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Background
Breast cancer is an increasing global burden, resulting in the highest incidence of cancers worldwide accounting for 11.7% of all cancer cases. Tissue diagnosis is essential for treatment planning of which stereotactic-guided breast biopsy has become an important method to biopsy suspicious microcalcifications or mass, focal asymmetry, asymmetry, or architectural distortion visualized on mammography with no sonographic correlate.

Educational Goals / Teaching Points
In this education exhibit, we discuss various concepts such as lesion localization and targeting, including coordinate axes, parallax shift and derivation of z coordinate, and different types of biopsy systems and biopsy needles including their advantages and disadvantages, which radiologists must know for a successful biopsy procedure. We explain how to select a biopsy approach and terminology such as biopsy excursion, stroke, stroke margin and compression. We also deconstruct and provide solutions for various difficult scenarios that are commonly encountered in the stereotactic breast biopsy procedure such as a thin compressed breast, lesion in the far posterior position or close to the nipple, blind spot images, localization errors and clip migration.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
It is essential to review screening and diagnostic mammography images before the biopsy procedure. The lesion of interest is centered according to the previous images, then angulated mammographic images of the lesion at +15 and - 15 degrees are acquired for calculation of the stereotactic coordinates. The biopsy gun is deployed to the calculated location and prefire images are acquired. If the position of the needle is satisfactory, the gun is deployed for sample collection and if not, the needle can be repositioned. After deploying the gun, postfire images are obtained to confirm the needle position. There are two types of biopsy tables - prone and upright, each with a set of advantages and disadvantages. There are primarily two types of biopsy needles based on size - standard needles for breasts with compression greater than 3 cm and petite needles with thin breasts which are less than 3 cm thick on compression. Various concepts related to biopsy needle and compression such as biopsy excursion, stroke and stroke margin play a role in the selection of the appropriate needle for breast biopsy. Biopsy from a thin breast can be done by increasing the thickness by manoeuvres such as injection of lidocaine or saline to increase the thickness or bolstering the breast or double paddle technique. Migration of biopsy clip by accordion effect can be improved by slight decompression. Far posterior lesions can be biopsied by arm-in-hole technique.

Conclusion
This educational exhibit can function as a practical guide to stereotactic biopsy for beginners and trainee radiologists. Sound knowledge of the principles and technique of stereotactic breast biopsy is important to accurately target and sample a breast lesion of concern. In addition, a radiologist should be aware of possible challenging stereotactic biopsy scenarios and techniques to address these situations appropriately.