The ACR recognizes that breast density has an impact on mammographic screening. The ACR’s BI-RADS® lexicon describes four categories of breast parenchymal density and instructs radiologists to include this density information in the medical report.
https://www.acr.org/Advocacy-and-Economics/ACR-Position-Stat...

Because of the importance of breast density, radiologists have developed a way to categorize density to
be more consistent in how it is reported, and therefore allow surgeons to use the information in caring
for their patients. Breast density is categorized as A, B, C or D.
https://www.massgeneral.org/cancer/assets/pdfs/FINAL Transcr...

I guess it is just a matter of how best to express the idea. As previous answers have already stated, in this case ACR refers to the American College of Radiology classification of brest density. In English it is quite many times obviated (the ACR BIRADS reference), expressing it as category or type C (also III or 3) breast density (cuidado no obstante de no confundir con la clasificación de gravedad/malignidad).

And so, for example,
corresponding to type C breast density / breast density category C

Or, if you do not want to obviate the ACR/BIRADS reference,
corresponding to ACR type C breast density


See some different examples;

If your mammogram shows that you have category C or D breast density, your mammography report will state that you have “dense breasts.” 
https://www.mainlinehealth.org/conditions-and-treatments/tre...

Breast parenchyma density: There are scattered areas of fibroglandular density (ACR type b).
https://radiopaedia.org/cases/complex-sclerosing-papillary-l...

BI-RADS type 3 breast density pattern/heterogeneously dense breast.
https://radiopaedia.org/cases/breast-carcinoma-5

Doctors worry about heterogeneously dense breasts, those with large areas of dense fibrous and glandular tissue combined with some fatty tissue. About 40% of women have this type C breast density. Extremely dense breast tissue — type D, which has very little fatty tissue — is seen in only 10% of women.
http://www.ucihealth.org/blog/2019/10/breast-density

An ultrasound exam was indicated for the following patient or case characteristics: dense breasts of ACR Type III and IV, palpable lesions (BI-RADS 0), or a suspicious lesion...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4045466/

Density ACR type III
https://books.google.es/books?id=5EWXxLthJ2YC&pg=PA605&lpg=P...

[Breast density] Type C (heterogeneously dense): another 40 percent of women will have this type of mammographic density, which is considered dense and may obscure small masses.
https://www.bcna.org.au/breast-health-awareness/mammographic...

classified breast tissue according to Breast Imaging and Reporting Data System (BIRADS) criteria as fatty (BIRADS density 1 or 2) or dense (BIRADS density 3 or 4)
https://www.medpagetoday.com/meetingcoverage/ascobreast/2253...