Tumor-related prognostic factors for breast cancer
Authors: W. L. Donegan
Journal: CA A Cancer Journal for Clinicians (1997)

The number of tumor-related features available to predict the prognosis of patients with breast cancer has grown impressively in recent years. Histology, tumor stage, and lymph-node status are now supplemented with measurements of steroid hormone receptors, ploidy, S-phase fractions, growth factors, oncogenes, and oncogene products. Cellular and molecular biology have not only advanced the understanding of carcinogenesis, but have provided a host of new biologic measures potentially related to clinical outcome. Interest in prognostic factors has been stimulated by the success of systemic adjuvant therapy for early-stage, operable cancer of the breast. Any feature of a tumor, or combination of features, that accurately indicates which patients are destined for recurrence and which are not is of considerable importance. Patients destined for recurrence can be selected for systemic adjuvant therapy, while patients who will not have a recurrence can be spared the morbidity of a treatment that offers no benefit. In addition, refinement of prognostic information facilitates improved clinical testing by ensuring comparability of treatment groups and providing markers to measure the success or failure of specific therapies. The literature devoted to prognostic factors for breast cancer is extensive. Scientific reports are supplemented by a multitude of letters, reviews, and metaanalyses.1, 2 Univariate and multivariate analyses are basic techniques. Variables are individually compared with measures of outcome, and those that are significantly related to outcome are used in multivariate analyses to determine if they have independent predictive value. These are then combined to form new prognostic categories. The mix of individual variables changes, however, and interrelationships are not always consistent. Measures of outcome are multiple, and relationships to outcome are subject to change with duration of follow-up.3, 4 Confirmation of projected outcomes with prospective studies is largely lacking for newer variables.5 The complexities are such that computer models are needed for integration of information. Computerized neural networks that are designed to learn from new data and predict individual patient outcome are under development to assist clinicians in making decisions about clinical management.6-8 The purpose of this article is to review tumor-related biologic factors of current interest and relate them to prognosis and treatment objectives. Ductal carcinoma in situ (DCIS) represents a small, but important, group of preinvasive breast cancers that can almost always be cured by local-regional therapy. DCIS made up 6.3 percent of 169,260 carcinomas (4.7 percent if lobular carcinoma in situ is excluded) reported to the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) national registry between 1973–1987.9 Only one percent of these early cancers are associated with metastasis to axillary nodes, and almost all (98 percent) are cured by local-regional therapy regardless of their size. Local recurrence, particularly after breast-conserving therapy, is associated with poor nuclear grade and comedo-type necrosis, but because dissemination is infrequent, systemic adjuvant therapy is unnecessary.10, 11 Invasive carcinoma carries with it the clear potential for metastasis and a diminished opportunity for cure. The term "microinvasive" is loosely used to describe the process of invasion in its earliest beginnings and to suggest that a tumor is still highly curable, possibly as curable as noninvasive carcinoma.12 No consensus exists on the definition of such tumors, and no consensus exists for the prognosis, other than that it is favorable. Distinguishing among in situ cancers, cancers with minimal signs of invasion, and those with a few millimeters of invasion is often difficult, and judgments among pathologists are not uniform. As a group, invasive carcinomas 5 mm or less in diameter have widely divergent rates of metastasis to regional lymph nodes, ranging from three to 28 percent in various reports.13-16 These rates are far higher than for noninvasive carcinomas and imply a poorer prognosis. A few special histologic types of invasive carcinoma pose a smaller risk of dissemination and death than other types of invasive ductal carcinoma. These types are pure mucinous, pure tubular, pure medullary, and pure papillary carcinoma.17-25 Mucinous carcinomas tend to occur later in life than the other three types.9 Typical medullary carcinoma is noteworthy because of the disparity between its aggressive histologic appearance and its less aggressive clinical behavior. However, it may be more aggressive than the other types mentioned.23 These special types form a small group, representing less than six percent of all invasive carcinomas.9 Most special types are localized when diagnosed. When nodes are involved, the number of involved nodes is usually three or less. In a large study, Rosen et al26 found that patients with special histologic types measuring 1.0 cm or less in diameter had a 10-year recurrence-free survival of 100 percent. The 10-year survival for all patients with tumors measuring 3.0 cm or less was 91 percent. Five-year relative survivals (survival corrected for normal mortality) of patients in the SEER registry treated for each of these special forms of invasive carcinoma are about 95 percent compared with 80 percent for all other types of invasive ductal carcinoma.9 A number of additional histologic features of invasive ductal and lobular carcinomas have prognostic value when considered in isolation. They include histologic grade, nuclear grade, tumor borders as stellate or circumscribed, peritumoral lymphatic and blood vessel invasion,27, 28 and necrosis within the tumor.29-31 Many of these lose relevance in multivariate analyses. Particular attention has been given to tumor differentiation as a morphologic indicator of tumor aggressiveness. Grading of differentiation can be performed without special equipment and is to some degree independent of axillary node status.24, 32-35 Histologic grade is currently based on the degree of tubule formation, number of mitoses, and nuclear pleomorphism in routine sections. These are combined as the Bloom-Richardson (B-R) grade or Scarff-Bloom-Richardson grade. Grades from 1 to 3 indicate progression from well differentiated (low or good grade) to poorly differentiated (high or poor grade). In general, well-differentiated tumors are a distinct minority. Histologic grade is found to increase with tumor size and with advancing anatomic stage.36 Histologic and nuclear grade are subordinate to node status and tumor size as prognostic features, but both are significant predictors of overall mortality for node-positive and node-negative patients.29, 33 In a multivariate analysis by Fisher et al,37 neither histologic grade nor any of nine additional pathologic variables related univariately to prognosis had independent predictive value for 15-year survival of 620 stage I and II patients treated with radical mastectomy after the number of positive axillary nodes, tumor size, and nipple involvement were taken into account. In a similar analysis of 859 cases, Shek and Godolphin38 found that histologic type and degree of histologic differentiation had no incremental value for predicting death rates after the number of involved axillary nodes, tumor-node-metastasis (TNM) stage, estrogen-receptor status, and presence of tumor necrosis were considered. Among 138 patients with tumors less than 1.0 cm in diameter, Stierer et al39 found that high histologic grade, high mitotic rate, and high nuclear polymorphism, but not node status, were significant predictors for recurrence and survival. However, only 12 percent of these patients had nodal metastasis. Poor histologic and nuclear grade may indicate responsiveness to adjuvant chemotherapy. Adjuvant chemotherapy has been observed to produce a greater improvement in prognosis among node-positive and node-negative patients with poorly differentiated tumors than among such patients with well-differentiated tumors.31, 40 This may diminish the difference in outcome between well-differentiated and poorly differentiated cases treated with chemotherapy. Among node-negative patients, the prognostic influences of nuclear and histologic grades are clearly evident.5 Treatment failures are few among the small group of histologic grade 1 cases.33 In an analysis of 1,157 node-negative patients, Fisher et al41 found that nuclear grade was a more important determinant of outcome than estrogen receptor or progesterone receptor status. Tumor size was not included in the analysis, and neither nuclear grade nor histologic grade identified a subgroup with five-year disease-free survival greater than 85 percent. Histologic grade was second only to tumor diameter as a predictor of disease-free survival among aneuploid tumors according to Winchester et al.35 These investigators reported a disease-free survival of 100 percent for a small group of node-negative patients with nuclear grade 1 tumors or with histologic grade 1 aneuploid tumors. An estimate of tumor grade is often included in a pathology report, but despite the clear influence of histologic and nuclear grading on prognosis and their ready availability, problems with tumor heterogeneity and with interobserver inconsistency continue to generate uncertainty about their role as routine prognostic indicators.42-44 Tumor angiogenesis as a prognostic assay is predicated on the evidence that ingrowth of blood vessels is a necessity for sustained tumor growth and metastasis. Interest in neovascularity as a prognostic factor was stimulated by the work of Judah Folkman on tumor angiogenesis and by the potential for treatment with antiangiogenic agents. The prognostic relevance of tumor angiogenesis was first reported by Weidner et al,45 who counted microvessels (veins and arteries) in the most densely vascularized areas of 49 invasive carcinomas and found their number and density significantly increased in cases with nodal and distant metastasis. The frequency of distant metastasis increased with increase in the vessel count. Subsequently, other investigators have produced varying results.46-48 in a carefully performed, blinded study of 220 cases of invasive breast carcinoma, Axelsson et al49 found considerable variability in microvessel counts in different parts of the same tumor and between the readings of two evaluators. These investigators found no significant correlation between microvessel count and other tumor factors of prognostic value and no significant correlation between microvessel count and survival or metastasis-free survival. They concluded that microvessel assay in its present state was unsuited for general application in the management of patients. The anatomic extent of a cancer determined clinically or histologically is a classic and reliable indicator of prognosis, but an imprecise one. The general staging categories include localized (confined to breast tissue), regional (direct invasion to extramammary tissues or metastasis to regional lymph nodes), and distant (metastasis beyond regional tissues). These categories identify three general groups with distinctly different probabilities for survival after diagnosis and treatment but allocate potentially curable cases into only two categories.50 In both of these categories, a substantial proportion of cases are not cured. According to a statistical report from the NCI, the five-year relative survivals of localized and regional cases diagnosed during the years 1960 through 1964 were 84 and 53 percent, respectively, a difference of only 31 percent.51 For the three stages, Gardner and Feldman52 reported ten-year survival rates of 54, 28, and five percent for 1,024 patients diagnosed before 1981. The five-stage TNM staging system is an improvement over general staging. It categorizes noninvasive carcinomas, which are highly curable, as stage 0. For staging invasive carcinomas, it places emphasis on size of the primary tumor and the extent of nodal metastasis. Excluding the incurable cases with distant metastasis (stage IV), the remaining stages of invasive carcinoma (stages I, IIA, IIB, IIIA, and IIIB) have an increasing likelihood of treatment failure and death. Five-year survivals for stages I to IIIB are 90, 80, 65, 50, and 40 percent, respectively.53 Staging based on clinical or pathologic information is limited to providing a static picture of the disease. Within each stage are cases with differing biologic potential and speed of progression and a broad spectrum of prognoses. The most important components of anatomic staging are the size of the primary tumor and the extent to which regional lymph nodes are involved. These two variables are independent, but they are closely related. The probability of metastasis to regional nodes (both axillary and internal mammary) and to distant sites increases progressively as tumors enlarge.15, 54, 55 As continuous variables the number of nodal metastases and tumor size allow more precision for assessment of prognosis than does anatomic staging. Clinical examination of axillary lymph nodes is notoriously inaccurate for determining the presence of metastases (unless they are large and advanced) and provides a poor criterion for staging. In a representative study, examinations yielded false-negative results in 38.6 percent of cases and false-positive results in 27.3 percent.56 The presence of metastasis to axillary lymph nodes on histologic examination of the nodes provides proof that a tumor with the capacity to metastasize has done so and may have metastasized to distant sites as well. Multivariate analyses regularly indicate that the presence or absence of metastasis to axillary lymph nodes is the single most influential predictor of posttreatment recurrence and death (Table 1). In the absence of systemic adjuvant therapy, the chance of recurrence within 10 years is 24 percent for patients without nodal metastasis on histologic examination and 76 percent for patients with nodal metastasis.57 Rotter's interpectoral nodes, a subgroup of axillary nodes, contain metastases when all other axillary nodes are normal in only 0.5 percent of cases, and their contribution to prognosis is considered of little importance.58 While axillary metastasis is the most important determinant of prognosis in operable cases, the fact that a quarter of patients without axillary metastasis are not cured by local-regional therapy and some patients with metastasis are alive and well after many years (at 10 years, 30 percent overall and 17 percent for patients with metastasis to four or more nodes) indicate that they are an imperfect sign of systemic disease.52, 59 More prognostic information is derived from axillary lymph nodes than the fact of involvement alone. The most important information is the number of involved nodes. The absolute number of involved nodes provides a prognostic continuum that is directly related to the prospects for recurrence and indirectly related to survival.15, 29, 59-63 In one large study of 1,741 cases, the 10-year survival of patients with 0, 1 to 3, 4 to 9, and 10 or more involved nodes was 75, 62, 42, and 20 percent, respectively.29 The total number of nodes examined, and by inference the percentage of nodes involved, does not alter the prognostic importance of the absolute number of nodes that contain metastases, provided that sampling is sufficient to detect all positive nodes. Surgical removal or pathologic examination that is too limited is likely to provide misleading information.64 A level II axillary dissection is considered necessary to obtain reasonably accurate information.56 The size of metastases, the growth of metastases through the capsule of the lymph nodes, and the highest axillary level reached by the metastases can be related individually to prognosis, but all are interrelated in a complex manner and generally tend to be a function of the total number of involved nodes. Metastasis to increasing numbers of nodes results in larger metastases, extracapsular growth of metastases, and a higher axillary level of involvement.62 For example, extracapsular growth is seen only with macrometastases and influences prognosis adversely only when three or more nodes are involved.65, 66 When the number of nodes with metastases is constant, the level of axillary involvement has no additional predictive value.62, 67 Micrometastases (⩽2 mm in diameter) are more favorable than macrometastases (>2 mm), and micrometastases rarely involve more than three nodes.68, 69 Micrometastases found on routine sections of nodes or as occult metastases on multiple recuts of nodes initially considered free of metastasis are of interest because they potentially identify the subgroup of node-negative cases likely to relapse. A number of retrospective studies have found little or no difference in survival of patients with micrometastases compared with patients without metastases,70-72 but a recent prospective study by the International (Ludwig) Breast Cancer Study Group using multiple step sections of nodes showed a significantly poorer prognosis of patients with micrometastases.73 With this encouragement, more sensitive methods of detecting micrometastases in axillary nodes are being investigated in a continuing attempt to identify high- and low-risk populations of node-negative cases. The techniques being used include immunohistochemical methods with tumor-seeking monoclonal antibodies and polymerase chain reaction technology, which can detect minute amounts of messenger RNA related to human breast cancer.74 Models predict that monoclonal antibody techniques can detect one cancer cell among one million normal cells.72 Flow cytometry may be equally or even more sensitive. Monoclonal antibodies are also being used to detect micrometastasis in bone marrow samples of patients with early breast cancer, which may prove even more relevant to early distant relapse.75 Internal mammary lymph nodes are also a primary lymphatic drainage basin of the breast. They are not routinely examined for pathologic staging, but they are involved in nine percent of cases when no metastases are found in axillary nodes, unmasking a high-risk group of "node-negative" cases.54 For this reason, histologic proof that they are free of metastasis provides additional evidence that a tumor is locally confined. Metastasis to these nodes has the same overall prognostic importance as metastasis to axillary nodes. However, they are less accessible for examination, and their small number provides less potential for quantifying prognosis. Internal mammary node metastasis is second only to axillary node metastasis as a prognostic variable, and it has been found more important than DNA ploidy and ERBB2 (c-erb B-2) expression.76, 77 Metastasis to both axillary and internal mammary nodes indicates a worse prognosis than metastasis to either axillary nodes or internal mammary nodes alone, reducing ten-year survival from about 55 percent when either group is involved down to 30 percent when both are involved.54 It is likely that metastasis to internal mammary nodes simply indicates more widespread disease. Five involved regional nodes may have the same prognostic significance whether they are found only in the axilla or represent the total from both sites. Metastasis to supraclavicular nodes implies extensive involvement of axillary nodes, but it can occur in the absence of axillary involvement, suggesting passage through internal mammary nodes or blood-borne passage. The prognosis for patients with metastasis to this site is equated in the current TNM staging system with general dissemination of cancer (i.e., stage IV).53 While patients with supraclavicular metastasis regularly develop distant dissemination, their survival is not always as poor as that of patients with distant metastasis at the outset. The five-year survival of the latter rarely exceeds 18 percent.53 The five-year survival rates for patients found initially with ipsilateral supraclavicular metastasis in two reports were 30 and 34 percent, suggesting a prognosis intermediate between TNM stages III and IV.78, 79 The importance of tumor size as a prognostic variable in cases of invasive carcinoma is robust enough to survive measurements derived variously from clinical estimates, mammograms, and gross and histologic sections. In many analyses it is second only to axillary node status as an independent prognostic factor. Tumor size is directly related to an increasing probability of regional metastasis, an increasing average number of involved axillary lymph nodes,16, 62, 80 and an increasing probability of recurrence and death. The favorable prognosis of nonpalpable invasive carcinomas relative to palpable ones and of screening-detected versus nonscreening-detected cancers is easily explained by their smaller size.81 In one report cancers 0.1 to 5 mm and 6 to 10 mm in diameter produced axillary metastasis in only 7.7 and 12.5 percent of cases, respectively.82 However, the incidence of positive nodes can range up to 21 percent for both of these size groups.15 Tumors of equal size are prognostically similar whether they are palpable or not and regardless of how they are detected.83, 84 The influence of primary tumor size on prognosis can be appreciated in both node-negative and node-positive cases. This relationship probably reflects increasing vascular and lymphatic dissemination with progressive tumor growth (Figure).85 of particular interest are node-negative cases, where tumor size provides a readily available means for identifying patients at low and high risk for recurrence. Nodal status and primary tumor size are independent influences on survival of 24,740 patients with breast cancer. These two variables identify prognostic groups with five-year relative survivals from 99.2 to 45.5 percent. Relative survival is actuarial survival adjusted for age- and race-related natural mortality. Adapted from Carter et al.15 Tumors 1.0 cm or less in diameter have an especially low risk of recurrence. The five-year disease-free survival of node-negative patients with tumors 1.0 cm or less in diameter is 92 to 96 percent.86, 87 The 10-year relapse-free survival of 47 cases of invasive carcinoma measuring 1.0 cm or less on gross section or on mammograms reported by Tinnemans et al82 exceeded 90 percent. A large study at Memorial Sloan-Kettering Cancer Center found a 10-year relapse-free survival of 91 percent.26 Only 12 percent of 171 patients with tumors 1.0 cm or less in diameter had recurrence within 20 years of their primary treatment in a study by Rosen et al.25, 80 Survival was significantly superior to that of patients with tumors 1.1 to 2.0 cm in diameter, and it was estimated that 80 percent of patients with tumors 1 cm or less were cured at 20 years. Tumor sizes were taken from the gross description of the specimen or from measurement on histologic sections. These and other studies support the contention that patients with the combination of node-negative disease and a tumor diameter of 1 cm or less represent a favorable subset of patients who would not benefit significantly from systemic adjuvant therapy.88 As failure rates are high enough in any case with macrometastasis to axillary lymph nodes to justify systemic adjuvant therapy, investigations have largely focused on the ability of other biologic variables to further define the prognosis of node-negative individuals, a group that is increasing due to more widespread use of screening mammography. Intracellular steroid-hormone receptor proteins, primarily estrogen receptor (ER) and progesterone receptor (PR), have received intensive study both as indicators of prognosis and as guides to hormone and endocrine therapy.63 About 50 to 85 percent of breast cancers contain measurable amounts of ER. The frequency with which tumors contain ER and the concentration of ER increase with patient age, both reaching their highest levels in postmenopausal patients. Concentrations of 10 fmol/mg or more of cytosol protein are generally considered positive for clinical purposes, and upper levels can reach more than 1,000 fmol/mg. The presence of ER implies that normal cellular mechanisms for processing estrogen have been maintained despite malignant change, particularly if PR is present. PR is expressed only after transcriptional activation of its gene by a functional ER-estrogen complex. The clinical importance of ER relates principally to the fact that its presence identifies hormone-sensitive tumors. About 50 to 60 percent of patients with significant amounts of ER in their tumors respond favorably to hormone or endocrine therapy. A higher percentage respond if ER levels are high and if both ER and PR are positive. Patients with ER-positive tumors have prolonged disease-free survival after primary treatment, superior overall survival, and longer survival after recurrence compared with patients with ER-negative tumors, and this advantage is independent of axillary node status. However, the value of ER status as an independent prognostic variable is diminished by its association with other established indicators of favorable prognosis and by its relationship to successful hormone therapy. ER-positive cancers generally have low-grade histology, favorable nuclear grade, a low S-phase fraction, a normal complement of DNA, a low proliferative index, and a low thymidine labeling index.63 The influence of therapy on prognosis is difficult to exclude because ER-positive patients so regularly receive and benefit from either adjuvant or palliative hormone therapy. In some studies the lengthened disease-free survival and survival of patients with ER-positive tumors are seen only in the presence of hormone therapy.89 Often the effect of ER-positive status as a discriminant is lost after several years, further suggesting a temporary influence of treatment.90-93 When node-positive patients not receiving adjuvant hormone therapy were studied, five-year disease-free survival was 20 percent higher for ER-positive patients than ER-negative patients. However, the five-year survival of the most favorable subgroup, patients with one to three positive nodes and ER-positive tumors, did not exceed 60 percent.94 Among node-negative patients, hormone therapy is less of a confounding factor, and small but statistically significant differences in disease-free survival and overall survival from eight to 12 percent have been found between ER-positive and ER-negative cases after various periods of follow-up.41, 95 A multivariate analysis of prognostic factors by McGuire96 that included ER and PR status for more than 3,000 cases shows ER status to be more important for prognosis than tumor size in node-negative cases but not in node-positive cases. Fisher et al40 found ER status to be less important for the prognosis of disease-free survival or overall survival than number of positive nodes and nuclear grade. The indications are that ER status is a weak prognostic indicator and that PR status provides no important advantage.97 Both are probably more reflective of growth rate than of metastatic potential. ER status alone or in combination with axillary status fails to identify a node-negative or a node-positive subset that has a rate of recurrence low enough to exclude systemic adjuvant therapy. Enzyme immunoassay and immunohistochemical methods of measuring ER in tissues depend on colorimetric reactions or on scoring systems based on visual estimates of the number and intensity of stained cells. These semiquantitative methods show high correlations with the results of biochemical ligand-binding assays.98-101 Similar correlations have been demonstrated with response to hormone therapy and with prognosis.102, 103 The pS2 protein appears to identify a subset of ER-positive tumors with a particularly favorable outlook. pS2 protein is an estrogen-regulated secretory protein of unknown function, but it probably indicates a more intact cellular estrogen-processing mechanism. It is expressed predominantly by ER-positive tumors. When found in ER-negative tumors, it is in much lower concentrations. It is not expressed in other normal human tissue (except stomach). Values above 11 ng/mg are associated with increases in disease-free survival and overall survival of patients with ER-positive tumors. After adjustment for tumor size, lymph node status, and ER status, negative pS2 status is still associated with early recurrence and death. Positive pS2 status in ER-positive patients indicates improved prognosis in both node-negative and node-positive patients. In patients with ER-positive/pR-positive tumors, positive pS2 status was associated with a five-year survival of 97 percent versus 54 percent for negative pS2 status. In node-negative patients, pS2-positive and pS2-negative cases had overall survivals of 89 and 58 percent, respectively. In node-positive patients, pS2-positive and pS2-negative patients had overall survivals of 88 and 34 percent, respectively.104 pS2 status may be an even stronger predictor of adjuvant hormone responsiveness than ER status.105 Flow cytometry with laser-stimulated DNA fluorescence makes automated measurement of the DNA content of individual cells and the number of cells in each phase of the cell cycle readily available. It is possible to determine whether the DNA of each cell is normal (diploid versus nondiploid) compared with a control and to determine the fraction of cells actively synthesizing DNA. Normally, diploid cells are in the resting phase (G0) or in the first gap phase of the cell cycle (G1). Cells with twice the normal DNA content are in either the G2 or early mitotic phase (M), and cells with intermediate amounts of DNA are in the synthesis phase (