine if the phantom image quality evaluation should be performed separately. (If there is a change, this should also be documented in your QC logs.)
AEC or Manual Exposure Performance Assessment: The ACR 1999 Stereotactic Breast Biopsy Quality Control Manual requires that the medical physicist performs a test for Automatic Exposure Control (AEC) or Manual Exposure Performance Assessment. This is difficult to accomplish with an add-on device since the needle apparatus remains in the way. The equivalent test of automatic exposure control that is annually performed for the digital mammography unit may be used to satisfy the requirements for the stereotactic breast biopsy unit. It should be representative of stereotactic breast biopsy performance.
Collimation Assessment for Filmless Facilities
Most facilities that have digital stereotactic breast biopsy equipment have gone totally filmless, removing all chemical processing. When wet processing of film is unavailable, there are several alternatives for the Collimation Assessment test in the Medical Physicist’s section of the ACR 1999 Stereotactic Breast Biopsy QC Manual:
CR cassettes may be substituted for screen-film cassettes
ISP – GAFCHROMIC XR-QA film is a self-developing radiochromic film. This film may be cut to size and exposed directly without the need of a cassette.
Polaroid Corporation provides sheet film that can be adapted for this use.
A revised test equipment list includes:
One or two mammography screen-film cassettes and sheets of mammography film, OR ISP – GAFCHROMIC XR-QA cut to appropriate size OR Polaroid film
Ruler with mm scale
System Limiting Resolution: Screen Film Versus Digital
The ACR understands that many stereotactic breast biopsy units are in a total filmless environment. In addition, overall “system resolution” is the most critical test. The Medical Physicist’s Stereotactic Unit QC Test Summary (see excerpt below) indicates that you only need to perform the evaluation related to the modality used, film or digital.
In addition to assessing the “consistency of system-limiting spatial resolution over time and in comparison to acceptance testing results”, the ACR also recommends that the results be compared against the manufacturer’s stated specifications.
Automatic Exposure Control Test on 8 cm Phantom
The Stereotactic Breast Biopsy Quality Control Manual’s Automatic Exposure Control (AEC) test calls for performance testing at phantom thicknesses of 2, 4, 6 and 8 cm. The performance criteria for digital systems specifies that “the signal value should remain within ±20% of the signal obtained for the 4 cm phantom…if it does not meet this criterion, the medical physicist should develop a technique chart which meets this criterion.”
Some units do not provide enough signal to pass this test for an 8 cm phantom. Some radiologists use manual modes with lower signals on very thick breasts, and don't find the lower signal a problem. Although the 1999 manual recognizes that it may be difficult to maintain signal at 8 cm under AEC (and thus allows for a manual technique at 8 cm), more recent experience has shown that these techniques can be quite long, and in some cases, not permissible on the equipment. In addition, medical physicists are finding that radiologists are used to “reading through” noisier images of very thick breasts. Consequently, in order to pass this test, only 2, 4 and 6 cm must meet the ±20% criteria. However, if the 8 cm test does not meet the criterion, the medical physicist should develop a technique chart for 2, 4, 6 and 8 cm showing 2-6 cm within range and 8 cm being as close as possible using a manual technique (and clinically acceptable).
Average Glandular Dose Test and Digital Image Receptors
The performance criteria of the Average Glandular Dose test in the Medical Physicist’s section of the ACR 1999 Stereotactic Breast Biopsy Quality Control Manual specifies that the “average glandular dose to an average (4.2 cm compressed) breast should not exceed 3 mGy (0.3 rads) per view for screen-film or digital image receptors.” For ACR accreditation, the intent is that the dose be assessed at the mode used clinically. The manual directs the medical physicist to use the technique that is clinically used to assess dose:
Page 78 – “Measure the typical entrance exposure for standardized breast thickness and composition (approximately 4.2 cm compressed breast thickness – 50% adipose, 50% glandular composition), calculate the associated average glandular dose, and assess short term exposure reproducibility.”
Page 78 – “4. Select the exposure technique normally used for stereotactic localizations.”
If the facility typically uses the 1024 matrix for stereotactic breast biopsies, the dose requirement applies to that mode; if the facility typically uses the 512 matrix for stereotactic breast biopsies, the dose requirement applies to 512.
Localization Accuracy Test
When performing the Localization Accuracy Test a targeting phantom with appropriately small (<5mm diameter) targets (i.e., gelatin or improvised phantom) should be used for the most clinically used approach. The ACR understands that manufacturers are not routinely providing gel phantoms to facilities for use with acceptance testing. The medical physicist may follow the manufacturer’s methodology for localization in air to verify accuracy in lieu of targeting in a phantom/patient for approaches other than the most clinically used approach. Documentation must be included to verify these approaches were tested and passed. The medical physicist has the option to use a phantom; gelatin phantom, an improvised phantom (ie: fruit with a radiopaque marker) or the manufacturer’s supplied localization in air phantom. If an appropriate phantom is not available for the most clinically used approach, then the physicist may use a clinical procedure performed by the staff to verify accuracy.
The Radiologic Technologist’s Test #3 (Hardcopy Output Quality Test) in the ACR 1999 Stereotactic Breast Biopsy Quality Control Manual requires that a printed SMPTE pattern be reviewed monthly by measuring the optical density at “four consistent locations on the film.” For facilities using breast biopsy attachments on Hologic Dimensions mammography units, the Hologic Dimensions QC manual has a similar, weekly DICOM Printer Quality Control test with the SMPTE pattern, but only three locations are evaluated. If the equipment manufacturer provides a different QC process and if it is reviewed and approved by the facility’s medical physicist, the facility may follow the manufacturer’s QC process for their hardcopy output device instead of the one provided in the Stereotactic Breast Biopsy QC Manual.
Similarly, the 1999 ACR Stereotactic Breast Biopsy Manual recommends a maximum compression force of at least 25 lbs (and between 25 to 40 lbs under power drive). Some Hologic prone units’ automatic compression can only reach a maximum of 15 lbs, manual compression can provide nearly 30 lbs of compression force. This is acceptable. Other Hologic prone units’ automatic compression can reach a maximum of 45 lbs. This is also acceptable (the MQSA FDA rules for mammography units allow for a maximum compression force under initial power drive to be as much as 47 lbs). Your facility should watch to see that these numbers do not change significantly over time (both during compression and over the years), and that the compression meets the manufacturer’s specifications.
The Stereotactic Breast Biopsy Quality Control Manual’s Digital Receptor Uniformity test calls for the SNR value at the corners of the image field of a uniform absorber to be within ±15% of the center. To pass this test, some Lorad service engineers have suggested a protocol for obtaining the SNRs using 32x32 ROI boxes centered well away from the corners of the 512x512 image (these coordinates are: 100,100; 100,400; 400,100; and 400,400). Many Lorad-based Hologic systems exhibit a signal intensity gradient at one or more edges of the field, which leads to a SNR gradient. Since the non-uniformity occurs only at the edge of the field, it should not compromise patient imaging. Also, it is gradient, not isolated inhomogeneities that might be mistaken for abnormalities. Throughout the Stereotactic Breast Biopsy Quality Control Manual, we have consistently deferred to manufacturer's specifications in the absence of hard data on how performance variations might affect image quality in many areas. Therefore, one can accept the manufacturer's test conditions and action limits. That being said, it may be possible to improve the uniformity on your Lorad system. Some medical physicists have found, and Lorad engineers have confirmed, that the gradient problem may arise because of the way flatfielding (a service engineer's task) is done. The digital image receptor is "flatfielded" without the steel compression paddle (which is also an X-ray beam-limiting aperture) in place to allow calibration of the image receptor to its edges. However, in phantom testing and in clinical use, the compression paddle/beam aperture is always in place, and the resultant reduced-size X-ray beam may have a different symmetry with respect to the edges of the receptor than when it was flatfielded. This effect can be minimized if the collimation at the X-ray tube is adjusted so the Xray beam is larger than the compression paddle aperture by precisely the same amount on each of the 4 edges. Lorad recommends between 5-10 mm for this margin.
GE Pristina Serena
The AEC (AOP) system in the General Electric Pristina Serena used for stereotactic biopsy is designed to optimize the CNR for a given dose rather than standardizing the signal level uniformity as a function of attenuator thickness. Consequently, the ±20% criteria in the 1999 QC Manual is not applicable to this system. Facilities using the GE Pristina Serena may use the test procedure and performance criteria provided by the manufacturer. Visit the GE Healthcare Customer Documentation Portal or consult with your vendor representative for the AEC (AOP) test procedure and performance criteria in the Pristina Serena QC Manual.
Revision History for this Article
Description of Revision(s)
Article created; FAQs incorporated; No criteria changes
Added Localization Accuracy Test
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