Multiple examples show the range of percent breast volume occupied by dense tissue in cases from fatty replaced (10%) to densely opacified breast tissue (90%).

Images:
1: 10% R,L CC
2: 20% R,L CC
3: 30% R,L CC
4: Male 30% R,L LM
5: 30% R, 40% L CC
6: 40% R,L CC
7: 50% R,L CC
8: 60% R,L LM
9: 70% R,L OBL
10: 70% R, 80% L OBL
11: 80% R,L CC
12: 90% R,L OBL


DIAGNOSIS:
Bilateral views from multiple different cases show the range of percentage of breast volume occupied by dense tissue in normal and a few abnormal cases.

FINDINGS:
1. 10% of breast volume occupied by dense tissue on right and left CC views. Normal.

2. 20% of breast volume occupied by dense tissue on right and left CC views. Normal.

3. 30% of breast volume occupied by dense tissue on right and left CC views.

4. 30% of breast volume occupied by dense tissue on right and left LM views. This male patient had recent enlargement of the left breast with firm tissue palpable in the retroareolar region. This was found to be benign gynecomastia on the left. A mammogram of the contralateral breast was done only for comparison to the palpable left breast lesion (checking for asymmetry). The right breast unexpectedly contained occult intraductal carcinoma with extensive microcalcifications. The patient had only a biopsy for the gynecomastia on the left but had a mastectomy for the incidentally detected breast carcinoma on the right.

5. Mild asymmetry on CC views with 30% of the right and 40% of the left breast volume occupied by dense tissue. Note the abnormally prominent interstitial pattern on the left. The findings on the left are typical radiation therapy changes after treatment for breast cancer.

6. 40% of breast volume occupied by dense tissue on right and left CC views.

7. 50% of breast volume occupied by dense tissue on right and left CC views.

8. 60% of breast volume occupied by dense tissue on right and left lateromedial views.

9. 70% of breast volume occupied by dense tissue on right and left OBL views.

10. 70% of the right and 80% of the left breast volume is occupied by dense tissue. The slight asymmetry on these oblique views was due to an large, complicated cyst located in the central left breast and retroareolar region.

11. 80% of breast volume occupied by dense tissue on normal right and left CC views. Only a few small areas of fat lucency remain.

12. 90% of breast volume occupied by dense tissue on normal right and left OBL views (almost no fat lucency remains visible).

DISCUSSION:
Routinely estimating the percent (10% to 90%) of breast volume occupied by dense tissue can be useful in several mammographic situations. The most common is hormone replacement therapy (HRT) induced mammographic changes. HRT can cause an increase in 10% to as much as 50% in breast density from before to after starting treatment. Most of this change usually occurs in the first 6 months to 4 years. Attempting to quantitatively estimate this percent will usually lead to the awareness that new nodularity or neodensity is not unilateral (worrisome) but is bilateral and diffuse. If the patient history data sheet shows a clinical history of starting HRT in the past 4 years, bilateral increase in density is less worrisome.

For radiation therapy patients, the percent density of the treated breast can increase 10% to 40% during the initial months of therapy. Reporting the percent change may be clinically useful because the density will normally begin to gradually decrease over the next 1 to 3 years. It may or may not eventually return to pretreatment density depending on dose and other factors. If this sequence varies, a recurrence of the carcinoma or other complication may be developing.

The percent density is only reproducible plus or minus 10% so it is not useful to report it with smaller percentages. Variation of 10% can be due to routine minor variations in radiographic technique and equipment. Year to year comparisons of one patient's annual exams can be an easy way to check long term quality assurance and reproducible technique. A radiologist or resident trying to decide if one breast differs from the other by as little as 10% in density may sometimes discover that a subtle asymmetric neodensity is causing the difference. This may lead to detection of an early breast cancer and avoid a missed lesion. The percent density can be accurately measured on digitally acquired images or digitized film mammograms using a pixel histogram of gray scale values. Skin, muscle, and implants densities should be excluded from the estimate.

Submitted by:
Ralph L. Smathers, M.D.
Mammography Specialists Medical Group Inc.
Los Gatos, California