Breast Cancer Is A Heterogeneous Disease
The lay press and much of the medical literature refers to breast cancer as a single disease. The term Breast Cancer refers to a variety of cancers with different characteristics, growth rates, and potential for spread. All begin in the smallest functional part of the breast, the terminal ductal lobular unit and are different in growth patterns, appearance under the microscope, potential for early spread, and age range in which they are usually found.
The Concept of Sojourn Time
In the life of any disease process there is point before which the presence of the disease cannot be recognized by any currently available means. Before the advent of Screening Mammography the recognition of the presence of breast cancer was delayed until a mass or lump was felt in the breast, there were changes in the skin overlying the breast, or until a patient had symptoms or signs of distant spread.
Screening mammography has drastically changed this. Although there is still a period of time from a breast cancers' start until it can be recognized, most (approximately 85-90%) but not all can be identified by mammography before they are detected by physical examination.
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It is this time period between which a breast cancer can be first identified by mammography and the time it is identified as a lump that is referred to as Sojourn Time. |
Sojourn Time Varies With Age1
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| Type of Breast Cancer | Mean Sojourn Time in Years | |
| 40-49 Year Age Group | 50-69 Year Age Group | |
| Invasive Lobular Carcinoma | 2.3 | 2.0 |
| Tubular Carcinoma | 3.2 | 7.1 |
| Mucinous | 1.9 | 2.9 |
| Medullary Carcinoma | 1.2 | 1.2 |
| Invasive Ductal Carcinoma Grade 1 | 2.0 | 7.7 |
| Invasive Ductal Carcinoma Grade 2 | 1.7 | 3.0 |
| Invasive Ductal Carcinoma Grade 3 | 1.7 | 3.1 |
| DCIS | 3.2 | 5.1 |
Invasive ductal carcinoma accounts for about 60% of all invasive breast cancer. A summary of Invasive Ductal Carcinoma Grades 1,2, and 3 broken into two age groups is informative.1
| Age Group | Adjusted Mean Sojourn Time (in years) | Proportion of tumors with potential of progressing during the sojourn time |
| 40-54 | 1.73 | 91% |
| 55-69 | 3.79 | 37% |
The goal of screening is to identify a treatable abnormality as early as possible in the sojourn time interval. The majority of breast cancers are of the Invasive Ductal Carcinoma subtype2. In the 40-54 year old age group, mammography on a yearly basis is necessary for screening to have an impact on deaths due to these breast cancers. As a generalization, these women also frequently have the densest breast tissue making small cancers more difficult to detect.
Note that this group cancers with the shortest window of opportunity for early detection affects younger women during their most active years.
Degree of Malignancy Relates to Breast Cancer Size
Tumor grade is a measure of the degree of malignancy shown by a tumor.
In general, the lower the number the less aggressive a tumor may be expected
to act. The distribution of cancer grade related to size in the population
actively studied in the Swedish Two-Country study is given:1
| Age Group | Size (millimeter) | Percent (%) of cancers with | |
| Grade 1-2 | Grade 3 | ||
| 40-49 | 1-19 | 56 | 45 |
| 20+ | 38 | 62 | |
| 50-59 | 1-19 | 75 | 25 |
| 20+ | 45 | 55 | |
| 60-69 | 1-19 | 74 | 26 |
| 20+ | 43 | 57 | |
This table shows that in each age group the smaller the tumor the larger the percent of cancers with the lower grade and therefore, the expectation of a better survival. This expectation is the experience in Sweden where formal screening programs have been in place for 20 years.
Note that the younger age group (40-49 years) has a smaller percentage of cancers in the 1-19 millimeter size. This is due to the short sojourn time (1.73 years) in this age group and the use of a two year screening interval in the scientific study.
Also note that the larger the cancer the higher the grade in all age groups. This relates to the faster growth of the more malignant portion of each breast cancer and is referred to as dedifferentiation or phenotypic drift.
Conclusions
| 1 | Different forms of breast cancer act differently with different growth rates, potential to spread, and length of the window of opportunity to identify it early when effective treatment may be less radical. |
| 2 | The most frequently occurring subtype of breast cancer has one of the shortest sojourn times and this short sojourn time occurs in the younger population eligible for screening. Therefore the recommendation is for screening to occur yearly beginning at age 40 years. |
| 3 | Breast cancer "malignancy" can be expressed as a grade and the larger tumors tend to have a higher grade which translates to more potential to lead to distant spread and death. |
1Tabar L, Fagerberg G, Chen H.H., Duffy S.W., Gad A. Tumour Development, Histology and Grade of Breast Cancers: Prognosis and Progression. Int. J. Cancer; 66, 413-419 (1996)
2Kopans DB, Breast Imaging 2nd Ed. Philadelphia. Lippincott-Raven.
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Updated March 8, 2004