Breast cancer is a common female cancer, responsible for a large proportion of deaths. Its early and accurate detection is therefore a matter of intense research. The most frequently utilized method for this purpose is X-ray mammography, which uses ionizing radiation to generate a picture of the fibroglandular tissue of the breast. This has high sensitivity, but the limit of ionizing radiation to which a person can be exposed each year is well established, which rules out repeated use in the same patient.
As a result, other methods to screen for this cancer have been investigated. One of the most recent is the use of microwaves to image breast tissue.
Mammography achieves good resolution in breast imaging, and is therefore the most sensitive way to detect a non-palpable tumor, but at the cost of using radiation at higher frequencies. Microwave imaging (MWI) uses frequencies between 1 and 30 Hz. Though this reduces the resolution as compared to X-ray mammography, it is not associated with tissue damage.
Principle of MWI
The basis of MWI is the transparency of normal breast tissue to microwaves. Malignancies which have a higher vascular content have a higher permittivity and conductivity to microwaves, which increases the backscattering. Thus a MWI system depends upon using microwave antennas in arrays designed to fit around the breast so as to detect any small area which produces a larger backscatter compared to the surrounding medium. The scattered signals are then picked up by the same or separate antennas. These are then analyzed by powerful software to reconstruct the tumor within the breast. The microwaves used in MWI of the breast have a thousand times lesser intensity than that used by a mobile phone.
Advantages of MWI
- MWI offers reasonably accurate images without exposing the patient to ionizing radiation and the risk of damage to healthy tissue.
- It can be used to detect tumors of the breast in younger women, which are typically missed on mammography in Asia and other developing regions.
- New advancements in MWI such as ultrawide band microwave or microwave tomography make it more comfortable, less expensive, more sensitive, have higher specificity for tumor detection, and yield more useful data.
- MWI avoids the need for breast compression to achieve optimal imaging, which is an important advantage in that such compression of tender breast tissue is typically uncomfortable and often painful, making women unwilling to comply with the screening guidelines.
- It is a technically simple procedure which poses few risks to the woman’s health
Drawbacks of X-Ray Mammography
While mammography using X-rays is certainly the gold standard for tumor detection in breast cancer, it does have some potentially serious shortcomings:
- It has a high false-negative rate, especially in younger women who have radio-dense breasts, leading to the missing of up to 30% of cancers more than 5 mm wide.
- It has a positive predictive value of only 15-40%, which means unnecessary and costly testing is performed in three out of every four patients with mammographic abnormalities, often leading to psychological, physical, and financial trauma.
- Variation in interpretation between different radiologists also leads to differences in the results of mammography.
- Ionizing radiation is associated with a small risk of future malignancy. This means the benefits of frequent mammography must be weighed against the potential risks.
- Finally, mammography is best at detecting malignancies of the breast which are advanced and calcified, which means the chances of successfully treating it are very low.
Caveats
The dielectric contrast in MWI varies significantly between different people and at different ages which requires further study. The contrast between normal fatty and fibroglandular breast tissue, and malignant tissue, may be as low as 10% in some cases, and at this level the resolution is significantly low, causing the image to be blurred.
Again, the microwaves undergo scattering and reflection at multiple points inside the breast tissue, which makes it more difficult to reconstruct an accurate three-dimensional picture of the breast, requiring additional tests to achieve an acceptable level.
Thus MWI is extremely appealing as a technique for the detection of breast cancers in a non-invasive and more patient-friendly manner, but the technology needs more refinement for it to become a universal alternative to X-ray mammography.
Further Reading
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