Breast tumors can be classified into slow-growing and fast-growing tumors. The main characteristics of fast-growing tumors are low doubling time, histological grade 2 or 3 (G2, G3), and high Ki-67 antigen value. Among slow-growing tumors, a significant proportion of invasive breast cancers may remain asymptomatic throughout life and another fraction may spontaneously regress. ^[1] The main characteristics of slow-growing tumors are high doubling time, histological grade 1 or 2 (G1, G2), and low Ki-67 antigen value.

Doubling time (DT) is defined by the number of days required for a tumor to double its volume. The doubling time for breast cancers has been reported to range from 44 to 1869 days. ^[2] Slow-growing tumors have high doubling time and fast-growing tumors have low doubling time,  as illustrated in Figure 1.

The Ki-67 antigen is a cellular proliferation marker during the active phases of the cell cycle. Slow-growing breast tumors have a lower Ki-67 value (<10-15%) and fast-growing breast tumors have a higher Ki-67 value (>15%)^[3], as illustrated in Figure 2.

Histological grade (Nottingham) combines criteria of tubule formation, nuclear pleomorphism, and mitotic count. Nottingham G1 (well-differentiated), G2 (moderately differentiated), or G3 (poorly differentiated). ^[4] Slow-growing breast tumors have histological grade G1/G2 and fast-growing tumors have histological grade G2/G3, as illustrated in Figure 3.

Slow-growing breast tumors are usually smaller at diagnosis and less likely to manifest clinical symptoms. Therefore, most slow-growing breast cancers are diagnosed through screening  ^[5]. 

The slow-growing breast cancers are one of the main causes of false negatives related to interpretation errors. ^[6] This is due to two main reasons. They can present as benign-looking lesions on imaging, making detection more difficult. Examples include: developing asymmetries (invasive lobular carcinoma). Additionally, they may appear relatively stable over several subsequent exams. When older exams are available for comparison, it is easier to appreciate that a slow-growing cancer is increasing in size over time, as illustrated in Figure 4.

Invasive breast  carcinoma of non-special (IBC-NST) luminal A is the most common subtype of these types of breast cancer. It is more likely to occur in elderly patients who have had greater exposure to estrogen during their lifetime and is often associated with ductal carcinoma in situ (DCIS). Regarding imaging findings, they can be characterized as follows: mammographic findings - spiculated mass (most common), architectural distortion, developing asymmetry; ultrasonographic findings - hypoechoic and  irregular mass with spiculated margins; and magnetic resonance imaging (MRI) - irregular mass, with heterogeneous enhancement. ^[7] Cases of this type of  tumor with discussions about their diagnosis over time are illustrated in figure 5, 6 and 7.

Ductal carcinoma in situ (DCIS) is the second most common subtype of slow-growing tumors. DCIS is a clonal proliferation of malignant epithelial cells originating in the terminal duct-lobular unit without invasion of the myoepithelial layer or basement membrane. It is a non-obligate precursor of invasive cancer. Generally affects asymptomatic patients, around 50 years old, with most detections by mammographic screening. Regarding imaging findings, they can be characterized as follows: mammographic findings - suspicious calcifications (most common), irregular mass, developing asymmetry; ultrasonographic findings - non-nodular lesions, irregular mass, intraductal mass, calcifications in ducts; MRI - non-mass linear, ductal or segmental enhancement. ^[8] Cases of this type of  tumor with discussions about their diagnosis over time are illustrated in figure 8 and 9.

Invasive lobular carcinoma (ILC) is the third most common subtype of slow-growing tumors. ILC is composed of non-cohesive cells dispersed or organized in a single-file linear pattern in a fibrous stroma. Most patients present with a poorly defined palpable mass. The disease is commonly multifocal, multicentric and bilateral, and mammographic and ultrasonographic findings underestimate the extent and size of the tumor. Regarding imaging findings, they can be characterized as follows: mammographic findings - spiculated and irregular mass, developing asymmetry, architectural distortion; ultrasonographic findings - hypoechoic and irregular mass, non-mass lesions; MRI - spiculated irregular mass with heterogeneous enhancement, focal non-mass enhancement.  ^[9] Case of this type of  tumor with discussions about their diagnosis over time is illustrated in figure 10.

Invasive tubular carcinoma (ITC) is the rarest subtype of slow-growing tumors. ITC is a special type of invasive ductal carcinoma, composed of well-formed tubules with open lumens lined by a single layer of neoplastic cells. It is more likely to occur in elderly patients, detected incidentally by mammographic screening and tends to be small. ITC is a rare tumor, representing 2% of breast tumors and has an excellent prognosis. Regarding imaging findings, they can be characterized as follows: mammographic findings - irregular spiculated mass, architectural distortion, asymmetry, calcifications; ultrasonographic findings - hypoechoic, irregular, and spiculated mass; MRI - spiculated and irregular mass, early enhancement and "washout". ^[10] Case of this type of  tumor with discussions about their diagnosis over time is illustrated in figure 11.

Mucinous Carcinoma (MC) is characterized by clusters of epithelial tumor cells suspended in abundant pools of extracellular mucin. MC is an uncommon subtype of slow-growing tumors. Most patients present a median age of 71 years. As for imaging findings, they can be characterized as follows: Mammographic findings: oval mass with circumscribed or microlobulated margins, irregular mass with indistinct margins; Ultrasonographic findings: solid and isoechoic mass with circumscribed or microlobulated margin, solid-cystic mass; MRI- high signal on T2/STIR due to the large mucin component, avid heterogeneous enhancement at the margin, with persistent or plateau kinetics. ^[11].Case of this type of  tumor with discussions about their diagnosis over time is illustrated in figure 12.

Invasive Papillary Carcinoma (IPC) is defined as a neoplasm with a solid growth pattern and delicate fibrovascular cores interspersed, resulting in a solid-papillary architecture. IPC is a rare tumor, representing less than 2% of all breast cancers, with a median age of 65-70 years. As for imaging findings, they can be characterized as follows: Mammographic findings: oval mass with circumscribed margin, solitary microlobulated mass or group of masses; Ultrasonographic findings: solid or complex solid-cystic mass with high vascularization in the solid areas; MRI findings: oval mass or complex solid-cystic mass with irregular mural nodules with enhancement; variable kinetic curves. ^[12]Case of this type of  tumor with discussions about their diagnosis over time is illustrated in figure 13.