Anemia in malignancies: Pathogenetic and diagnostic considerations

Abstract

Objectives

The aim of this paper is to review the pathogenesis and diagnostic approaches to anemia in cancer patients.

Methods

PubMed was queried for various combinations of anemia and cancer-related terms using appropriate filters for articles and practice guidelines published in the last 5 years. Specific searches were conducted for individual pathogenetic mechanisms and malignancies of specific anatomic sites.

Results

Anemia is the commonest hematological manifestation of cancer, afflicting 40–64% of patients treated for malignancies. Pathophysiologically, cancer-related anemia can be classified into four broad but overlapping categories: hypoproliferative anemia including the common anemia of inflammation/chronic disease, hemolytic anemia, miscellaneous etiologies, and uncertain etiologies. Anemia incidence increases with the administration of chemotherapy/radiotherapy. It reduces the quality of life and shortens survival in cancer patients. A positive correlation is observed between anemia and tumor hypoxia. Experimentally, hypoxemia enhances tumor growth and resistance to therapy by stimulating angiogenesis, acquisition of genomic mutations, and increasing resistance to apoptosis as well as to the killing effects of chemo/radiotherapy-generated free radicals.

Discussion

Diagnostic approaches to the anemic cancer patient begin with a detailed clinical history and physical examination. Peripheral blood morphology and reticulocyte count are also helpful. Patients with unexplained anemia are evaluated by standard approaches also used in patients of similar age without malignancy. Serum iron profile and bone marrow examination are often required in difficult cases. This review focuses on major aspects of the pathogenesis of the individual entities. Diagnostic approaches and uncommon causes including hemophagocytic lymphohistiocytosis, acquired hemoglobinopathies, and myelodysplasia are also discussed.

Keywords:

• Anemia
• Cancer
• Chemotherapy
• Chronic disease
• Etiopathogenesis
• Hypoxia
• Malignancy
• Radiotherapy

Introduction

Cancer is the commonest cause of mortality in developed countries and the second leading cause of mortality in the developing world. Global estimates for 2008 pointed to 12.7 million cancer cases and 7.6 million cancer deaths.^1,2 Anemia is a major public health problem affecting 1.62 billion people worldwide. A World Health Organization (WHO) Global Database on Anemia for 1993–2005 pegged the prevalence of anemia worldwide at 25%.³

Anemia is the commonest hematological manifestation of cancer and a majority of cancer patients are anemic.⁴ Anemia may at times be the sole manifestation of cancer and a diagnostic work-up of anemia may unmask a hidden malignancy like a gastrointestinal tract adenocarcinoma (refractory iron deficiency anemia (IDA) may be sole manifestation), hairy cell leukemia (HCL), or a myelodysplastic syndrome (MDS). The commonest anemia in patients with malignancies is anemia of inflammation/chronic disease,⁴ although other causes depending on the site may be also common (e.g. blood loss in stomach, bladder, uterine, and cervical malignancies). Multiple etiologies are encountered in many cases. In the current era of multimodality cancer therapy, the prevalence of treatment-related anemia is likely to be increasing even though definite estimates are difficult.

We review here the pathogenetic and etiological approaches to anemia in cancer patients. For this, we queried PubMed for various combinations of the terms anemia, cancer, malignancy, diagnosis, and pathogenesis using the following filters: reviews, systematic reviews, articles published in the last 5 years, and practice guidelines. Subsequently, specific searches were conducted for the individual pathogenetic mechanisms (e.g. chemotherapy-related anemia) and malignancies of specific anatomic sites.

Pathophysiological classification of anemia in malignancies

Anemia in cancer can be broadly classified into four major categories: hypoproliferative anemia, hemolytic anemia, anemia of miscellaneous etiology, and anemia of uncertain etiology (Table 1). The rubric hypoproliferative anemia includes anemia of inflammation or chronic disease/iron sequestration anemia, anemia of iron deficiency and other malnutritive/malabsorptive states, pure red cell aplasia (PRCA), megaloblastic anemia, myelophthisic or leukoerythroblastic anemia, myelodysplasia, and anemia due to marrow aplasia or hypoplasia. Hemolytic anemias include autoimmune hemolytic syndromes and microangiopathic hemolytic anemia (MAHA). Anemias of miscellaneous etiologies include a wide variety of causes including those due to hypersplenism, secondary hemophagocytic syndrome, acquired (especially alpha) thalassemia, and blood loss. At times, the cause of anemia cannot be established where the term anemia of uncertain etiology is more appropriate. These entities are discussed further below.

Table 1. A pathogenesis-based classification of etiologies of anemia in patients with malignancies

Hypoproliferative anemia

Anemia of inflammation or chronic disease/iron sequestration anemia

It is the commonest cause of anemia in cancer patients.^4,5 Among the malignancies, solid tumors account for majority of cases. Mechanisms of anemia of inflammation/chronic disease in cancer are both functional as well as iron deficiency-related.⁵ Though these two processes are not mutually exclusive, the pathogenesis of anemia of inflammation/chronic disease needs separate discussion.

The master regulator of iron homeostasis is hepcidin.⁶ This polypeptide hormone with antimicrobial activity acts by binding to ferroportin and causing its degradation. This prevents iron from being exported out of the cell and results in functional iron deficiency. Bone morphogenetic protein-6 (BMP6) and its downstream signaling pathway mediated by second messengers act as key physiological regulators of hepcidin. Hypoxia regulates hepcidin production directly through hypoxia-inducible factor or indirectly by increased erythropoietin production.⁷ Cytokines, especially interleukin-6 (IL-6) increase hepcidin production via signal transducer and activator of transcription-3 (STAT3), as well as by other pathways.^6–9 Recently, the role of activin receptor-mediated pathways has been unraveled and dysregulation of these pathways too appears to contribute to the development of iron sequestration anemia in malignancies.⁹

Iron deficiency anemia

Absolute iron deficiency commonly observed in cancer may be nutritional, related to blood loss, insufficient iron uptake from food, or increased utilization. Solid tumors comprised of 22%, and hematological malignancies 8% of the total causes of intravenous iron requirement for IDA.¹⁰ Other causes of IDA are bleeding associated with malignancies of gastrointestinal, urogenital, and respiratory tracts. Bleeding may also occur due to acquired platelet dysfunction in malignancies like the myeloproliferative neoplasms and in paraproteinemias.¹¹ In patients with tumors requiring gastric resection, iron deficiency develops because of decreased iron absorption. An underlying malignancy should be strongly considered in any patient presenting with IDA which is unexplained by common approaches.

Red cell hypoplasia and PRCA

PRCA is a syndrome characterized by a severe normocytic anemia, reticulocytopenia, and the absence of erythroblasts from an otherwise normal bone marrow. Cancer is an important cause of secondary PRCA. Both hematological neoplasms as well as various solid malignancies have been implicated although the exact pathogenesis remains unclear. PRCA has rarely been reported in association with lymphoproliferative disorders and among the hematological malignancies chronic lymphocytic leukemia (CLL), T-cell large granular leukemia (LGL)/natural killer (NK) cell leukemia, plasma cell myeloma, non-Hodgkin lymphoma, MDS, and acute lymphoblastic leukemia (ALL) have been reported.^12–14 PRCA might precede, may present simultaneously, or might follow the lymphoproliferative disorder, either in relapse or even in remission.

Various pathogenetic roles of autoreactive B-cells, T-cells, and NK-cells in erythroid hypoproliferation have been described. B-cells mainly act via production of IgG antibodies.^12,14,15 IgG binds complement, may be directly cytotoxic via type II hypersensitivity reaction, targets erythroblasts in vitro, or inhibits hemoglobin synthesis. IgG produced following a viral or bacterial infection cross-reacts with erythroid precursor cells or erythropoietin. Anti-erythropoietin antibodies form immune complexes with erythropoietin, causing functional inactivation. Obstruction of the erythropoietin receptor by anti-erythroblast antibody has also been described.¹⁵ The role of T-cells is mainly by major histocompatibility complex-I (MHC-I)-restricted CD8+ cytotoxic T-lymphocytes and NK-cells is via MHC-unrestricted CD3+ T-LGLs with T-cell receptor (TCR)-expressing killer-cell immunoglobulin-like receptors.¹⁶ There is also an interplay between all the cell types wherein antibodies cross-link the TCR of the T-LGLs with the Fc receptor on target cells or cross-links the Fc receptor on the T-LGLs or NK-LGLs (CD16) with any specific ligand for the antibody on the target cells.^15,17,18

Megaloblastic anemia in malignancies

Megaloblastic anemia is a less common form of hypoproliferative anemia encountered in cancer patients. Folic acid or vitamin B12 deficiency may be nutritional or may result from anorexia and decreased dietary intake. Increased requirements for folic acid or vitamin B12 that cannot be replenished by the regular dietary intake may occur due to increased cellular proliferation and high-cell turnover as in leukemias with hyperleukocytosis. Gastric tumor patients who have been treated by total or subtotal gastrectomy are at increased risk for the development of megaloblastic anemia because of vitamin B12 malabsorption. Folate antagonists, nucleoside analogs, and other drugs interfering with DNA synthesis including hydroxycarbamide, cytarabine, methotrexate, 5-fluorouracil, thioguanine, azathioprine, and 6-mercaptopurine frequently produce megaloblastosis.^19,20

Myelophthisic or leukoerythroblastic anemia

Myelophthisic or leukoerythroblastic anemia refers to anemia due to replacement of the normal marrow elements by (in this context) a neoplastic process thereby resulting in hypoproliferation of the normal marrow elements including the erythroid precursors. Bone marrow examination is always indicated in persons with anemia of obscure etiology, even occasionally yielding the first clue to the presence of an occult malignancy.²¹ The neoplastic process can be in the form of frankly malignant cellular elements as well as tumor diatheses in the form of fibrosis, osteomyelosclerosis, and rarely necrosis or granulomatous responses. Both metastatic (e.g. breast, prostate, renal carcinomas, etc.) as well as hematological malignancies (e.g. HCL, Hodgkin lymphoma, primary myelofibrosis, and other myeloproliferative neoplasms) commonly produce secondary myelofibrosis.

Myelodysplastic syndromes

MDS are a heterogeneous group of clonal hematopoietic stem cell disorders that are characterized by ineffective hematopoiesis/bone marrow failure, peripheral blood cytopenias, and a propensity for leukemic transformation (usually into acute myeloid leukemia, AML). The WHO classification system currently recognizes distinct pathological subtypes of MDS based on morphological features, percentage of nucleated bone marrow cells that are blasts, and the number of hematopoietic lineages affected.

The diagnosis of MDS in a patient with a known malignancy should however be made with extreme caution as myriad tumor and therapy-related influences can mimic the morphological appearances of MDS. Cytogenetic studies and, to a lesser extent, flow cytometry can be useful in this distinction although clinical and hematological follow-up including repeat bone marrow examinations is always judicious.²²

Hypoplastic/aplastic anemia in patients with cancer

More appropriately termed ‘secondary’ hypoplastic/aplastic anemia; this is nearly always therapy-related. Even so, hematological malignancies (and concomitant diminished hematopoietic reserves) as well as their treatments are more common causes than non-hematological malignancies. The pathophysiology of marrow cell destruction (direct cytotoxicity) has been inferred from the results of treatment in humans, with substantial support from in vitro and animal models.²³

Hemolytic anemia in cancer patients

Autoimmune hemolytic anemia

Cancer-associated autoimmune hemolytic anemia (AIHA) can be broadly divided into warm and cold AIHA. Warm AIHA occurs in both lymphoid a well as non-lymphoid malignancies. CLL and lymphomas account for nearly 80% of secondary warm-antibody AIHA cases.²⁴ In addition, some non-lymphoid malignancies like ovarian tumors have also been shown to produce warm AIHA.

IgG1, the most commonly encountered subclass, and IgG3 autoantibodies appear to be the most effective in shortening red blood cell (RBC) life spans.^25,26 Destruction of autoantibody-coated RBCs is mediated primarily by sequestration and phagocytosis in macrophages of the splenic Billroth cords and to a lesser extent in hepatic Kupffer cells. These macrophages express cell surface receptors for the Fc region of IgG, especially IgG1 and IgG3 and also for fragments of the complement proteins C3 and C4. When present together on the RBC surface, IgG and complement fragments act synergistically to enhance RBC trapping and phagocytosis.

Most RBC sequestration in warm-antibody AIHA occurs in the spleen, but trapping in the liver occurs in the presence of large quantities of RBC-bound IgG or the concomitant presence of complement proteins on the RBC surface. Trapped RBCs may be partially or completely ingested by a macrophage. Partial phagocytosis is most common and leads to the formation of spherocytes. Since membrane loss exceeds volume, the escaped RBC assumes a spherical shape, a sphere being the geometric shape with the lowest ratio of surface area-to-volume. Certain cancer chemotherapeutic drugs are well-known causes of hemolytic anemia. Carmustine (bis-chloroethylnitrosourea) reduces glutathione reductase and results in hemolysis in patients who are glucose 6-phosphate dehydrogenase (G6PD) deficient. Doxorubicin generates reactive oxygen species and results in oxidative hemolysis, also marked in patients with G6PD deficiency.²⁷ Pentostatin, used in HCL, CLL, and graft-versus-host disease, may also cause hemolysis.²⁸

MAHA including disseminated intravascular coagulation

Cancer-related MAHA is a paraneoplastic syndrome characterized by Coombs-negative hemolytic anemia with schistocytes, thrombocytopenia, elevated plasma hemoglobin, and lactate dehydrogenase and frequently, renal failure or dysfunction.²⁹ Cancer-related microangiopathy is a rare cause but frequent terminal cause of secondary MAHA.³⁰ A recent review of cancer-related MAHA found that 154 cases were associated with solid cancers and 14 with lymphoma.³¹ Over 90% of the cancers were metastatic, and in 19.4% of solid cancers, the MAHA occurred at recurrence/relapse. In general, adenocarcinoma was the commonest histological type of cancer in MAHA, in part due to the erythrocytopathic effects of mucins produced. Gastric carcinoma was the commonest cancer site in that review, followed by breast, prostate, lung, lymphomas, unknown primary, abdominal, genitourinary, and endocrine cancers. Lymphomas associated with MAHA included Hodgkin disease, angiotropic lymphoma (especially T-cell types), diffuse large B-cell lymphomas, and plasma cell myelomas.³¹

A special feature of MAHA in malignancies is that, unlike in other paraneoplastic syndromes, a relatively high proportion of MAHA cases occur at the time of cancer recurrence. In the review cited above, clinical and laboratory findings revealed that only a minority of cases presented with the features of thrombotic thrombocytopenic purpura (TTP) or atypical hemolytic uremic syndrome (aHUS), with the exception of prostate cancer, whereas aHUS was a common presentation and compared to hereditary or immune TTP or aHUS.³¹ Bone marrow was the commonest site of metastasis in cancer-related MAHA. About one-third of patients with MAHA had laboratory signs suggesting disseminated intravascular coagulation (DIC). Pulmonary symptoms were more common in cancer-related MAHA as compared to non-malignant TTP/HUS. In the vast majority of cancer patients (a disintegrin and metalloproteinase with thrombospondin-like motif-13), ADAMTS13 levels were >20% in contrast to hereditary and acquired TTP where ADAMTS13 activity is more or less reduced.

The pathogenesis of MAHA in cancer is largely speculative. In solid cancers, the most likely cause may be red cell fragmentation and platelet destruction in small vessels of cancerous tissue, in particular, in bone marrow, lung, or other organs. Cytokine production by tumor cells may also play a role, for instance, in tumor necrosis factor (TNF)-mediated red cell damage. Massive tumor necrosis could release tissue factor and initiate the coagulation cascade leading to thrombotic microangiopathies.^31,32 Endothelial cells too are likely to play a role. Clinically, MAHA is a serious complication of malignancy with a very poor prognosis. Nearly half of all patients (46.5%) in the aforementioned study died within a month with or without treatment.³¹

Miscellaneous hemolytic processes in patients with malignancies

These include hemolytic reactions (acute or delayed) as well as alloimmunization leading to reduced life span of transfused RBCs in patients requiring blood transfusions, co-incident infections like malaria, shigellosis, clostridial sepsis, leishmaniasis,³³ etc., and hemolysis associated with extra-corporeal circulatory devices and dialysis membranes.

Anemias of miscellaneous etiology in cancer patients

Hypersplenism

Hypersplenism secondary to lymphoproliferative disorders and less commonly, myeloproliferative neoplasms, is a rare cause of anemia. In myeloproliferative neoplasms, the primary pathology is in bone marrow whereas in lymphoproliferative neoplasms, hypersplenism might play a significant role. Anemia is caused by RBC pooling in the enlarged spleen, decreased survival of RBCs, and hemodilution resulting from an increased plasma volume.^34,35

Secondary hemophagocytic syndrome/hemophagocytic lymphohistiocytosis

Malignancies presenting with prominent hemophagocytosis are rare and anemia resulting from malignancy-associated hemophagocytic lymphohistiocytosis (HLH) is exceedingly rare. Only a few case series are available in the literature and in virtually all the anemia is associated with other cytopenias. In the series by Shabbir et al.³⁶ in 18 adult patients, hematological malignancies and stem cell transplantation constituted 33% of all HLH cases and no solid tumors were noted. Celkan et al.³⁷ analyzed 29 pediatric malignancy-associated HLH and observed that 60% were due to hematological malignancies while 40% were due to solid tumors. ALL constituted 83.3% of the hematological malignancies. Rhabdomyosarcoma and neuroblastoma constituted 50% each of the solid malignancies. Both genders were equally represented. They postulated that presumably, malignant cells secrete the proinflammatory cytokines TNF-α and IL-6 that contribute to immune dysregulation.

Regardless of the underlying condition, the hallmark in many studies of HLH in malignancies appears to be acquired NK-cell dysfunction. Lymphoma-associated HLH, harbor either a bi-allelic or mono-allelic mutation in the perforin gene.³⁸ Soluble CD163 is a scavenger receptor exclusively expressed in the monocyte–macrophage system and increased shedding of its extracellular domain has been demonstrated with various inflammatory stimuli. Very high levels of soluble interleukin-2 receptor alpha (sIL-2R-alpha) are rarely seen in conditions other than HLH. Measurement of both sIL2R-alpha and sCD163 can aid in making a timely diagnosis of HLH, especially when the presentation is not classical. Hence, HLH should be borne in mind as a diagnostic possibility in sick, febrile patients with cytopenias, and organomegaly where commoner causes of anemia are unlikely.

Acquired thalassemias

Acquired hemoglobinopathies including acquired hemoglobin H disease and other α-thalassemias are characteristically seen in patients with hematological malignancies. They are usually associated with MDS or less commonly, with myeloproliferative neoplasms and erythroleukemia.^39–41 These relatively rare disorders may cause diagnostic dilemmas with the commoner inherited hemoglobin abnormalities.

Molecular pathogenesis for acquired α-thalassemia includes inactivating somatic mutations of trans-acting chromatin-associated factor on the X-chromosome (ATRX) that down-regulate α-globin gene expression and, less commonly, acquired deletion of the α-globin gene cluster on chromosome 16.^39–41

Effects of anemia and tissue hypoxia on tumor growth and sensitivity to therapy

Tumor hypoxia in anemic patients is well-known clinically to reduce the effectiveness of radiation therapy. It has also emerged, through experimental studies, as a major factor influencing malignant cell proliferation and tumor progression.⁴² Although hypoxia can also negatively impact tumor cell growth, in vivo it more often leads to hypoxia-driven responses that enhance malignant progression and aggressiveness, ultimately resulting in increased resistance to therapy and poorer long-term prognosis. Malignant progression associated with tumor hypoxia appears to be mediated by several mechanisms, including changes in gene expression, inactivation of tumor suppressor genes, activation of proto-oncogenes, genomic instability, and clonal selection.^43,44

These genetic changes further result in increased angiogenesis, cellular resistance to apoptosis, and to oxidative free radicals generated by chemotherapy and radiotherapy finally leading to enhanced tumor growth and escape from therapy. A meta-analysis of 19 clinical studies of cancer-related anemia and 8 studies on tumor hypoxemia in 2005 attempted to determine the effect of these variables on loco-regional control and survival in patients with cancer.⁴³ Despite varying definitions of anemia and hypoxemia, all studies indicated positive correlations between reduced hemoglobin and higher levels of tumor hypoxia with inferior prognosis. Attempted radiosensitization by improved tumor oxygenation or hypoxic cell sensitization through hyperbaric oxygen, electrophilic radiosensitizer pharmacological agents, and mitomycin had limited success.

Diagnostic approach to anemia in malignancy

Although heavily dependent on the clinical background and historical and examination findings, certain generic approaches can be defined for the anemic cancer patient. These are outlined in Fig. 1. Specific malignancies have differing incidences and features of anemia. Data on a few of the common ones are presented in Table 2.

Figure 1. An algorithmic approach to anemia in a cancer patient. [Note: The black boxes highlight key investigations often vital to the differential diagnosis.]

Table 2. Salient findings in anemia in common malignancies

Conclusions

1.	Anemia is a common manifestation of malignancies and is estimated to occur in up to 60% of cancer patients.
2.	Anemia in cancer is usually multifactorial, with anemia of inflammation/chronic disease being the commonest component. Other common causes include nutritional deficiencies, blood loss, and anemia as a consequence of anti-neoplastic therapy.
3.	Diagnostic approaches are similar to those in anemic non-cancer patients, albeit with special consideration to exclude causes peculiar to malignancy. A detailed history and examination along with the complete blood count including a reticulocyte count are of paramount importance.
4.	A pathogenetic approach to investigating the cause of anemia in a cancer patient is often clinically more rewarding than a pure blood film morphology-based scheme.
5.	Anemia in cancer patients affects the quality of life, survival, and possibly also the tumor response to chemo- and radiotherapy.

Disclaimer statements

Contributors PS conceived of the idea. BLG and PS performed the literature search. BLG wrote the first draft of the manuscript. PS and RD edited the manuscript for critical intellectual content. All authors read and approved the final submission. PS is guarantor for the manuscript.

Funding None.

Conflicts of interest None.

Ethics approval Ethical approval was not required for this review article which contains no identifying information from any specific patient.