Background: Progesterone receptor (PR) is an estrogen receptor (ER)-regulated gene and has critical roles in both normal mammary and breast cancer development. Loss of ER expression in breast cancer is associated with older patients, larger tumors, higher grade tumors, and tumors over-expressing HER2. We previously showed that loss of PR expression predicts for higher Recurrence Scores (RS) for the 21-gene assay (ODX) and higher risk for bone metastasis. Recently, a 20% or less PR was proposed as a classifier to separate Luminal B from Luminal A subtype of breast cancer. Limited information is known for PR in ductal carcinoma in situ (DCIS). Here, we intend to study the relationship between the different levels of PR expression and various pathologic features in DCIS of the breast.<br>Design: 886 cases of primary DCIS of the breast from our institution and diagnosed between 1997 and 2013, had an exact percentage documented for ER and PR IHC scoring, were included in the study. The clinicopathological features, including patient age, tumor size, nuclear grade and expression of ER and PR, were collected for each case. The relationships between the different levels of PR expression (<1%, 1-20%, 21-50%, 51-70% and 71-100%) and these pathologic features were analyzed.<br>Results: Among the 886 cases of DCIS, the mean age was 85.5 years, mean tumor size was 2.4cm, 84.5% were positive for ER, and 76.2% were positive for PR. 24%, 14%, 13, 11%, and 38% of these DCIS had PR expression of <1%, 1-20%, 21-50%, 51-70% and 71-100%, respectively, similar in distribution as was seen in IDC (data not shown). Significant differences in patient age, nuclear grade and ER expression were observed only between the PR<1% and PR 1-20% subgroups, with less PR expression associating with younger patients, higher nuclear grade and lesion ER expression in DCIS; these difference was not observed between the PR 1-20% and PR 21-50% subgroups, the PR 21-50% and 51-70% subgroups, and the PR 51-70% and PR 71-100% subgroups. In ER positive tumors, the only significant difference noted between different PR subgroups was nuclear grade.<br>Conclusions: The significant pathological difference observed in between PR<1% and PR 1-20% suggests the cutoff of 1% for PR appears to be biologically important. Further studies are warranted to investigate the roles of PR in local/regional of DCIS of the breast.
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