How relevant should my past LCIS dx be in light of this new one?

I'm so sorry you are going through this conundrum.

As in almost all things concerning LCIS, there is controversy. A lot of the controversy happens because LCIS is unusual and hard to study. Most studies of women with LCIS don't involve more than 100 women, often many less than this. LCIS doesn't reliably show up on imaging. The only way we can be sure some breast tissue is LCIS currently is by biopsying it, thus removing the tissue from the body. We don't know how many women are walking around with LCIS and don't know it.

Because they used to routinely do bilateral mastectomies to treat LCIS, they found most LCIS is multicentric (there are many spots of LCIS in one breast) and frequently bilateral (in both breasts.) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2783988/?tool=pubmed

Subsequently, it is not surprising that subsequent breast cancers in women with LCIS were bilateral at an almost equal rate.

As expected, IBCs were detected with approximately equal frequency overall in patients having right-sided (6.8%) versus left-sided LCIS (5.5%). http://jco.ascopubs.org/content/23/24/5534.long

Since most breast cancers have been in the breast for some 4-10 years before they can be detected by ANY means, it is not possible to say when your IDC started to grow, and whether or not the LCIS affected the IDC's presence or growth.

This recent paper looked at women with invasive breast cancer with and without LCIS at the surgical margins, and concluded that there was no increase in locoregional recurrance in women with LCIS. http://www.ncbi.nlm.nih.gov/pubmed/24064317 However, it only looked at 5 years. In other papers, LCIS risk continues throughout one's lifetime. http://jco.ascopubs.org/content/23/24/5534.long

This Chuba paper states

Mixed results regarding the recurrence rates of IBCs that are associated with LCIS have been reported. Sasson et al³⁵ found that IBTR occurred in 57 (5%) of 1,209 patients without LCIS compared with 10 (15%) of 65 patients with associated LCIS (P = .001). Abner et al⁴¹ found an 8-year recurrence rates of 13% for moderate or marked LCIS adjacent to the tumor in 137 patients and 12% for 1,062 patients without LCIS. They concluded that neither the presence nor extent of LCIS associated with IBC should influence management decisions.⁴¹ http://jco.ascopubs.org/content/23/24/5534.long

In this paper, they found that _some_ LCIS probably eventually evolves into invasive breast cancer. http://www.ncbi.nlm.nih.gov/pubmed/17380381

Although this paper doesn't directly address your situation, it found that women with invasive breast cancers that had both lobular and ductal features (in other words, both IDC and ILC) did not have a significantly different survival outcomes than either individually. http://www.ncbi.nlm.nih.gov/pubmed/18404368

This 2001 paper proposed that DCIS may be the precursor in patients with LCIS who go on to get IDC. http://www.ncbi.nlm.nih.gov/pubmed/11484500

This paper found no evidence for a common low grade precursor (LCIS and low grade DCIS) for ILC and IDC. http://www.ncbi.nlm.nih.gov/pubmed/21935747

Well, this post isn't very well organized, but I hope it gives you some idea about how little we know about the natural history of LCIS and IDC. I don't know if I've given a representative sample of opinions either. But it sounds like there isn't an easy, one choice-fits-all answer for many LCIS questions. Gotta go.

Thinking of you! Best wishes.