While mammography remains the most effective tool for early detection, no screening process is perfect, and a small percentage of cancers do become noticeable during the interval between appointments. A tumor could emerge and grow aggressively in the time between screenings, leading to understandable concern. Understanding the underlying biology of breast cancer growth and the definition of a rapidly developing tumor provides a factual basis for the science behind tumor growth rates.
Defining Interval Breast Cancer
The medical community uses a specific term for a cancer found between screenings: an interval breast cancer. This is defined as an invasive breast cancer diagnosed after a negative mammogram result but before the next scheduled screening appointment, which is typically one or two years later depending on guidelines. Interval cancers are significant because they tend to have a worse prognosis than cancers found directly by screening, often due to their inherently aggressive nature or rapid growth rate. These cancers account for about 20% to 30% of all breast cancer diagnoses in screened populations. The cancers are often categorized further, distinguishing between a “true” interval cancer that developed since the last screen and a “missed” cancer that was present but not visible or recognized at the time of the previous mammogram.
Biological Factors Influencing Growth Rate
The speed at which a breast tumor develops is determined by its cellular biology, primarily its proliferation rate. This rate is measured by the tumor volume doubling time (TVDT), which is the time it takes for a tumor to double its mass. While the average TVDT for breast cancer is often cited in months, aggressive subtypes can have much shorter doubling times, allowing them to appear quickly.
A key marker doctors use to gauge this speed is the Ki-67 proliferation index. Ki-67 is a protein present in the nucleus of cells that are actively growing and dividing. A higher Ki-67 percentage indicates a larger proportion of cancer cells are multiplying, suggesting a faster-growing tumor and a more aggressive clinical course.
Tumor grade is another factor reflecting growth speed, with Grade 3 tumors demonstrating high cellular abnormality and rapid division compared to slower-growing Grade 1 tumors. Cancers that lack hormone receptors, such as estrogen receptor (ER) and progesterone receptor (PR), also tend to grow faster than hormone-positive tumors. Interval cancers are significantly more likely to be high-grade and estrogen receptor-negative than screen-detected cancers, confirming their tendency toward faster growth.
Specific Aggressive Subtypes
The most rapidly developing breast cancers often fall into specific molecular subtypes defined by their receptor status. Triple-Negative Breast Cancer (TNBC) is a prime example, lacking the estrogen receptor, progesterone receptor, and the HER2 protein. This absence means TNBC lacks targets for common hormone therapies and is known for its high proliferation rate, high histological grade, and tendency to be diagnosed as an interval cancer.
Another subtype known for fast growth is high-grade HER2-positive breast cancer. This cancer overexpresses the HER2 protein, which promotes rapid cell growth and division. Although treatments targeting HER2 have improved outcomes, the inherent biology of these tumors is aggressive, often showing shorter doubling times than hormone receptor-positive cancers.
Inflammatory Breast Cancer (IBC) is a particularly aggressive and rare type that often manifests rapidly and is frequently diagnosed between screenings. IBC typically does not present as a lump but instead causes sudden, noticeable changes to the skin, such as redness, swelling, and a pitted texture resembling an orange peel. Its unusual presentation and tendency to quickly involve the skin and lymph system demand prompt medical attention regardless of the screening schedule.
What to Look For Between Screenings
Given that some cancers can develop and grow quickly, maintaining breast awareness is paramount, even after a recent negative mammogram. This involves knowing the normal look and feel of your breasts and noticing any persistent or new changes. A new lump or mass, whether painful or not, is the most common sign and should always be evaluated promptly.
Changes to the skin’s appearance can also signal a rapidly growing tumor. These include swelling, dimpling, irritation, or a thickening that may resemble an orange peel texture. Other signs requiring immediate medical follow-up are changes to the nipple, such as turning inward (retraction), persistent pain in one area, or any discharge other than breast milk. If any of these symptoms are noticed, contact a healthcare provider without delay, as a recent mammogram does not rule out the possibility of a new, quickly developing cancer.