USG in breast cancer: the old and the new

The traditional role of ultrasound in imaging of the breast is to further evaluate masses or asymmetries and to help differentiate a solid mass from a cyst. In breast cancer, it has been used as a diagnostic follow-up to an abnormal screening mammogram. The addition of ultrasonography to mammography increases sensitivity for small cancers but decreases specificity. Ultrasound (US) is also used to provide guidance for biopsies and other interventions. It is the first line of imaging in a woman who is pregnant or less than 30 years old with focal breast symptoms or findings, where both the sensitivity of ultrasound and negative predictive value for malignancy were 100 per cent. US of the axilla also detects suspicious lymph node metastasis, especially in the obese. 

Results of a recent clinical trial from American Radiology Services Inc, conducted at the John Hopkins Institute and published in JAMA 2008, showed that addition of US to mammographic screening of the breast will yield an additional 1.1 to 7.2 cancers per 1000 high-risk women. However, this study also reported a higher incidence of false positives leading to more number of biopsies/ cytology in the screened population.
 

The use of US to guide core needle biopsies (CNBs) from a breast lesion for diagnosis of nonpalpable lesions is faster and better tolerated by some patients than stereotactic mammography. The use of US requires that the lesion can be well visualised by ultrasound and confidence that the ultrasound finding and mammographic finding represent the same thing. With US-guided core needle biopsy, passage of the needle through the lesion can be directly visualised and confirmed; as a result, fewer samples (usually three to five) are needed to provide diagnostic material. If the lesion is better visualised mammographically and is difficult to reproduce reliably on US, then stereotactic guidance is the preferred method. 

More recent uses of US in the management spectrum of breast cancer have been in the area of interventions during neoadjuvant chemotherapy and breast conservation surgeries for patients who have large or locally advanced tumours for which neoadjuvant (induction) chemotherapy is considered, careful anatomic localisation is critical to ensure that the surgeon can localise the area of tumor after neoadjuvant therapy. Typically, the lesion is measured both clinically and through ultrasonograpy, reported in terms of size, the “o’clock” location on the breast surface, and the distance of the lesion from the nipple. The use of radio-opaque clips placed at the time of biopsy to localise the primary tumour in case there is a complete clinical and radiographic response to induction therapy can also be tried under sonographic guidance.
 

Ultrasound is useful in evaluating the local extent of breast cancer and can identify additional tumour in the same breast thereby altering surgical management (mastectomy verses breast conservation) in up to 18 per cent of women. 

Ultrasound is appropriate in evaluating implants in a woman with contradictions to MRI or where MRI is not available. It can be used to check the integrity of a silicone implant capsule. Leak of silicone to the surrounding breast tissue causes a typical “snowstorm” appearance. Similarly, intracapsular rupture can be diagnosed on ultrasound by a characteristic “stepladder” appearance. 

Newer modalities of USG 

Real time tissue elastography: Conventional uses of mammography for breast screening over the years have put forth some interesting caveats. For example, in Japan where the average age of diagnosis of breast cancer is a woman in her 40s, the specificity of mammography as a single tool of screening is unacceptably less, because the breast tissue is quite dense in this age group. Also, the global trend of breast cancer diagnosis is at present towards a younger population of women, who are mostly in their 30s, where simply getting a mammogram done won’t be sufficient as a screening tool. All this has created a need for a tool which can add to, and increase the specificity of mammography, and if necessary which can also be used as a standalone modality of screening or diagnosis. 

Elastography was described at the beginning of the 1990s by Ophir et al. In 1997, Garra et al published the first clinical study showing the potential of elastography in the detection and characterisation of breast lesions. But it was only after 2004 that it became possible to use this technique simultaneously with conventional sonography, thanks to new equipment with probes that allowed both real-time B-mode sonographic and elastographic studies.

Ultrasonography works as an essential tool in diagnosing breast cancer, especially in women with dense breasts and in detecting small cancers of the breast. In recent years, real-time tissue elastography has come up in a big way as an auxillary tool in the evaluation of cancer of the breast. This method uses colour evaluation of the degree and distribution of tissue strain, induced by tissue compression with an ultrasound device. Compared with conventional B-mode sonography, it scores a lot higher in specificity, so much so that it might replace histological proof of some breast lesions in the near future. It can also be useful in the preoperative assessment of the margins in breast cancer. Further development of ultrasound elastography is expected. 

Method: Real time elastography visualises the degree and distribution of strain induced by light compression in a real-time manner when artificial light compression is applied to breast tissue. Results are noted when soft tissue receives greater strain, while stiff tissue receives less (Shiina et al., 2002). The images are classified into five patterns (Itoh et al., 2006, Tsukuba Elastography Score, Itoh, 2007). 

Interpretation: Scores from one to three showing green images of strain with and without tense blue images are judged to be benign. Scores four and five showing blue images are diagnosed as malignant. 

Score 1. Strain in the entire hypoechoic area
Score 2. Strain not seen in part of the hypoechoic area
Score 3. Strain only in the peripheral areas and not at the center of the hypoechoic area. Score 4. No strain in the entire hypoechoic area; and
Score 5. No strain either in the hypoechoic or surrounding areas
 

Preoperative assessment of margins in breast cancer 

With increasing awareness and use of breast conservation surgery, evaluation of the margins of excision of a lesion has become one of the important parameters of management. A precise grasp of the degree of invasion into the breast tissue and the presence and degree of the extensive intraductal component is important to assess margins of excision. As a tool for preoperative assessment of the extent of cancer, 3D MR mammography which utilises MRI is superior to mammography or ultrasound (Esserman et al., 1999; Nakamura et al., 2002). On the other hand, to achieve negative margins in the operating room, the usefulness of ultrasound with its easy portability and improvements in technology is well known (Henry-Tillman et al., 2001). Furthermore, ultrasound is superior in specificity but inferior to MRI in sensitivity (Tamaki et al., 2002). Considering the above facts, application of ultrasound elastography to assess the extent can be considered in both pre and intraoperative setting.
 

Ultrasound tomography 

A recent study from Karmanos Cancer Institute, Detroit, US by Duric et al describes the construction and use of a prototype tomographic scanner and reports on the feasibility of implementing tomographic theory in practice and the potential of US tomography in diagnostic imaging. 

Method: Data were collected with the prototype by scanning two types of phantoms and a cadaveric breast. A specialised suite of algorithms was developed and utilised to construct images of reflectivity and sound speed from the phantom data. 

Results: The basic results can be summarised as follows: 

1. A fast, clinically relevant US tomography scanner can be built using existing technology.
2. The spatial resolution, deduced from images of reflectivity, is 0.4 mm. The demonstrated 10 cm depth-of-field is superior to that of conventional ultrasound and the image contrast is improved through the reduction of speckle noise and overall lowering of the noise floor.
3. Images of acoustic properties such as sound speed suggest that it is possible to measure variations in the sound speed of five m/s. An apparent correlation with x-ray attenuation suggests that the sound speed can be used to discriminate between various types of soft tissue.
4. Ultrasound tomography has the potential to improve diagnostic imaging in relation to breast cancer detection.