Interval breast cancer is diagnosed in the period between a normal screening mammogram and the next scheduled one. The cancer is not found by the screening itself but is discovered when a person develops symptoms.
This diagnosis can be confusing, as it may imply an error was made in the previous screening. However, the development of an interval cancer does not automatically mean a mistake occurred. The term is used by healthcare systems to monitor the effectiveness of breast cancer screening programs and helps researchers identify individuals who might need different screening schedules or methods.
Causes and Risk Factors
Interval cancers are grouped into categories that explain their appearance. Some are “true” interval cancers, which are new, fast-growing tumors that developed entirely since the last mammogram. Another category includes “missed” cancers that were present but too subtle to detect on the previous mammogram. A third type, “occult” cancers, are not visible on mammograms even in hindsight and are found through other methods.
High breast density is a primary risk factor. Dense breast tissue, which has more glandular and fibrous tissue than fat, appears white on a mammogram. Since tumors also appear white, this creates a masking effect that can make it difficult to spot a developing cancer.
Other risk factors include age, as younger women often have denser breasts. A family history of breast cancer or the presence of specific genetic mutations also elevates a person’s risk. These factors can contribute to cancers that grow more aggressively or are harder to see on a mammogram.
Clinical Characteristics and Prognosis
Interval breast cancers often have distinct features compared to screen-detected cancers. They tend to be larger at diagnosis because they are found due to symptoms, which may not appear until the tumor has grown. The tumor cells are also more likely to be of a higher grade, meaning they look more abnormal under a microscope and tend to grow faster.
These biological characteristics influence the patient’s prognosis. For instance, there is a higher probability that an interval cancer has spread to nearby lymph nodes by the time it is found. This fact, combined with a larger tumor size and higher grade, means the overall prognosis can be less favorable than for cancers detected through screening.
While these trends exist, they do not apply to every case. The prognosis for any patient is ultimately determined by the specific biological features of their individual cancer, not just the timing of its discovery.
Detection and Diagnosis
Detection of an interval breast cancer begins with recognizing new symptoms, which should prompt a visit to a healthcare provider. Common signs include:
- A new lump or mass in the breast or underarm
- Swelling in all or part of the breast
- Skin dimpling or irritation
- Changes to the nipple, such as retraction, redness, or discharge
Individuals should be aware of how their breasts normally look and feel, even after a normal mammogram. Promptly reporting any changes is the most direct way to ensure a timely diagnosis.
After a symptom is reported, a doctor performs a clinical breast exam followed by diagnostic imaging. This is more targeted than a screening mammogram and may include a diagnostic mammogram for more detailed X-rays or a breast ultrasound. If imaging reveals a suspicious area, the final step is a biopsy, where a small tissue sample is removed for examination to confirm a cancer diagnosis.
Treatment Approaches
Treatment for interval breast cancer is determined by the tumor’s specific characteristics, not the timing of its discovery. Doctors analyze the cancer’s stage (size and spread), grade, and its hormone receptor and HER2 status. This information provides a detailed profile of the cancer’s biology.
Based on this profile, a personalized treatment plan is developed using standard approaches. These can include surgery to remove the tumor, radiation therapy, and systemic treatments like chemotherapy, hormone therapy, or targeted therapy.
Because interval cancers may be diagnosed at a more advanced stage or have more aggressive features, the treatment plan might be more intensive. For instance, a larger tumor or one that has spread to the lymph nodes may require a more comprehensive combination of treatments compared to a cancer found during a routine screening.