A new study focusing on pure mucinous breast cancer, also known as colloid carcinoma, finds that multicentric or multifocal disease (i.e., multiple tumors in one breast) is much more common in this type of breast cancer than previously thought. Mucinous breast cancer has been reported to have a very favorable prognosis and treatment guidelines tend to recommend minimal effective therapy. The study included 264 women diagnosed with pure mucinous carcinoma between 1965 and 2005. Median age at diagnosis was 57 years (range 25-89), which was lower than reported in previous studies. The median follow-up period was 14 years. Breast conserving therapy (lumpectomy) was performed on 44%; the remainder underwent mastectomy. Just over half (51%) of the patients received whole breast radiation therapy.

While only 10% percent of the women had originally been diagnosed with multicentric/multifocal disease, a detailed pathology review performed by the authors found a 38% multifocal/multicentric rate. The additional tumors had not been detected by mammography or ultrasound initially. The five-year overall survival rate for all the women in the study was 95%, the 10-year overall survival rate was 88%, and the 15-year overall survival rate was 83%. Distant metastases-free survival rates were 97%, 95%, and 92% for the same periods. Local-regional control rates were 97%, 94%, and 85%. No statistically significant differences in these rates were found based on whether the women had breast conserving surgery or mastectomy. Similarly, whole breast radiotherapy after surgery resulted in no significant improvement in overall survival or distant metastases-free survival rates. However, patients who received radiation treatment had fewer local recurrences compared to women who did not undergo radiation.

The authors conclude that pure mucinous breast carcinoma demonstrates a potentially favorable prognosis, as previously reported. However, they suggest that the significant occult multicentricity/multifocality found in their study should influence the evaluation and treatment of this rare form of breast cancer. At the moment, given its apparently favorable prognosis, pure mucinous breast cancer may be managed with no additional treatment after surgery or with partial breast irradiation. However, the possible existence of difficult-to-detect additional tumors in the same breast suggests that multidisciplinary management that includes pathology and diagnostic imaging is needed. In other words, before the decision is made to omit whole breast irradiation, patients should have pathologic and radiologic intraoperative correlation. In addition, MRI should be a considered in an effort to identify potentially hidden multifocal disease.

Trend toward minimal treatment of mucinous breast cancer

The authors of the above study point out that treatment guidelines for pure mucinous breast cancer (i.e., not a mixed tumor with only a partial mucinous component) have been based on the apparently favorable prognosis of this form of breast cancer. However, treatment in the past included whole breast irradiation when it was deemed necessary to perform treatment beyond surgery. Current innovations and trends that minimize treatment, such as partial breast irradiation, could result in suboptimal treatment of multifocal disease if any additional tumors are not detected.

Similarly, some authors have suggested that lymph node staging is not necessary for this type of breast cancer under some circumstances. However, multifocal disease that is not detected could underestimate the combined overall size of the tumor, resulting in a mistaken decision not to perform lymph node staging. Since the number of positive lymph nodes has been found to be the most reliable indicator of prognosis for this disease (and therefore would guide decision-making with respect to any additional therapy), this could result in under treatment in some cases where the disease is more aggressive than usual.