breast_book_cover.gif (5551 bytes)    MUCINOUS CARCINOMA
Mucinous carcinoma (also known as colloid carcinoma) is another special-type cancer that is associated with a relatively favorable prognosis. The reported incidence of mucinous carcinoma varies depending on the histologic criteria. Most studies have indicated that fewer than 5% of invasive breast carcinomas have a mucinous component, and of these, fewer than one-half represent pure mucinous carcinomas.

Clinical Presentation

The mean age at presentation for patients with mucinous carcinoma is in the seventh decade in most studies (range, 21 to 94 years) and is older than that for patients with breast cancers of no special type. Most patients with mucinous carcinoma included in published reports presented with palpable tumors. Some reports, however, suggest that a substantial proportion of patients with mucinous carcinoma (30% to 70%) present with nonpalpable mammographic abnormalities.

On mammography, mucinous carcinomas are most often poorly defined or lobulated mass lesions that are rarely associated with calcification. Wilson et al. reported that pure mucinous carcinomas were more often associated with a circumscribed, lobulated contour than with the irregular borders characteristic of tumors with a mixture of mucinous and nonmucinous components (mixed mucinous tumors).In addition, mammographically occult mucinous carcinomas are not infrequent, accounting for 4 of 23 cases (17%) in one study. On ultrasonography, mucinous carcinomas are typically hypoechoic mass lesions.

Gross Pathology

Mucinous carcinomas average approximately 3 cm in size, with a wide range reported in the literature. In some studies, tumors composed exclusively of mucinous features are smaller, on average, than mixed tumors.Mucinous carcinomas have a distinctive gross appearance. These lesions are typically circumscribed and have a variably soft, gelatinous consistency, and a glistening cut surface. Lesions with a greater amount of fibrous stroma may have a firmer consistency, however.

Histopathology

The hallmark of mucinous carcinomas is extracellular mucin production. The extent of extracellular mucin varies from tumor to tumor, however. Typically, tumor cells in small clusters, sheets, or papillary configurations are dispersed within pools of extracellular mucin. This characteristic histology should comprise at least 90% of the tumor (or 100%, according to some)6 for the tumor to qualify for the diagnosis of mucinous carcinoma. Mucinous neoplasms intermixed with other nonmucinous histologic features are classified as mixed mucinous tumors. The cellularity of mucinous carcinomas is variable. Some tumors are relatively paucicellular; in these cases, the differential diagnosis includes mucocele-like lesions, which are benign lesions characterized by cystically dilated ducts associated with rupture and extravasation of mucin into the stroma.

The expression of various biological markers in mucinous carcinomas generally reflects the good prognosis associated with these lesions. Estrogen-receptor positivity has been reported in 86% to 90% of tumors and progesterone-receptor positivity in 63% to 67%. DNA studies of 26 pure mucinous carcinomas revealed that 25 (96%) were diploid, compared with only 8 of 19 mixed tumors (42%). The rate of diploidy among the mixed tumors was comparable to that seen in breast cancers of no special type.155 In a review examining the karyotypic analysis of 20 mucinous carcinomas, 17 exhibited chromosomal aberrations that were simple in comparison with the complex aberrations typically associated with breast cancers of no special type.128 In addition, mucinous carcinomas usually do not overexpress the HER-2/neu oncoprotein (0% to 4% of cases) or show p53 protein accumulation (18% of cases).

Clinical Course and Prognosis

The incidence of axillary lymph node metastases in pure mucinous carcinomas, although variable (range, 4% to 39%; average, 15%), is significantly less than the incidence of node positivity seen in mixed mucinous tumors (38% to 59%) or in breast cancers of no special type (43% to 63%).Some investigators have questioned the necessity of performing lymph node dissections in patients with mucinous carcinoma, particularly if 100% of the tumor shows typical mucinous histology.

With regard to survival, 38 patients with mucinous carcinoma were enrolled in the NSABP B-06 trial, and they experienced the same significantly increased survival rate as patients with tubular carcinoma, particularly those in the node-negative group.Similar results were reported by Ellis et al. in their retrospective series; however, the patients in their study were not stratified by nodal status. Survival data reported in most other retrospective reports suggest that, to a variable degree, patients with mucinous carcinoma experience lower recurrence rates and greater short- and long-term survival than patients with mixed mucinous carcinomas and breast cancers of no special type.Several studies have noted that a significant number of late recurrences are seen in patients with mucinous carcinoma138,143,157; one report documented a recurrence 30 years after initial treatment. Results from the SEER database were published comparing 20-year survival data from 3,356 patients with mucinous carcinoma and  patients with invasive ductal carcinoma diagnosed between 1973 and 1990. Similar to the studies cited earlier, this report indicated that patients with mucinous carcinoma present with localized disease more commonly than patients with invasive ductal carcinoma (78.1% versus 53.1%). In addition, even after prolonged follow-up, only 25.1% of patients with mucinous carcinoma died as a consequence of breast cancer, compared with 58.3% of patients with invasive ductal carcinoma. These results were highly statistically significant, even after correction for potentially confounding variables, such as age, year of diagnosis, race, stage, and grade.147 In addition, two series—one examining node-negative early-stage breast cancer patients treated with mastectomy (with a 20-year follow-up) and the other examining early-stage patients treated with breast-conserving therapy (with a 10-year follow-up)—reported that patients with mucinous carcinoma had significantly lower rates of distant recurrences compared with patients with invasive ductal carcinoma.

Three studies have examined the use of conservative surgery and radiation therapy in a total of 38 patients with mucinous carcinoma and report no significant differences in local recurrence rates for these patients compared with patients with invasive ductal carcinoma. Given the relatively good prognosis of patients with mucinous carcinoma, some authors have raised the question of whether radiation therapy can be safely omitted after breast-conserving surgery in patients with this tumor type.At this time, however, not enough data exist to support such a recommendation.

Mucinous carcinomas have rarely been associated with unusual metastatic manifestations, including mucin embolism resulting in fatal cerebral infarcts and pseudomyxoma peritonei.