Evidence base and efficacy
Randomized controlled trials (RCTs) served as the basis for implementing mass screening programs, as their design eliminates lead-time bias (lead-time bias). The largest Swedish studies involving a quarter of a million women and long-term follow-up demonstrated an overall mortality reduction of 15%.
According to the Marmot report (2012), participation in a 20-year screening program reduces the risk of death from this disease by 20%. For every 10,000 women invited, the program prevents 43 deaths. At the same time, the detection rate of pathology is about 8.4–8.5 cases per thousand examined. A significant proportion of findings are small invasive tumors (less than 15 mm) and carcinomas in situ.
Selection of age intervals
Age directly affects the feasibility of examination and the so-called lead-time (lead-time):
- Ages 39 to 49: Mortality is reduced by 17%. The lead time is 2.2 years, so the optimal interval between examinations is 12–18 months. However, mammography specificity and sensitivity are lower in this group, leading to frequent false-positive recalls and raising questions about cost-effectiveness.
- Ages 50 to 69: The optimal group. The lead time is 3–4 years, so a three-year interval is justified (although moving to a two-year step could provide additional benefit by reducing the number of interval cancers).
- Over 70 years: There is no reliable evidence of benefit from RCTs. Due to the high risk of overdiagnosis, patients are not routinely invited, but may undergo mammography on their own initiative (self-referral).
Organization of the screening process
The examinations themselves are performed by radiographers in mobile or fixed units. Only specialists with a high volume of practice (more than 5,000 examinations per year) are allowed to interpret the images, as their specificity and sensitivity rates are statistically higher.
A mandatory standard is double reading (double reading). If the opinions of the two doctors differ, consensus (joint discussion) or arbitration (involving a third independent expert who makes the final decision) is used.
About 5% of patients are recalled for in-depth evaluation. At this stage, additional X-ray projections, ultrasound, and core-needle biopsy are used. As a result, cancer is confirmed in approximately one out of seven recalled women. The final decision (referral for treatment, return to routine screening, or short-term follow-up) is made at a multidisciplinary team meeting.
Risks of screening: interval cancers and overdiagnosis
Any population-based program faces two unavoidable challenges:
- Interval cancers (interval cancers) are tumors that produce symptoms in the interval between scheduled visits (about 3 cases per 1,000 women). In 80% of cases, previous images show no signs of the disease. Sometimes the pathology is misinterpreted (most often calcifications and architectural distortions are missed). The prognosis for such tumors is similar to symptomatic cancer and worse than for screen-detected cancer.
- Overdiagnosis (overdiagnosis) is the detection of indolent lesions that would never have manifested clinically or shortened a patient's life. This primarily applies to tubular cancers and low-grade ductal carcinoma in situ (low-grade DCIS). The detection of such lesions entails unnecessary, excessive treatment (overtreatment). There is a 1% risk that a patient will receive treatment for a cancer that would never have harmed her, yet the benefit of lives saved is considered more significant.
Briefly: answers to topical questions
What standard X-ray projections are used in screening mammography?
Two main standard projections are used in screening mammography.
- Mediolateral oblique view (mediolateral oblique view — MLO) is the only projection that displays all breast tissue in a single image. For correct positioning, the nipple must be in profile at the level of the lower edge of the pectoralis major muscle.
- Craniocaudal view (craniocaudal view — CC) is performed by directing the X-ray beam from top to bottom. It visualizes almost all medial and most lateral tissue, with the nipple also in profile.
Screening balances three "I"s: Intervals (choice depends on age), Interpretation (mandatory double reading), and Interval cancers (tumors arising between visits).
Class and exam questions
Why is double reading of mammograms necessary?
This approach reliably increases the cancer detection rate while maintaining an acceptable level of recalls. In case of disagreement, consensus or independent arbitration is used.
What is lead-time bias?
This is the apparent increase in survival time due to earlier detection of the disease (the natural history of the disease itself may not change). Randomized trials help eliminate this bias and prove a real reduction in mortality.
What is the risk of detecting low-grade ductal carcinoma in situ (low-grade DCIS)?
Only about 40% of such lesions eventually progress to invasive cancer. The detection and treatment of the remaining 60% is a classic example of overdiagnosis and overtreatment.
What else to study on this topic
- Classification of interval cancer subtypes according to the NHSBSP system
- Algorithm for recall and assessment when anomalies are detected
- Watch and wait strategy for age-related indolent tumors
- The role of high-grade ductal carcinoma in situ (high-grade DCIS) as a precursor to invasive cancer
- Cost-effectiveness of screening in women aged 39–49 years
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