The term “ductal carcinoma in situ” (DCIS) of the breast represents a heterogeneous group of neoplastic lesions histologically confined to the breast ducts.

While DCIS made up roughly 3% of breast cancers detected prior to the advent of screening, the diagnosis of DCIS increased dramatically following its introduction and now comprises approximately 20% of all mammographically diagnosed breast cancers [1]. Pathologically, DCIS refers to the proliferation of neoplastic epithelial cells within the tubulo-lobular system of the breast; these abnormal epithelial cells have the morphological features of invasive carcinoma of the breast but, critically, differ from invasive carcinoma by being confined by the myoepithelial cells and basement membrane of the ducts, with no invasion of the stroma or of lymphatic or blood vessels [2].

The traditional vision of DCIS has been comprehensively summarized by Silverstein as such:

DCIS of the breast is a proliferation of presumably malignant epithelial cells within the ducto-lobular system of the breast without evidence by light microscopy of invasion through the basement membrane into the surrounding stroma. I say ‘presumably,’ because not every untreated DCIS lesion will go on to become an invasive breast cancer. Although we consider DCIS to be cancer, it lacks two components of the fully expressed malignant phenotype, i.e., invasion and metastasis. It is these defects, along with its distribution, generally within a single ductal system, that makes DCIS a surgical disease [3]; preventing local recurrence is important because about one half of such recurrences are invasive cancers with the potential to metastasize [4].

Accordingly, the present management of DCIS is focused on the prevention of local recurrence and therapeutic approaches parallel the treatment for invasive breast cancer (with exclusion of chemotherapy) including surgery, radiation therapy, and adjuvant endocrine treatments. A thorough discussion of the treatments for DCIS is articulated in other chapters of this book, but, in summary, patients with DCIS undergo local treatment with mastectomy or breast-conserving therapy (BCT) ; BCT consists of lumpectomy (also called breast-conserving surgery, wide excision, or partial mastectomy) followed in most cases by adjuvant radiation, even though radiation therapy may be reasonably omitted in a selected population of patients with low-risk disease. A sentinel lymph node biopsy can be avoided in most women, but it should be obtained in women with high-risk features for whom resection may compromise the ability to perform a future sentinel lymph node biopsy. Then, following local treatment, the decision to administer endocrine therapy to reduce the risk of subsequent cancers depends upon the choice of local therapy and the tumor hormone receptor status.

However, while Silverstein’s description of the “DCIS issue” holds true as a whole, gray areas exist in the treatment decision-making process, for several reasons.

First, although mastectomy achieves an excellent long-term survival and results curative for over 98% of patients with DCIS [5–9], with a local recurrence rate on the order of 1% [10–13], it provides an overly aggressive treatment for many women. Similarly, while BCT has less morbidity, it is associated with a higher risk of local recurrence, roughly threefold [1]; thus, the most suitable surgical option at the specific-patient level is not always straightforward. In facts, the progression rate varies widely according to specific DCIS subtypes. It is calculated that, overall, 14–53% of untreated DCIS will progress to invasive cancer [14, 15], but, in particular, it was estimated that the rate of progression from DCIS to invasive cancer across a 10-year period is 16% for low-grade DCIS (for patients older than 45 years with lesions larger than 2.5 cm) and 60% for high-grade DCIS (for patients younger than 45 years with lesions larger than 1 cm), which means that a nearly fourfold risk is implied by a different combination of clinical features.

Moreover, some observational data are in contrast with the assumed completely malignant-lacking phenotype of DCIS and should further motivate the ongoing quest for a deeper understanding of its biology.

It is noteworthy that among patients receiving a treatment for DCIS, even mastectomies resulted associated with a nonzero breast cancer-specific mortality [1]. Besides, women with in situ breast cancer, even after treatment, are at increased risk of subsequent invasive breast cancer compared to women in the general population [16]. In particular, such increased risk of developing a recurrence (either in situ or invasive) affects both the ipsilateral and contralateral breast, and, in a study from the National Comprehensive Cancer Network Outcomes Database [17], developing an ipsilateral second breast cancer resulted not significantly more likely than developing a contralateral recurrence (55% vs. 45%), as well as developing an invasive second breast cancer (ipsilateral or contralateral) was not significantly more likely than developing a noninvasive one (49% vs. 51%). These findings confirmed former observations that suggested a clonal relation between index DCIS and second ipsilateral DCIS events [18] and extended the relation also to DCIS and contralateral recurrence, raising the intriguing possibility that both DCIS characteristics and patient traits influence the type of breast cancer that may subsequently develop in either breast, particularly in terms of grading and estrogen receptors expression; the awareness that women with DCIS may be at risk of developing a recurrence of a similar phenotype may have the potential to inform clinical decision-making at the time of the index DCIS diagnosis.

Finally, from a biological perspective, some authors were able to demonstrate the presence of disseminated tumor cells in the bone marrow of patients (21.1%) with pathology-proven pure DCIS [19], albeit in a small case series.

From a clinical standpoint, despite the presumption that early treatment for DCIS would reduce cancer incidence and mortality [20], a small proportion of patients with DCIS ultimately die of breast cancer [21]; while some patients experience an in-breast invasive recurrence prior to death, some women die of breast cancer without first receiving a diagnosis of local invasive disease [22–24]. Therefore, it is unclear to what extent mortality from breast cancer after DCIS is the direct consequence of an invasive recurrence or whether fatal cases of DCIS have high malignant potential from the outset. In particular, it has not been shown that preventing invasive recurrences by means of radiotherapy or extensive breast surgery ( mastectomy) reduces the risk of breast cancer-specific mortality, as long-term epidemiology studies have demonstrated that the removal of 50,000–60,000 lesions annually has not been accompanied by a reduction in the incidence of invasive breast cancers, and this is in contrast to the experience with colonic polyps or intraepithelial lesions of the cervix, for instance [20].

As stated above, an observational study [1] of women who received a diagnosis of DCIS from 1988 to 2011 in the Surveillance, Epidemiology, and End Results (SEER) database reported a 20-year breast cancer-specific mortality rate after a diagnosis of DCIS of 3.3%, meaning an increase by 1.8 times compared with the general population in the USA. Of interest, mortality rates appeared to be actually lower than rates reported in the past and reasonably due to a more precise diagnosis; it is unlikely that the decline in mortality was due to more effective treatments because mortality rates did not vary with specific treatment. Apart from lower mortality, many known basic epidemiologic data were confirmed in the abovementioned study, as women with DCIS who developed an ipsilateral invasive in-breast recurrence were 18.1 times more likely to die of breast cancer than women who did not; the use of radiotherapy confirmed to reduce to the half the risk of developing an ipsilateral invasive recurrence (from 5 to 2.5%) but did not reduce breast cancer-specific mortality at 10 years. Similarly, patients who underwent unilateral mastectomy had a lower risk of ipsilateral invasive recurrence at 10 years than patients who underwent lumpectomy but had a higher breast cancer-specific mortality which, however, did not result significant after adjustment for tumor size, grade, and other tumor-related factors.

Surprisingly, other much less straightforward findings emerged from the same study and can be added to earlier data depicting as incomplete the current understanding of DCIS: the majority (54.1%) of women with DCIS in the cohort who died of breast cancer did not experience an invasive in-breast recurrence (ipsilateral or contralateral) prior to death.

Formerly, in the Early Breast Cancer Trialists’ Collaborative Group overview [25], there were 54 patients with DCIS (1.4% of all patients) who were reported to have experienced a distant or regional recurrence without any prior ipsilateral or contralateral in-breast cancer. In another study of more than 2000 women with DCIS who were treated at the University of Texas MD Anderson Cancer Center, 25 women developed distant metastases after a median follow-up of 4.5 years, of whom 16 had an intervening invasive recurrence but 9 did not [26].

Taken together, these data seem to suggest that cases of DCIS have more in common with small invasive cancers than previously thought and that considering DCIS as a preinvasive neoplastic lesion that is not lethal in itself should be revisited.

However, as death from breast cancer after DCIS is too rare to be used as an end point in randomized clinical trials, information on the lethality of DCIS must be indirectly derived from the features of its potential recurrence. Notably, for the vast majority of cases, a unilateral or contralateral recurrence of DCIS has no impact on mortality while an invasive cancer does (18-fold for unilateral and 13-fold for contralateral), leading to the accepted conclusion that nearly all risk depends on whether an invasive disease presents [20].

Fortunately, the absolute risk of dying from breast cancer is low, and the 3.3% mortality observed in the study by Narod [1] is not very different from the 2.69% risk of dying from breast cancer that an average woman faces during her lifetime according to the American Cancer Society [27]. So, the current conundrum in the management of DCIS patients actually comes down to the issue of whether to recommend adjuvant treatment to patients in addition to surgery, because it must be taken into consideration that there is a trade-off between treatments: aggressive treatment usually decreases the possibility of recurrence, but has difficult side effects, ranging from those caused by radiation and/or hormone therapy to the disfigurement caused by mastectomy.

The one-size-fits-all approach of adjuvant treatment for all patients with DCIS seems counterintuitive due to both the molecular heterogeneity of DCIS and the increasing trend toward individualized cancer treatment; so to aid in the decision-making, a number of factors are taken into account, including patient age and tumor margins, grade, and size, but the evidence to support these and other potential features as prognostic is variable.

In a meta-analysis based on 12 studies including more than 10,000 patients [28], Wang et al. evaluated the effects of several important characteristics such as biomarkers, focality, and method of detection as predictors for DCIS recurrence. Their findings suggest that women whose features of DCIS include positive margin, comedonecrosis, higher tumor grade, large tumor size, or multifocality or who are diagnosed due to a palpable mass or nipple discharge are associated with a higher risk of ipsilateral breast tumor recurrence. Also, although some features are statistically insignificant, patients whose DCIS is ER-negative, PR-negative, or HER2/neu-receptor-positive have a higher probability of ipsilateral breast tumor recurrence than those who do not present these features.