o.k. I will post my opinion this time as a suggestion to the answer.

suggestions for:
(A)
it has invaded into the interstitium
an interstitial extension has occured

(B)
a infiltrating mass
a infiltrative mass

(You need to do research to find best words to make your choice or find better words for them)

I need to see several sentences prior to this to be sure but I assume this is talking about a behaviour of non-infiltrating (or non-invasive) intraductal mammary carcinoma, which is considered to occurs in very early stage of mammary carcinoma where cancer tissue is confined to inside the ducts, still not yet broken through the basement membrane and enter the interstitial space. So when the cancer tissue break through the membrane and enter the interstitium, there is no barrier (or something hinders the extension of the cancer cells), they grow as a mass. Here is my biggest assumption (and it might get clearer if I read all of Mr cinefil's references), when it came to the point at which those cancer tissue spread to interstitium, they don't bother growing along inside the ductules, and prefer or easier to grow in the interstitium, or so one might want to think. But here the sentence is telling you that it is not the case, but you still see great growth of cancer tissue inside ducts (inside the ducts of the mass itself and also) further extending out from the mass, through the duct system (because they run long distance).

Well, this is my take on the thingy, and it takes many more hours of readings to be sure of the whole subject for me, so I rest here and wish you the best and good luck !