Relation of Mean Platelet Volume (MPV) with Cancer: A Systematic Review with a Focus on Disease Outcome on Twelve Types of Cancer

Inflammatory proteins activate platelets, which have been observed to be directly related to cancer progression and development. The aim of this systematic review is to investigate the possible association between Mean Platelet Volume (MPV) and cancer (diagnostic capacity of MPV, relation to survival, the severity of the disease, and metastasis). A literature review was performed in the online database PubMed and Google Scholar for the period of 2010–2022. In total, 83 studies including 21,034 participants with 12 different types of cancer (i.e., gastric cancer, colon cancer, esophageal squamous cell carcinoma, renal cancer, breast cancer, ovarian cancer, endometrial cancer, thyroid cancer, lung cancer, bladder cancer, gallbladder cancer, and multiple myeloma) were identified. The role of MPV has been extensively investigated in several types of cancer, such as gastric, colon, breast, and lung cancer, while few data exist for other types, such as renal, gallbladder cancer, and multiple myeloma. Most studies in gastric, breast, endometrium, thyroid, and lung cancer documented an elevated MPV in cancer patients. Data were less clear-cut for esophageal, ovarian, and colon cancer, while reduced MPV was observed in renal cell carcinoma and gallbladder cancer. Several studies on colon cancer (4 out of 6) and fewer on lung cancer (4 out of 10) indicated an unfavorable role of increased MPV regarding mortality. As far as other cancer types are concerned, fewer studies were conducted. MPV can be used as a potential biomarker in cancer diagnosis and could be a useful tool for the optimization of treatment strategies. Possible underlying mechanisms between cancer and MPV are discussed. However, further studies are needed to elucidate the exact role of MPV in cancer progression and metastasis.


Introduction
Cancer is an emerging epidemic with economic, social, and psychosomatic effects [1]. It is a life-threatening disease characterized by abnormal cell growth and division [1]. Most cancers are curable when diagnosed early and treatment begins soon after diagnosis [1]. To develop appropriate treatment strategies against cancer, research into easily detectable and accessible biomarkers is required [2]. Platelet-based markers are potential candidates for cancer diagnosis and monitoring, given the emerging role of platelets in cancer biology [3,4].
Platelets are the smallest but highly active morphological components of blood [5]. They are produced by the megakaryocytes of the bone marrow and under normal conditions are 157.000-351.000 in women and 135.000-317.000 in men per microliter of blood [5]. The average lifespan of platelets is 5-9 days [5]. They play a major role in the coagulation process and also participate in fibrosis, normal hemostasis, and other pathophysiological processes [5]. Platelets accumulate at the site of damage, and changes in their morphology are observed upon their activation with inflammatory markers [6] and several agonists such as Platelet-Activating Factor (PAF) and Adenosine Diphosphate (ADP) in vitro and ex vivo [7,8]. The number of platelets is determined by the balance between the rate of production and consumption and genetic factors [5]. Platelets differ in functional activity and size, and the function of platelets is related to their size [5]. New and more active platelets are larger than old ones. In other words, larger platelets may be younger and more metabolically and enzymatically active than smaller ones, they aggregate more easily and could be more easily stimulated to release chemical mediators [9]. This suggests that platelet volume reflects platelet activation. Activated platelets play an important role during the formation and development of clots [10]. They are active in systemic inflammation and have a higher prothrombotic potential in health and disease [8,11].
Tumor cells secrete cytokines that contribute to a prothrombotic microenvironment, which includes platelet activation [4]. By secreting proinflammatory and growth factors, platelets play an important role in cancer progression and metastasis, since inflammation is a critical component of tumor progression [4]. Complicated interactions between platelets and cancer cells lead to tumor growth, neoangiogenesis, tumor cell dissemination, the release of adhesion molecules, and growth factors, all of which provide basic ingredients for tumor growth and metastasis [4].
Mean Platelet Volume (MPV) is one of the key platelet parameters, along with platelet count (PLT) and Platelet Distribution Width (PDW). MPV is a non-invasive, low-cost parameter, easily assessed and readily available in clinical practice, which shows the average size of platelets in the bloodstream and reflects their production rate and their degree of stimulation [10]. As a marker of platelet activation, MPV has attracted attention in recent decades, and many studies have evaluated its association with various malignancies [12][13][14][15][16][17][18][19][20][21][22]. The in-depth investigation of MPV alterations in cancer can reveal the potential usefulness of this index for cancer diagnosis, treatment response, and prognosis. This new perspective of a routine test may give additional information on the diagnosis and course of the disease, which is important given that several cancers may be asymptomatic until advanced stages [23].
The role of MPV in inflammatory diseases has been previously reviewed [10]. To our knowledge, there are few reviews on the relationship between MPV and cancer; including studies up to 2015, Ref. [24] is a recent review that has evaluated platelet indices with esophageal squamous cell carcinoma [25], and it includes a meta-analysis of 2421 patients, which has focused on the relation of MPV to survival in lung cancer patients [26].
Thus, the aim of this systematic review was to investigate the possible alterations of MPV in various types of cancer in relation to healthy subjects by extracting data from recent literature. In addition, the relation of MPV to disease outcome and the effectiveness of treatment was tested, and its prognostic value was assessed. Importantly, as a further goal, MPV could be used not only as a marker but also as a measure of intervention effectiveness in the disruption of tumor-platelet cross-talk.

Materials and Methods
A literature search has been made in the online databases PubMed and Google Scholar from 1 January 2010 to 31 December 2022. The review has been registered in the PROS-PERO database (the University of York, https://www.crd.york.ac.uk/PROSPERO/, ID CRD42023396237, accessed on 13 March 2023).
The search terms in PubMed were formulated with Boolean operators as follows: (Mean Platelet Volume OR MPV OR platelet activation) AND (cancer OR tumor OR tumor OR carcinoma OR myeloma OR neoplasm OR survivor) AND (diagnosis OR prognosis OR risk factor OR survival OR metastasis OR diagnostic indicator OR prognostic factor OR biomarker OR chemotherapy OR immunotherapy OR radiotherapy OR surgery). We also searched the references of relative reviews for additional articles.
The research question was formulated as follows: population (P), intervention (I), comparison (C), and outcome (O). Table 1 describes in detail the research question.

Inclusion and Exclusion Criteria
The study selection criteria were (1) published studies within the last 12 years (publication period between 2010 and December 2022) (2) published studies in English language (3) measurement of MPV and assessment of its correlation with disease parameters or survival (4) cancer types included the following: gastric cancer, colon/colorectal cancer, esophageal squamous cell carcinoma, breast cancer, ovarian cancer, endometrial cancer, renal cancer, thyroid cancer, lung cancer, multiple myeloma, bladder cancer, and gallbladder cancer.
Exclusion criteria were as follows: (1) studies performed on animals, (2) case reports, (3) other types of cancers not included in the inclusion criteria, (4) studies including only patients with metastatic cancer, (5) studies including only cancer survivors, (6) studies with mixed types of cancer in the study design, (7) studies assessing ratios or algorithms including MPV and not MPV alone, (8) studies assessing changes in MPV after medical treatment, (9) studies including patients with cancer and comorbidities (such as diabetes) in inclusion criteria, and (10) studies in children and/or adolescents.

Quality Assessment
The quality assessment of the studies is presented in Supplementary Table S1. The procedure was performed by two independent researchers (I.P. and S.P.) using the New Castle Ottawa pros scale (NOS) [27,28] for both cohort and case-control studies and the AXIS tool for cross-sectional studies [29] (Supplementary Table S2). Disagreements were discussed with a third researcher (P.D.).

Data Extraction
Four independent researchers (M.M., P.M., I.P., and S.P.) extracted the data in predefined excel spreadsheets with separate questions regarding MPV alterations and their relation to survival. Possible disagreements were discussed with an additional researcher (P.D.). The following items were extracted from each study: study details (first author, year of publication, design), sample details (number of participants, type of cancer, characteristics of the control group), results (increased or decreased levels in patients versus controls, outcomes about the prognostic ability of MPV, and associations with disease), and reported study limitations.

Results
The results from the first database (PubMed) revealed 3800 articles and from the second (Google Scholar) 17,500 of the most relevant articles were read. In total, 83 studies including 21,034 participants with 12 different types of cancer, i.e., gastric cancer, colon cancer, esophageal squamous cell carcinoma, renal cancer, breast cancer, ovarian cancer, endometrial cancer, thyroid cancer, lung cancer, bladder cancer, gallbladder cancer, and multiple myeloma were identified. The flowchart according to PRISMA guidelines [30] is given in Figure 1 and the corresponding checklist is provided as a supplementary file (Supplementary Table S1) [30]. The studies are presented in detail in Tables 2-7.

Data Extraction
Four independent researchers (M.M., P.M., I.P., and S.P.) extracted the data in predefined excel spreadsheets with separate questions regarding MPV alterations and their relation to survival. Possible disagreements were discussed with an additional researcher (P.D.). The following items were extracted from each study: study details (first author, year of publication, design), sample details (number of participants, type of cancer, characteristics of the control group), results (increased or decreased levels in patients versus controls, outcomes about the prognostic ability of MPV, and associations with disease), and reported study limitations.

Results
The results from the first database (PubMed) revealed 3800 articles and from the second (Google Scholar) 17,500 of the most relevant articles were read. In total, 83 studies including 21,034 participants with 12 different types of cancer, i.e., gastric cancer, colon cancer, esophageal squamous cell carcinoma, renal cancer, breast cancer, ovarian cancer, endometrial cancer, thyroid cancer, lung cancer, bladder cancer, gallbladder cancer, and multiple myeloma were identified. The flowchart according to PRISMA guidelines [30] is given in Figure 1 and the corresponding checklist is provided as a supplementary file (Supplementary Table S1) [30]. The studies are presented in detail in Tables 2-7.

Renal Cell Carcinoma
Regarding renal cell carcinoma, three studies were found including 681 patients from China and Poland [18,51,52] (Table 3). Patients with renal cell carcinoma had significantly reduced levels of MPV compared to patients with benign renal tumors and healthy controls [18]. Furthermore, surgical resection of the tumor led to a significant increase in MPV levels [18], and patients with low MPV had a significantly shorter survival time and worse prognosis than patients with high MPV levels [51,52].

Ovarian Cancer
Six studies relating MPV to ovarian cancer were identified with 906 patients [60][61][62][63][64][65] ( Table 4). Four studies were conducted in Turkey [61,[63][64][65] and two studies in China [60,62]. The results were mixed since some studies documented increased levels of MPV in patients with ovarian cancer [61,65], whereas others documented decreased levels [60] or levels that were no different [62,64] from the control groups. Moreover, in the study of Kokcu, no relation was found between MPV and cancer staging [63]. It is noted that no study assessed the relation of MPV to survival in patients with ovarian cancer.

Endometrial Cancer
Eight studies relating MPV to endometrial cancer were identified, including 1707 patients from China and Turkey [66][67][68][69][70][71][72][73] (Table 4). In all studies, including a control group, MPV was increased in cancer versus healthy patients [66,67,[69][70][71]73]. The relation of MPV with cancer staging was less clear, as in two studies, no relation was found [68,71] and in one study, MPV was negatively related to the cancer stage [67]. In two studies, MPV was tested against the overall survival with mixed results. In fact, MPV was either not related to survival [68] or was related to shorter survival (increased MPV related to lower survival) [72].

Bladder Cancer
Five studies relating MPV to bladder cancer were identified with 879 participants from China [97][98][99] and Turkey [100,101] (Table 7). The results were mixed. One study documented increased levels of MPV in bladder cancer patients compared to controls [99]. Regarding recurrence risk, one study found that increased MPV was related to recurrence risk [98], while two studies found a non-significant association [100,101]. One study assessed the relation of MPV to mortality and found that lower MCV was related to lower survival [97].

Gallbladder Cancer
Three studies assessed the relation of MPV to gallbladder cancer, including 473 patients from China [22], Turkey [102], and India [103] (Table 7). Gallbladder cancer patients (after surgery) had lower levels of MPV than controls [22,102]. Moreover, there was no correlation between MPV and the local dissemination and prognosis of gallbladder cancer [103].

Multiple Myeloma
Only one study, including 62 Chinese patients with newly diagnosed multiple myeloma, was identified [104] (Table 7). This study showed significantly decreased survival in patients with low MPV, compared to patients with high MPV [104].   • No data on inflammatory markers.   • Retrospective design of the study.    • ↑ MPV levels were found to be related to active inflammation, whereas ↓ MPV levels were found to be related to several chronic diseases. • ↓ MPV in cancer and dysplasia groups.
• Lacking data on the use of medication including preoperative use of anti-inflammatory therapy and the medical conditions of participants.

Alterations of MPV Values in Patients with Cancer and Relation to Survival
An increase in MPV is observed in many neoplastic diseases, although in some cancers, a decrease can be found. More particularly, most studies in gastric (3 studies out of 4), breast (3 studies out of 3), endometrium (6 studies out of 6), thyroid (4 studies out of 6), and lung cancer (4 studies out of 4) documented an elevated MPV in cancer patients. Data regarding an increased MPV in cancer patients were less clear-cut for esophageal cancer (2 studies out of 3), ovarian cancer (2 studies out of 5), and colon cancer (1 study out of 2). In contrast, reduced MPV was observed in renal cell carcinoma (1 study out of 1) and gallbladder cancer (2 studies out of 2), although the number of studies was small (see the "Results" section). These conflicting results denote that specific organs and different types of tumors or stages of cancer may affect MPV differently or that more data are needed to obtain a clearer image.
For the case of gallbladder cancer, the observed reductions in MPV may be a result of surgery, since participants had undergone surgery at the time of measurement [22,102]. In fact, surgery seems to reduce MPV in several cancer types, such as thyroid cancer [14], ovarian cancer [61], and gastric cancer [3,34], although some studies have shown no changes [32]. This postoperative decrease in MPV values may be due to a reduced systemic inflammatory response after the reduction of tumor volume. In addition, anesthetics used in surgery may have anti-inflammatory effects [105]. For example, propofol inhibits cyclooxygenase; thus, it restricts angiogenesis, which may have an effect on MPV [106]. It is noted that other therapies, such as chemotherapy, may also reduce MPV, reflecting a reduction in inflammatory burdens [56].
An increase in MPV was associated with reduced survival in most but not all studies. More particularly, most studies in colon cancer (4 out of 6) and fewer in lung cancer (4 out of 10) indicated an unfavorable role of increased MPV regarding mortality. It is noted that MPV was not related to overall survival in a recent meta-analysis of lung cancer patients [26]. As far as other cancer types are concerned, fewer studies were conducted. The available data suggest that high MPV is related to better survival in renal cancer (two out of two studies), bladder cancer (one out of one study), and multiple myeloma (one out of one study) (see the "Results" section).
The relation between cancer and MPV is bidirectional and is analyzed below (Figure 2). The relation between cancer and MPV is bidirectional and is analyzed below ( Figure  2).

Cancer-Related Inflammation, Platelets, and MPV
Any deviations in the platelets number, total mass, morphology, and function depend on the factors that directly affect the majority of megakaryocytes, the maturity of progenitor cells, and the activation and "use" of platelets during coagulation and inflammatory processes [10]. The course of an inflammatory state is associated with an increased percentage of large platelets, possibly due to intracellular synthesis of procoagulant and proinflammatory factors, degranulation of granules, and initiation of transmigration of platelets stored in the spleen [10]. At the same time, these cells rapidly move to the site of inflammation, where they undergo activation [10].
Thus, MPV alterations in cancer patients reflect cancer-related inflammation [10]. The proinflammatory cytokines released in cancer, such as interleukins IL-1, IL-3, and IL-6, can promote the proliferation of megakaryocytes and increase the presence of large platelets, causing their activation and aggregation and possibly leading to the gradual establishment of thrombocytosis [35]. In parallel, higher IL-6 levels are associated with increasing tumor stages, tumor sizes, metastasis, and reduced cancer survival [10,39].
Moreover, for stomach cancer, a chronic inflammation caused by Helicobacter pylori may be present, which is often leading to neoplastic transformation [3,13]. On the contrary, in some cancers, for example, renal cancer, a decrease in MPV was observed [18]. In this case, it can be hypothesized that the inflammatory state accompanying carcinoma may lead to excessive "usage" of platelets and consequently a decrease in MPV, which is reversed upon anti-inflammatory treatment [10,18,51].

MPV, Activated Platelets, Cancer Progression, and Metastasis
Platelets have an important metabolic role in cancer pathogenesis through their angiogenic, metastatic, and proteolytic activities in the context of inflammation [4,10]. Activated platelets facilitate cancer progression and tumor growth by promoting angiogenesis

Cancer-Related Inflammation, Platelets, and MPV
Any deviations in the platelets number, total mass, morphology, and function depend on the factors that directly affect the majority of megakaryocytes, the maturity of progenitor cells, and the activation and "use" of platelets during coagulation and inflammatory processes [10]. The course of an inflammatory state is associated with an increased percentage of large platelets, possibly due to intracellular synthesis of procoagulant and proinflammatory factors, degranulation of granules, and initiation of transmigration of platelets stored in the spleen [10]. At the same time, these cells rapidly move to the site of inflammation, where they undergo activation [10].
Thus, MPV alterations in cancer patients reflect cancer-related inflammation [10]. The proinflammatory cytokines released in cancer, such as interleukins IL-1, IL-3, and IL-6, can promote the proliferation of megakaryocytes and increase the presence of large platelets, causing their activation and aggregation and possibly leading to the gradual establishment of thrombocytosis [35]. In parallel, higher IL-6 levels are associated with increasing tumor stages, tumor sizes, metastasis, and reduced cancer survival [10,39].
Moreover, for stomach cancer, a chronic inflammation caused by Helicobacter pylori may be present, which is often leading to neoplastic transformation [3,13]. On the contrary, in some cancers, for example, renal cancer, a decrease in MPV was observed [18]. In this case, it can be hypothesized that the inflammatory state accompanying carcinoma may lead to excessive "usage" of platelets and consequently a decrease in MPV, which is reversed upon anti-inflammatory treatment [10,18,51].

MPV, Activated Platelets, Cancer Progression, and Metastasis
Platelets have an important metabolic role in cancer pathogenesis through their angiogenic, metastatic, and proteolytic activities in the context of inflammation [4,10]. Activated platelets facilitate cancer progression and tumor growth by promoting angiogenesis and tumor cell generation at distant sites through the secretion of angiogenic growth factors, such as the vascular endothelial growth factor (VEGF) [4,10,107]. Indeed, the platelet content of VEGF is significantly increased in cancer patients [108].
Thrombosis is one of the common causes of mortality in cancer patients and the clotting process is enhanced by activated platelets through their procoagulant surface [109]. Multifactorial complex interactions between platelets, endothelial cells, and leukocytes further stimulate the production of proinflammatory cytokines and lead to thrombosis [109]. Recent studies have provided ample evidence for the multifunctional nature of platelets, which are the first to accumulate at the site of injury, changing shape and exhibiting pseudopodia and local release of cytoplasmic granular contents [109]. When activated by classical agonists such as ADP, TXA2, PAF, inflammatory cytokines (e.g., IL-1, IL-6), tumor necrosis factor alpha (TNF alpha), and other growth, hemostatic factors and the adhesion molecules they aggregate [7,8] stimulate platelet production and lead to a hypercoagulable state and thrombogenesis [4,10].
Platelets also play an important role in cancer progression and metastasis [110]. Elevated thrombocytosis and platelet count are associated with advanced, often metastatic, stages of cancer as also supported in our results for colon and breast cancer [40,54,55,58], but not for thyroid cancer [80]. Activated platelets create a procoagulant microenvironment that allows cancer cells to become coated with platelets and evade the host's immune system. Encrusted with platelets, circulating cancer cells can more easily transport themselves into the bloodstream and cope with physical factors, such as shear stress when passing through the microvascular system [110]. Further in vivo studies have shown that platelets in cancer patients may mask cancer cells, making them unrecognizable by immune system cells, which facilitates metastasis [110].
Regarding the relationship between MPV and tumor aggressiveness, it is important to mention that the increased reactivity of larger platelets is due, among other things, to increased expression of integrin αIIbβ3 and glycoprotein (GP) Ibα [111]. αIIbβ3 may participate in platelet-tumor cell interaction in tumor metastasis through the binding of metalloproteinase domain-containing protein 9 (ADAM-9). This interaction could form a physical shield around cancer cells protecting them from natural killer (NK) cell lysis [112]. Additionally, it has been observed that activated platelets facilitate tumor cell adhesion to endothelial cells through αIIbβ3-associated mechanisms [113]. Moreover, activated platelets can facilitate tumor cell extravasation after the extracellular matrix degradation by matrix metalloproteinase-2 (MMP-2) [114]. Regarding the GPIba, GPIbα-mediated platelet adhesion to angiogenic vessels enhances angiogenesis and prevents hemorrhage from newly formed vessels contributing to cancer development and aggressiveness [115].

Other Factors Affecting MPV
Preanalytical factors may interfere in MPV determination, such as the anticoagulant used (ethylenediamine tetraacetic acid-EDTA or citrate with EDTA changing platelet shape), sample temperature (with high temperature leading to increases in MPV), and the interval between blood taking and testing [10,116].
Another issue that needs to be considered is the definition of a normal MPV range in healthy subjects, as the available literature presents different cut-off points depending on the method and the hematological analyzer used [117]. This clearly shows the need to establish reference values for MPV by laboratories and possibly a reference range of values in relation to gender, age, or ethnicity. Finally, the baseline values and differences of MPV in benign and malignant tumors, which have not yet been elucidated, also need to be determined [10,24].
Some researchers indicate that MPV should always be evaluated together with platelet count, as there is a non-linear inverse relationship between these blood indices [10]. It has been documented this ratio can have a high prognostic value in cancer patients [118]. To date, the effect of platelet count on MPV has not been fully understood in malignant tumors and the present review excluded studies using ratios of blood indices while not reporting results for MPV alone.
The value of MPV and other platelet indices can be affected by many factors, such as age [119], gender [120], and genetic factors [121]. Age was positively related to MPV in thyroid cancer patients [79]. In addition, age has been associated with tumor progression and recurrence risk and may modify the relation of blood indices (such as the neutrophilto-lymphocyte ratio) to the disease [100]. Lifestyle (including diet and weight status) and genetic factors may also affect MPV [122,123]. So lifestyle and its improvement may play a role in reducing platelet activation and may be an aspect of treatment in some patients [10]. Moreover, cancer treatment, such as hormone therapy [124], and Radioiodine Therapy [125] may alter MPV values. Concerning antithrombotic drugs, it has been shown that aspirin does not affect MPV, but no data are available on the possible effect of other antiplatelet drugs on MPV values [10].
Various conditions and diseases, such as diabetes mellitus, hypertension, hypercholesterolemia, smoking, and obesity, show higher levels of MPV [126,127]. Smoking also seems to increase MPV in cancer patients [92]. In addition, a high level of MPV may also be associated with various malignant tumors, as already mentioned [24]. It is noted that in the studies analyzed in this review, patients with comorbidities were excluded, which is better for the interpretation of the results.

Limitations
Most included studies were retrospective and had a small sample size. The majority of studies were conducted in China and Turkey, which may limit the generalization of the observed findings. In the methodology of the present review, we included most types of cancer but not all, due to large data management. We also excluded the articles which used only combinations of biochemical indices to predict overall survival, since the aim of the present review was to clarify the role of MPV in cancer. However, in real life, it is possible that combinations of indices and resulting algorithms could predict an outcome better. For example, cardiovascular risk is better predicted through several algorithms combining age, sex, smoking, lipid, and blood pressure measurements [128].
In our work, we did not provide quantitative information on MPV levels in the different studies. Different cutoffs were used by researchers and thus it is difficult to define an optimal "prognostic" value of MPV within the normal range. Yet, there are no standardized reference values [116]. We documented several alterations in MPV values in cancer patients compared to a "control" group. It is noted that in some studies, the "control" group included adults with benign tumors and not healthy adults [16,78], which may have affected the magnitude and/or orientation of comparisons.
In treated patients with cancer, the timing of MPV measurement in relation to treatment (such as surgery or chemotherapy) may be also important, which is not always available. Moreover, several variables that influence MPV, such as genetic polymorphisms, body composition, and diet, were not assessed, and these may affect both cancer prognosis [129] and platelet secretory molecules [130][131][132]. In addition, usual treatments may also induce changes in biological parameters [133]. Last but not least, cancer patients often take vitamins and oral nutritional supplements to cover their nutritional needs, which most often contain fat to increase caloric supply [134]. Interestingly, MPV has been correlated with platelet unsaturated phospholipids [135].

Conclusions
Over the last few years, several studies reviewed in the present work have investigated the association between MPV and cancer in terms of diagnosis and prognosis. MPV can be used as a potential biomarker in cancer diagnosis and could be a useful tool for the optimization of treatment strategies. However, further studies are needed to elucidate the exact role of MPV in cancer progression and responsible underlying mechanisms.