Invasive breast cancer is characterized by spread to the surrounding tissue. The most common type of invasive breast cancer is invasive ductal carcinoma (IDC). It makes up 70-80% of all breast cancers and is defined by its origination in the milk ducts of the breast. There are many different types of IDC: mucinous, mixed, tubular, medullary, papillary, cribriform, adenocystic, metaplastic, adenosquamous, and micropapillary. Since these are all types of IDC, they all originate in the milk ducts.[1]

Mucinous breast cancer, or colloid breast cancer, makes up less than 2% of all breast cancers. Mucinous breast cancer is characterized by a large amount of mucous in the cancer cells.[2] A breast tumor with both mucinous cells and more typical IDC cells is referred to as a mixed tumor breast cancer and makes up about 5% of invasive cancers.2,3 Mucinous breast cancer tends to be less aggressive than usual cases of IDC and is characterized as being low grade, hormone receptor positive, and HER2 (human epidermal growth factor receptor 2) negative.[2] Tumor grade refers to how abnormal the cancer cells look, with low grade meaning the cancer cells look similar to normal cells and high grade meaning the cancer cells look very different. Low grade cancer cells will look and act similar to normal cells, therefore growing more slowly.[4] HER2, a protein encoded by the HER2 gene, promotes growth; therefore, breast cancers with an excess of this protein tend to grow and spread quickly. The hormones estrogen and progesterone can increase cancer growth if the tumor has receptors for them.[5] Pure mucinous breast cancer is defined as having 90-100% mucous cells. If more than 10% of the tumor cells are more typical IDC cells, it is considered a mixed tumor. For a mixed tumor, the treatment plan must address both types of cells.[6]

Tubular breast cancer, making up 1-5% of IDC cases, is characterized by tube-shaped structures on the tumor.[7] With newer screening technology, it is now estimated that tubular breast cancer accounts for 8-27% of all breast cancer cases. This type of IDC is usually low grade and the tumor is usually 1 cm or less. Tubular breast cancer tends to be less aggressive than the typical case of IDC.[8]

Medullary breast cancer accounts for 3-5% of all breast cancer cases. The appearance of a medullary tumor is described as soft and fleshy, resembling the medulla of the brain. Medullary breast cancer has been described as high grade in appearance and low grade in behavior. This implies that while the cancer cells look very abnormal, it is not as aggressive as expected. For this reason, medullary breast cancer tends to have better prognosis than usual cases of IDC.[9]

Papillary breast cancer makes up less than 1-2% of IDC cases and is characterized by having fingerlike projections (papules) on the tumor.[10,11] Papillary tumors are often non-cancerous, but when cells begin to spread outside of the milk duct it is classified as cancerous.[11] Papillary breast cancer tends to be low grade to moderate grade and less aggressive than typical cases of IDC.[10,11]

Cribriform breast cancer is defined by the cancer cells invading the connective tissue in a nest-like structure between the milk ducts and lobules. This structure results in holes in the tumor, resembling swiss cheese. In about 5-6% of invasive breast cancers, at least a portion of the tumor is cribriform. Cribriform breast cancer tends to be low grade and less aggressive than the usual case of IDC.[12]

Adenocystic breast cancer makes up less than 1% of all breast cancer cases and is characterized by having tumor cells that appear similar to salivary cells. Adenocystic breast tumors are typically triple negative, which means they are negative for HER2 and the hormone receptors for estrogen and progesterone.[13] Triple negative breast cancer (TNBC) is typically aggressive due to its quick growth and limited treatment options since specific therapies targeting hormone receptors or HER2 cannot be used.[14] However, adenocystic breast cancers tend to have a better prognosis than typical cases of TNBC and IDC.[13]

Metaplastic breast cancer is defined by having tumor cells that have transformed into another cell type such as connective tissue and soft tissue. Metaplastic breast cancer makes up less than 1% of all breast cancer cases. Metaplastic breast tumors are generally triple negative, high grade, and large. Metaplastic breast cancer is typically a more aggressive type of IDC and is more likely to spread to other parts of the body.[15]

Adenosquamous breast cancer, considered a low-grade type of metaplastic breast cancer, accounts for about 0.3% of all breast cancer cases. Adenosquamous breast tumors also contain cells that have transformed into connective tissue and soft tissue as well as narrow spindle cells. Low grade adenosquamous breast cancer is less likely to spread than other types of IDC, but often has a high rate of recurrence.[16]

Micropapillary breast cancer makes up about 6% of all breast cancers and is characterized by having clear spaces where the tumor forms clusters. These distinctive clusters have led to micropapillary breast tumors sometimes being referred to as “hedgehog” tumors. Micropapillary breast cancer tends to be more aggressive than other types of IDC, with cancer spreading to the lymph nodes in 75-100% of cases. In about 20-23% of cases, micropapillary breast cancer spreads to the skin. Micropapillary breast cancer is also more likely to spread to distant parts of the body.[17]

References

1.     Invasive Breast Cancer (IDC/ILC): Types of Invasive Breast Carcinoma https://www.cancer.org/cancer/breast-cancer/understanding-a-breast-cancer-diagnosis/types-of-breast-cancer/invasive-breast-cancer.html (accessed May 5, 2021).

2.     Potter, M. Mucinous (Colloid) Breast Cancer https://www.hopkinsmedicine.org/kimmel_cancer_center/cancers_we_treat/breast_cancer_program/treatment_and_services/rare_breast_tumors/mucinous_breast_cancer.html#:~:text=Mucinous%20breast%20cancer%2C%20also%20called%20colloid%20breast%20cancer%2C,before%20spreading%20to%20the%20tissues%20around%20the%20duct. (accessed Apr 30, 2021).

3.     IDC Type: Mucinous Carcinoma of the Breast https://www.breastcancer.org/symptoms/types/mucinous (accessed May 5, 2021).