Malignant pleural effusion: from bench to bedside

Pleural localisation and propagation of malignant disease

Autopsy studies indicate that tumour cells metastasise to the pleura mainly through the blood-stream and initially invade the visceral pleura [11, 12]. Most lung carcinomas translocate to the ipsilateral visceral pleura via the pulmonary vessels [11]. Thereafter, secondary dissemination to the parietal pleural occurs by tumour seeding along adhesions or by exfoliated tumour cells floating in the effusion. The pleura may also be invaded through lymphangitic spread or even through direct extension of tumours infiltrating adjacent structures (i.e. lung, chest wall, mediastinum or diaphragm).

For the establishment of viable tumour foci on pleural surface, cancer cells contained in the fluid need to adhere to the mesothelium, escape local anti-tumour immune responses, invade the pleural tissue and obtain access to nutrients and growth stimuli. Tumour-mesothelial cell interactions and cancerous invasion of the pleura implicate adhesion molecules and proteolyic enzymes, although these processes are poorly investigated [13]. Mesothelial cells have been found to limit tumour cell proliferation, whereas the impact of cancerous cells on mesothelial cell growth is controversial. Co-culture studies demonstrated that they either stimulate proliferation or induce apoptosis of mesothelial cells, while in vivo observations suggest that mesothelial cells in MPEs had lower proliferation compared with those in benign effusions [14–16]. Cytokines that possess immune-inhibitory properties and are abundant in the malignancy-affected pleural cavity, as well as local accumulation of immunosuppressive and protumour CD4⁺ lymphocytes, contribute to immune evasion and facilitate tumour growth [17–21]. Relative to this, MPE is characterised by defective recruitment, activation and the cytotoxic potential of CD8⁺ cells [22, 23]. Macrophages from malignant effusions not only display reduced cytotoxic activity against autologous tumour cells but they also inhibit tumour cell apoptosis [24, 25].

After their detachment from a pleural tumour, cancer cells are deprived of the nutrients provided by the tumour vasculature and of the supportive cell–cell and cell–matrix interactions, thus becoming amenable to apoptosis [26]. However, floating tumour cells not only survive, but are still capable of forming secondary foci in other sites of the pleural cavity denoting that they either dramatically change their metabolic needs or use alternative sources of energy and growth factors. It is therefore possible, though still speculative, that pleural fluid that contains both nutrients and mitogenic/survival stimulators might support cancer cells from the time they exfoliate from a pleural tumour mass until they form secondary implantations that invade the subpleural tissues and obtain access to the host vasculature.

Formation of MPE: general aspects

Pleural fluid accumulates when its production overwhelms removal. The reason why some tumours cause effusions while others do not is unclear. Post mortem studies demonstrate that mediastinal lymph node invasion, but not the extent of pleural involvement, predicts the presence of an effusion [11]. In addition, it has long been believed that pleural fluid clearance occurs through the lymphatics that originate from the stomata of the parietal pleura and drains through the mediastinal nodes. Based on the above, it was initially assumed that impaired pleural fluid drainage, secondary to tumour invasion of the drainage system, is the primary mechanism of MPE formation. However, this notion was challenged by the following: 1) the rate of MPE accumulation is commonly higher than that expected if it is merely due to clearance of the fluid occurring secondary to lymphatic blockage; 2) in the majority of MPE, the protein content is higher than that in normal pleural fluid, suggesting the presence of plasma leakage; and 3) MPE occurs even in patients without parietal pleura involvement. It is therefore currently believed that a combination of increased fluid production due to fluid extravasation from hyper-permeable parietal or visceral pleural and/or tumour vessels and impaired lymphatic outflow underlie the development of MPE [7].

Cell and molecular biology of MPE

Since pleural fluid overproduction has been recognised as a key event in MPE formation, considerable effort has been invested in unravelling the molecular mechanisms of pleural tumour-driven fluid extravasation. Evidence from in vivo studies indicates that MPE formation is dictated by a complex tumour–host interplay which through paracrine and autocrine effects stimulates pleural inflammation, tumour angiogenesis and vascular hyperpermeability [7]. To trigger these events, tumour cells execute pro-inflammatory and pro-angiogenic transcriptional programmes which are controlled, at least in part, by transcription factors nuclear factor (NF)-κB [27] and signal transducer and activator of transcription (STAT) 3 [28, 29]. Autocrine tumour necrosis factor (TNF), interleukin (IL)-6 and osteopontin (OPN) participate in positive feedback loops regulating tumour NF-κB/STAT3 activation to promote MPE [29–31]. In addition, tumour-derived mediators, including vascular endothelial growth factor (VEGF), C-C-motif chemokine ligand (CCL) 2 and TNF, directly stimulate inflammatory cell influx and/or vascular changes [31–34]. Finally, host mediators perpetuate this malicious interplay by amplifying inflammatory and angiogenic signalling loops. To this end, host IL-5 promotes MPE by facilitating the pleural influx of eosinophils and tumour-promoting myeloid suppressor cells [35]. In addition, tumour-expressed OPN protects tumour cells from pro-apoptotic stimuli and host-secreted OPN promotes pleural inflammation and angiogenesis, while the cytokine from both origins provokes vascular leakage [30]. Recently, it has been proved that mast cells are required for MPE formation and by releasing tryptase AB1 and IL-1β they induce pleural vasculature leakiness and trigger NF-κB activation thereby fostering fluid accumation and tumour growth [34]. This novel concept on MPE pathogenesis was exploited in preclinical studies that demonstrated that a variety of pharmacological agents including VEGF, sulindac derivative, TNF and angiopoietin inhibitors, zolendronic acid, a dual VEGF receptor/sTie2 antagonist, bortezomib and endostatin curtail experimental MPE, and may now warrant clinical testing [33, 36–40].

Imaging

The role of imaging techniques is firmly established in the diagnostic workup of patients with suspected MPE. Nowadays, TUS is routinely used by respiratory physicians mainly for the guidance of pleural interventions to minimise complications [41]. The utility of the technique is strongly recommended by national and international guidelines [42]. Evidence also shows that TUS could provide important information on the diagnostic pathway of pleural effusion. Pleural or diaphragmatic thickening and nodularity on TUS are highly specific for malignancy and may therefore help to expedite timely investigation in those with these high-risk features [8, 43].

Contrast-enhanced thoracic computed tomography is the current gold-standard imaging modality for the pleura. It can also provide useful information on the pleural cavity as a whole as well as on the primary tumour site and stage. Leung et al. [44] suggested that the presence of nodular thickening, mediastinal, parietal and circumferential pleural thickening to have high specificity (88–94%) but low sensitivity (36–51%) in identifying malignancy. However, this modality is not perfect and data from a recent retrospective series suggest that approximately one in three patients with pleural malignancy may not demonstrate clear features of cancer on computed tomography [45]. Therefore, invasive investigation and careful follow-up are often warranted even if specific malignant characteristics are not identified in initial imaging studies. The role of positron emission tomography using 18-fluorodeoxyglucose for diagnosing MPE is still not well established. According to a recent meta-analysis of 14 studies including a total of 639 patients, the moderate accuracy of the technique precludes its everyday use to differentiate malignant from benign pleural effusions (table 1) [46].

Cytology–pathology

The methods used to definitively diagnose MPE have also evolved in recent years. Cytology is a well-established initial test with a mean sensitivity of 60%, but this depends on the underlying primary tumour, sample preparation and experience of the cytologist [47]. The diagnostic yield of pleural fluid cytology for mesothelioma is even lower and most international guidelines advocate the use of pleural biopsy as a preferred diagnostic method over fluid cytology, though the latter is sometimes sufficient in some experienced laboratories [48–51]. In conjunction with the above limitations of cytology, the expanding use of cancer therapies targeted to tissue-specific gene expression or receptor status which necessitates the acquisition of ample tumour tissue for immunohistochemistry and/or genotyping, has led to an increasing requirement for adequately-sized pleural biopsies [52].

Pleural biopsies can be obtained by different techniques: blind, radiologically-guided or through direct visualisation of the pleura. Based on the results of a randomised trial, blind pleural biopsies are no longer advocated given their lower diagnostic yield [53]. Over the recent years, medical thoracoscopy has been increasingly used by respiratory physicians for the investigation of suspected malignant pleural effusions with negative fluid cytology. Medical thoracoscopy has the advantage of a high diagnostic yield and is well established as a key diagnostic test in the investigation of an exudative pleural effusion of unknown cause. The technique can be performed under local anaesthesia with either rigid or semi-rigid scopes and yields a sensitivity of more than 90% for MPE [54]. Medical thoracoscopy has a favourable safety profile and is considered an overall safe procedure. The BTS reported 16 cases of death in 4736 cases across 47 studies, a mortality rate of 0.3% [1]. Another advance in pleural sampling is the development of physician-based ultrasound-guided pleural biopsies. Evidence suggests that on frail patients with tethered lungs who may be unfit for medical thoracoscopy, ultrasound-guided pleural biopsy is a valid alternative with a diagnostic yield of 94% [55].

Biomarkers

During the previous decade, research has mainly focused on pleural fluid proteins as biomarkers in order to obtain information on patients' diagnosis, prognosis or treatment outcomes. More recently, the development of high-throughput techniques has boosted the characterisation of nucleic acids in MPE. A plethora of different biomarkers have been measured and these include proteins expressed by cancer cells (e.g. mesothelin, CEA, CA15-3, CA125, CYFRA 21-1), surface receptors on immune cells (e.g. CD163⁺ on macrophages), extracellular matrix proteins (e.g. OPN, fibulin-3) and RNA/DNA levels and sequence [56–60]. Although the majority of nucleic acid biomarkers have not yet gained acceptance in clinical practice, analysis of epidermal growth factor receptor (EGFR), EML4-ALK and KRAS mutations have a valid role for the targeted treatment of patients with lung cancer. Interestingly, evidence indicates that EGFR mutations are abundant in patients with MPE compared to primary tumour samples [61].

The clinical implication of diagnostic biomarkers is limited due to the inadequacy of the validation of the results by subsequent studies. Mesothelin, first published as a useful biomarker in 2003, has however since been confirmed as a useful biomarker in serum in at multiple independent studies with very similar sensitivity and specificity levels, and its elevation in pleural fluid is also a useful guide to the presence of malignancy, including in initially cytology negative effusions [42, 62, 63].

There have been many, often contradictory, biomarker studies with varying specificity and sensitivity, mainly related to the differentiation of malignant from benign effusions [64]. The different results may be ascribed to variations in study populations, measurement techniques (i.e. different ELISA kits for the same cytokine), sample preparation (i.e. measurements in pleural fluid serum versus plasma), failure to include enough control samples in the initial studies or methodological inconsistencies that introduce systematic errors [65]. Promising biomarkers are emerging, but further research with larger studies and prospective validation is required before clinical application. Currently, there are at least two clinical studies pursuing that goal through the prospective collection of samples from patients with MPE (The efficacy of sonographic and biological pleurodesis indicators of malignant pleural effusion-SIMPLE; www.isrctn.com ISRCTN16441661) and mesothelioma (Diagnostic and Prognostic Biomarkers in the Rational Assessment of Mesothelioma- DIAPHRAGM; www.isrctn.com ISRCTN10079972) using a standardised approach on the timing and testing of biomarkers.

Therapeutic aspiration

In general, selection of the most appropriate treatment approach should be individualised, while also considering patient preference. Patients with poor performance status, minimal life expectancy, and an underlying chemotherapy-sensitive malignancy (e.g., lymphoma or small cell lung cancer) can be managed with therapeutic aspiration [4]. Thoracentesis is generally safe, especially if it is performed under TUS guidance. Re-expansion pulmonary oedema can occur infrequently after the removal of more than 1.5 L of fluid (<0.5% risk in large series) [69]. Although fluid drainage does not improve survival, it can substantially improve symptoms and avoid hospitalisation. The drawback of this approach is that over time the effusion is likely to recur, leading to repeated uncomfortable procedures and relative frequent hospital visits.

Pleurodesis

Pleurodesis is defined as parietal-to-visceral pleural fusion with concomitant obliteration of the pleural space. It can be accomplished via a variety of chemical or mechanical means, after complete drainage of the effusion that allows parietal-to-visceral pleural apposition. Instillation of the sclerosing agent is thereafter followed by a profound inflammatory response between the layers, which, in turn, result in fibrin accumulation and pleural fibrosis. A variety of different chemicals (talc, bleomycin, tetracycline, iodopovide and others) and bacterial products (from Corynobactum parvum, Streptococcus pyogenes, Staphylococcus aureus and others) have been used in clinical studies to achieve pleurodesis [70–73]. The profound inflammatory response they cause may result in adverse events such as pain and fever but it is believed that the level of inflammation correlates with the likelihood of successful pleurodesis [74].

The type of sclerosant, the method of administration and the method of selection of patients that will be treated with pleurodesis are still unclear. Talc is the most commonly used pleurodesis agent and a meta-analysis supports its use as the sclerosant of choice [4, 75]. Even within the context of large randomised trials, pleurodesis success rates remain under 80% and are almost certainly lower in everyday practice [9, 10, 76, 77]. It has been proven that graded preparations (as opposed to small particle talc) should be used to minimise systemic talc particle dissemination and the risk of acute respiratory distress syndrome [78, 79]. Talc can be administrated either as a slurry through the chest drain or as poudrage during medical thoracoscopy. Even though it was historically believed that talc poudrage was more effective than slurry, this has changed in light of three randomised studies [76, 80–82]. Results from the largest randomised trial in MPE revealed that the success rates of talc slurry and talc poudrage are not significantly different. A subgroup analysis of patients with lung and breast cancer suggested a better success rate for talc poudrage [76]. On the contrary, a recent metanalysis demonstrated that talc poudrage was superior to talc slurry in pleurodesis success [83]. It is expected that the results of TAPPS trial, which is a suitably powered multicentre, open-label, randomised controlled trial designed to compare the pleurodesis success rate of talc poudrage with talc slurry, will provide evidence on the most efficient way of pleurodesis [84].

Studies report a median length of hospital stay of 4–7 days for inpatient talc pleurodesis, which for patients with poorer survival represents a significant proportion of their remaining lives. Even today, no consensus exists about the optimal size of the chest drain and the time of removal of the tube post-pleurodesis. National guidelines advocate the use of small bore (<14 F) chest drains, but this is now challenged by the results of the largest study to specifically address tube size for MPE in terms of pleurodesis success [10]. Specifically, the TIME1 study showed that smaller chest tubes might be inferior to larger ones in terms of pleurodesis success [10]. The results need validation by future randomised trials powered to address the optimum size of chest drain for pleurodesis.

Indwelling pleural catheters

IPCs have gained popularity during the last decade as they offer ambulatory management, thereby minimising hospital stay and healthcare costs [2, 74]. An IPC is a silicone tube placed in the pleural cavity and tunnelled subcutaneously. The proximal end of the exposed tube has a one-way valve which connects to drainage bottles. Drainage is guided by symptoms and is patient-driven, offering a sense of control to most patients. The optimal schedule of MPE drainage through an IPC is still not clear. Two actively recruiting randomised studies (www.clinicaltrials.gov NCT00978939 and NCT00761618) are under way to address this question (aggressive daily versus three times weekly drainage).

Guidelines have advocated the use of IPCs in those patients with MPE that have failed pleurodesis or in those with trapped lung (unsuitable for pleurodesis) [4]. A meta-analysis of 1348 patients with MPE treated with IPCs revealed that 95.6% had symptomatic improvement and 45.6% achieved spontaneous pleurodesis after a median of 52 days [85]. In addition, the TIME2 randomised controlled trial showed that IPCs achieved control of breathlessness and quality of life comparable to talc pleurodesis, but significantly shortened the length of hospital stay [9]. Recent data also provided reassurance on the safety of IPC use, with a risk of death from pleural infection below 0.3% [86]. IPC-related pleural infection occurred in <5% of patients enrolled in another multicentre study and was well-controlled with antibiotics. Moreover, patients with an IPC who develop an associated pleural infection appear to have high rates of post-infectious pleurodesis and also prolonged survival times, as suggested by a recent observational study [87]. There is ongoing research on possible combinations of IPC with sclerosant agents in order to enhance pleurodesis success [88]. As IPC offer long-term access to the pleural cavity, they represent ideal potential portals for local drug delivery. In addition to IPC-delivered bacterial moieties that have been evaluated as potential immunomodulators against MPE, IPC will hopefully facilitate further research in this field [82].

Ambulatory management

Pleural-specific services and technologies are expanding, and this has led to a greater drive towards ambulatory management [89]. The traditional approach of admitting patients with MPE for talc pleurodesis is now outdated. Recent advances in IPC and medical thoracoscopy have created the potential for an attractive “one-stop” approach to diagnosis and management. Combinations of medical thoracoscopy with IPC at the end of the procedure to assure drainage of fluid or sclerosant delivery through IPC could streamline patients' pathways for the limitation of hospitalisation. Clinical trials are currently under way to establish the use and benefit of combined treatment methods.