Diagnosing/findin all lcis foci in bilateral breasts.

Yes, LCIS is not reliably found on mammograms, ultrasounds, clinical exams, or MRI.  (Some LCIS spots are found this way, but most aren't.)

LCIS is usually an incidental finding on an imaging abnormality.  LCIS is often found not _at_ the site of the imaging abnormality, but near the imaging abnormality.

Most LCIS is multifocal, meaning there are usually multiple spots of LCIS in one breast.  LCIS is usually bilateral, meaning it is often (not always) found in both breasts.  How do they know this?  Before the ~1990s, it was routine practice for a woman with LCIS to have bilateral mastectomies as a treatment.  They examined the removed breasts for LCIS.

We have  very little knowledge how LCIS is involved in breast cancer development.  LCIS does NOT always itself become breast cancer.

Unless a woman has prophylactic bilateral mastectomies (and probably not even then), an LCIS woman never knows if they have removed all the LCIS.  Unlike DCIS, the purpose of an excision after LCIS is found on a core biopsy is _not_ to remove the LCIS, but to see if there is anything worse (i.e. DCIS or invasive breast cancer) in the vicinity.  If LCIS is found on a core biopsy, then roughly 20% of these women, after an excisional biopsy, will be found to have DCIS or invasive breast cancer.  (Some studies give about 20%, some more and some less.)

The goal of surgery is not to remove all of the LCIS.  You are right: they cannot know where all your LCIS is without having PBMs (and even when you have PBMs, you can't remove 100% of the breast tissue.)  

If you take a woman who has had LCIS, and wait several years (so they are fairly sure that the woman didn't have an invasive breast cancer at the time she was diagnosed with LCIS), and look at those women who went on to get invasive breast cancer, most of these breast cancers were found in places in the breast that did _not_ have any previous imaging abnormalities:  these areas looked perfectly normal.    So at least some experts feel that LCIS may be an indicator of some other risk factor for breast cancer, but usually the LCIS itself does not itself become breast cancer.  I think of it as sometimes it uses 'action at a distance', though I've never seen any papers that use these terms.

They think that _some_ LCIS women do have their LCIS actually develop into invasive breast cancer (from studies about clonality.)  In women who get simultaneous invasive breast cancer and LCIS, they have looked at the genetics of the invasive breast cancer and the genetics of the associated LCIS.  A sizeable portion of these women have LCIS that is _not_ genetically related to their invasive cancer.  Sometimes their LCIS is related, but sometimes not.

Roughly, in women without a significant family history of breast /ovarian cancer, most LCIS women will be given approximately a 25-40% lifetime incidence of getting DCIS or invasive breast cancer. So, in the majority of LCIS women, their LCIS will _never_ in their lifetime go on to become DCIS or invasive breast cancer, even though they almost certainly have LCIS spots that were never excised.  Most of the breast cancers that LCIS women go on to get will be IDC, not ILC, even though LCIS women get a lot more ILC than the average woman who gets breast cancer.  (Of the breast cancers that LCIS women go on to get, roughly 40% are ILC, whereas, in the general population, about 10% are ILC.)

LCIS acts differently than DCIS.  LCIS is very difficult to study. LCIS is weird. We don't know how many women are walking around with LCIS and don't know it because they've never had a breast biopsy.  They don't know how LCIS is involved in increased breast cancer risk.  Sometimes LCIS probably becomes invasive breast cancer, but they think that sometimes it does not.  There are a lot of unknowns in LCIS.