Thrombocytosis as a Prognostic Marker of Survival in Patients with Head and Neck Tumors

The correlation between cancer-related thrombocytosis and worse survival has been described with a variety of solid neoplasms...


Introduction
Each year, 500,000 patients are diagnosed with head and neck tumors worldwide [1]. This condition, which is the 8th most common cause of death, contributes 6% of all malignancies [2]. Head and Neck Squamous Cell Carcinomas (HNSCC) require individualized therapy with three components-that is, surgery, irradiation, and chemotherapy. A prognostic marker for predicting therapeutic response or survival would be of great help in developing an appropriate treatment strategy. Possible candidates include conventional haematological indices such as Complete Blood Count (CBC) for example [3]. Haemoglobin levels have been found predictive of the primary response to irradiation, of the chance of local recurrence, as well as of overall (OS) [4], and of Disease-Free Survival (DFS) [5]. Recently, an association between elevated platelet count and earlier metastasis has been observed in a variety of solid neoplasms, including colorectal [6][7][8][9][10], gastric [11,12], cervical [13], ovarian [14][15][16], pancreatic [17,18], lung [19][20][21], and breast [22] cancers. The exact pathomechanism behind the relationship between thrombocytosis and neoplasia is yet unknown to date. The most popular hypothesis postulates the presence of a paraneoplastic pathway in the background. The authors of this hypothesis found elevated serum Interleukin-6 (IL-6) levels in ovarian cancer. By enhancing the release of thrombopoietin from the liver, IL-6 enhances megakaryocytosis and this leads to thrombocytosis [23]. We studied the prognostic significance of platelet counts determined before initiating treatment in patients with oral, pharyngeal, or laryngeal malignancies in order to ascertain whether thrombocytosis is associated with a worse prognosis also in these tumors.

Materials and Methods
The study conforms with The Code of Ethics of the World Medical Association (Declaration of Helsinki), printed in the British Medical Journal (18 July 1964 When describing the characteristics of the study population, we calculated means and Standard Deviation (SD) for normally distributed data, and medians with Interquartile Range (IQR) for data with non-normal distribution. We applied cross-tabulation and non-parametric tests (Wilcoxon rank-sum test, Kruskall-Wallis test) when comparing the characteristics of subgroups within the sample. We analysed survival data using the Kaplan-Meier method, and compared the survival estimates of the subgroups with high or low platelet counts using the log-rank test. We defined 300 G/L as the cut-off value for high platelet count. When determining the optimal cutoff value, we followed the following process: we run logistic regression models with 3-year survival status as the dependent variable, and cut-off-values from the entire range of platelet-count rounded to 10. We calculated the area under ROC curve for each cut-off-value and selected the optimal cut-off based on the largest area under ROC. With this method, we selected 300 G/l as the optimal cut-off-value, a level somewhat lower than the clinical threshold of 400 G/l for thrombocytosis. We analysed 5-year overall survival and disease-free survival using multivariable Cox-proportional hazard-regression models, treating platelets as continuous, as well as discrete categorical variables. We tested the effect of multiple-category covariates using the Wald-test. We tested the proportional hazard assumption for the Cox-regression models according to the method proposed by Grambsch and Terneau [24] and tested goodness of fit on 4 quantiles of risk as proposed by May and Hosmer [4]. All statistical analyses were performed with the Stata 14.2 statistical software package [25].

Results
Platelet-count and overall survival data were available for 312, whereas information for calculating disease-free survival was known for 299 patients (95.8%). Mean age (SD) was 57 years  Table 1. We found significant differences between various locations in terms of age, gender, primary tumor and lymph node status, the occurrence of thrombocytosis, and 5-year survival rate. The    Figure 3.   Note: Vertical ticks on survival curves indicate censored cases.

Discussion
The relationship between thrombocytosis and a worse survival has been observed in several types of solid tumors [26,27]. The underlying pathomechanism is still unknown yet, but several hypotheses have been proposed, based on the following observations. On one hand, platelets can enhance tumor growth and stimulate angiogenesis by secreting proangiogenic cytokines [28]. On the other hand, platelets may be involved in the formation of metastases by cloaking tumor cells and thereby protecting them both from mechanical damage [29,30], and from the immune defences of the body [31,32]. By expressing immunoregulatory proteins on their surface, the platelets attached to the tumor cells can defend the latter against Natural Killer (NK) cells [33,34].

Moreover, platelets also express Major Histocompatibility Complex
Class I (MHC-I) in abundant quantities. That is, adherent platelets confer a false phenotype to the tumor cells, and thereby interfere with the recognition of the malignant cells by the immune system [33]. This process is based on a paraneoplastic pathway, the starting point of which is the elevation of serum IL-6 level [23]. Because the pathomechanism has not yet been fully clarified, the possibility of reactive thrombocytosis cannot be excluded either. In order to eliminate this option, we excluded patients with leukocytosis or reduced CBC from our study. We confirmed the known correlation of reduced CBC and survival in patients with HNSCC. In general, a certain degree of anaemia develops before treatment in 30 to 50 per cent of cancer patients, and this proportion may increase to 50-70% during anti-tumor therapy. Anaemia impairs the patients' quality of life on one hand, and reduces therapeutic efficacy on the other -thus, it may lessen the chances for cure and survival [35,36].
However, it is still controversial whether anaemia might be the underlying cause of thrombocytosis and poor survival in cancer patients. According to CHEN et al. [37], anaemia, monocytosis, and thrombocytosis were independent risk factors in their study population of patients with head and neck tumors -our study also confirmed this finding. Using 300-G/L platelet-count as the cut-offvalue, higher red blood cell count was associated with a reduced 5-year mortality risk. Besides this, multivariable Cox-proportional hazards regression analysis, after adjustment for the patients' red blood cell counts, showed that platelet-count was a significant to draw an overall conclusion [46]. Remarkably, RACHIDI et al. [41], as well as CHEN et al. [37] found a correlation between both lower and higher platelet counts and poor survival. The head and neck region is regarded as an integrated system (consisting of the nasal cavity, the paranasal sinuses, the oral cavity, the salivary glands, the pharnyx and the larynx) and the predominant malignancy in this region is squamous cell carcinoma (HNSCC).
Notwithstanding this, the behaviour of tumor-associated thrombocytosis appears to be different in specific parts of this organ system. According to some observations, platelet counts are lower in patients with tumors of the oral cavity, or of the larynx (sub/supraglottic or vocal cord lesions) than in those with meso-or hypopharyngeal tumors. In the latter (hypopharyngeal) location, the correlation between platelet count and diseasespecific survival has been found to be particularly strong [38]. By contrast, other authors reported worse survival in patients with oral [39], or nasopharyngeal tumors [40]. Our study also found different platelet counts and 5-year survival rates depending on the anatomical location of the tumor. However, the multivariate Cox regression analysis did not confirm a significant relationship between anatomical location and survival. In view of the small number of cases, we did not perform a location-specific analysis of the relationship between thrombocytosis and survival. Male gender was associated with an increased 5-year mortality, though the association was not significant. LIN et al found similar difference in prognosis between males and females. High platelet count was associated with decreased OS in males but not in females.
They performed gene expression analyses as well, which showed that females have higher immune cell infiltration in the tumor microenvironment [47]. Other studies have suggested greater activity in male platelets, which may lead to worse cancer prognosis [48][49][50]. In summary of our findings, we can conclude that -in a patient cohort with tumors with a variety of possible anatomical locations, stages, and treatment modalities -we found significant correlation between platelet count and worse survival in patients with head and neck tumors. However, the variable, anatomical location-dependent influence of platelet count may be a possible clue worth for further investigation, as this might bring us closer to understanding the relationship of thrombocytosis, solid tumors, and poor survival.