For the first part, R12, R3, this possibly refers to the position using hands of a clock:

Yo tampoco he visto nunca esa forma de clasificar la densidad del tejido mamario.
Cuando he visto en un informe R11 (o R-cualquier número) se refería a radial, una forma de indicar la la localización de una lesión en la mama según las agujas del reloj.
https://www.doctoralia.es/preguntas-respuestas/que-es-un-tej...

In English:
Left breast shows ill defined speculated lesion with architectural distortion and central lucency in the 11 o'clock position, zone C.
https://radiopaedia.org/cases/radial-sclerosing-lesion-rsl-w...


Well circumscribed heteroechoic lesion in right - EPOS™https://epos.myesr.org › mediagallery
USG: Well circumscribed heteroechoic lesion in right breast at 8 O'clock , circle 3, zone C position. Significant internal vascularity noted. BIRADS IV.

I think the LA/LC could refer to zones:

Most ultrasound departments use the clock method to label breast images. In addition, ABC and 123 notation is helpful to label the exact location of a lesion
ABC notation: A is used if a lesion is near the surface or close to the transducer. B is used if a lesion is midway down and represents the mammary zone in the breast. C is used in the setting of lesions located against the chest wall
https://radiologykey.com/breast-ultrasound-4/

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Note added at 7 days (2023-03-10 15:01:05 GMT) Post-grading
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You're welcome, glad to be of help!