What Type of Cancer Causes Fluid in the Lungs?

In this video, I explain what a pleural effusion is, which cancers cause it, how malignancy leads to fluid buildup, and what treatment options are available. If you're experiencing these symptoms or supporting someone who is, I hope this gives you some clarity.

What is a Pleural Effusion?

A pleural effusion occurs when excess fluid builds up in the space between the lung and the chest wall. This space, called the pleural cavity, normally contains just a small amount of pleural fluid to help the lungs move smoothly when you breathe. When extra fluid accumulates in this pleural space, it can make breathing difficult and cause chest discomfort.

When this fluid buildup is caused by cancer, we call it a malignant pleural effusion. It's one of the more distressing symptoms that patients with lung cancer or other cancers may experience. But here's what's important to know: it can be treated, and symptoms often improve within 24 hours of drainage.

If you or someone you're caring for is experiencing unexplained difficulty breathing, chest pain, or a persistent cough, especially with a history of cancer, it's crucial to speak to a doctor quickly. The sooner this is assessed, the sooner relief can be found.

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What Type of Cancer Causes Fluid in the Lungs?

Lung cancer is the most common cause of malignant effusion, but it's not the only one. Any cancer that spreads to the lung or affects the pleura (the lining around the lung) can lead to fluid buildup in the chest cavity.

Cancers that commonly cause pleural effusions include:

• Lung cancer (both small cell and non-small cell lung cancer)

• Breast cancer that has spread to the lungs or pleura

• Lymphoma affecting the chest

• Ovarian cancer in advanced stages

• Gastrointestinal cancers that have metastasised to the lungs

The fluid doesn't build up inside the lungs themselves. Instead, it collects in the pleural space between the chest wall and the lung, which causes the lung to compress. This separation is what leads to breathlessness, coughing, and discomfort when lying flat.

If you are currently a patient and you've been diagnosed with any of these cancers and you're noticing new or worsening breathlessness, this is something your medical team needs to assess urgently.

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Understanding Different Types of Pleural Effusion

Not all pleural effusions are caused by cancer. It's helpful to understand the different types, as this affects how they're diagnosed and treated.

Exudative vs Transudative Effusions

Pleural effusions are broadly classified into two categories:

Exudative effusions occur when the pleural membrane becomes inflamed or damaged, causing protein-rich fluid to leak into the pleural cavity. Malignant pleural effusions are a type of exudative effusion. Other causes include lung infection, pulmonary embolism, and conditions causing pleural disease.

Transudative effusions happen when fluid filters through normal pleural membranes due to imbalances in pressure or fluid retention. These are typically caused by heart failure, liver disease, or kidney problems. Pulmonary oedema (fluid in the lung tissue itself) is different from a transudative pleural effusion and has separate causes.

It's the level of protein in the fluid that makes the effusion a transudative effusion (low protein) or exudative effusion (high protein).

Other Types of Effusions

Parapneumonic effusions develop alongside pneumonia or other chest infections. If infected fluid accumulates, this can progress to a pleural empyema, which requires urgent drainage. Pneumonia is a common cause of an exudative pleural effusion.

In some cases, patients develop bilateral pleural effusions, meaning fluid builds up on both sides of the chest. This can be particularly uncomfortable and requires careful assessment by a chest physician.

When you have a chest X-ray or CT scan, your doctor will look at the pleural fluid to determine which type of effusion you have. Sometimes a pleural aspiration (removing a sample of fluid) or pleural biopsy is needed to analyse the pleural fluid and confirm the diagnosis. Tests like pleural fluid LDH (an enzyme level) help distinguish between exudative and transudative causes.

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How Does Malignancy Cause Pleural Effusion?

When cancer involves the pleura, it disrupts the normal balance of fluid production and drainage. Normally, a small amount of fluid is made and drained away continuously without any issues. But cancer interferes with this process in two main ways.

First, cancer cells can block the lymphatic drainage system, which is responsible for clearing pleural fluid from the chest cavity. When drainage is blocked, fluid has nowhere to go and begins to accumulate.

Second, cancer causes inflammation in the pleura, making the tiny blood vessels in the lining "leaky". This allows more fluid than usual to seep into the pleural space. The body keeps producing fluid, but it's not being drained properly, so the buildup worsens over time. In some cases, this chronic inflammation can also lead to pleural thickening, where the lining becomes scarred and less flexible.

This can happen when lung cancer spreads directly to the pleura, or when cancers like breast cancer or lymphoma spread to the lungs from elsewhere in the body. Sometimes patients notice a pleural friction rub (a scratching sound when breathing) before fluid accumulates significantly.

What frustrates many patients and families is how long it can take to get scans, reports, and a clear treatment plan. In some cases, people wait two or three weeks just to understand what's happening, all while living with worsening breathlessness and chest pain. The good news is that diagnosing and treating a pleural effusion can happen very quickly when it's prioritised.

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How Do You Treat Malignant Pleural Effusion?

There are several effective ways to treat malignant pleural effusion, and the right option depends on your situation. The first priority is always to relieve your symptoms, particularly breathlessness and chest discomfort.

Thoracentesis (Fluid Drainage)

The most immediate pleural effusion treatment is called thoracentesis. This is a straightforward procedure where a needle is carefully inserted between the ribs to drain the fluid. When done to relieve symptoms, we call it therapeutic thoracentesis. It sounds more uncomfortable than it usually is. Most patients feel significantly better within hours of having the fluid removed.

If you haven't yet been diagnosed with cancer, this fluid can also be tested to help make or confirm a diagnosis through pleural fluid analysis. It's worth asking your medical team to send the fluid for all the appropriate tests at the time of drainage.

Chest Drain or Chest Tube

Sometimes, particularly if there's a large volume of fluid or a complicated parapneumonic effusion, a chest drain (also called a chest tube) may be inserted for a day or two. This allows continuous drainage while you're monitored in hospital.

Indwelling Pleural Catheter

If you have a recurrent pleural effusion, where the fluid keeps coming back, one option is to have a small, soft plastic pleural catheter inserted. You can go home with this in place, and it allows you or a district nurse to drain the fluid regularly at home. This gives you more control and avoids repeated hospital visits.

Pleurodesis (Pleural Sclerosis)

Another option for recurrent pleural effusions is pleurodesis, sometimes called pleural sclerosis. This involves draining all the fluid and then using a special substance to make the two layers of the pleura stick together. Once they're stuck, fluid can't accumulate in that space anymore. It's a minor procedure, but it can prevent the effusion from returning.

In some cases where pleural disease is extensive or drainage procedures haven't worked, thoracic surgery may be considered, though this is less common for malignant effusions.

Treating the Underlying Cancer

Ultimately, the most important step is treating the cancer itself. Chemotherapy, immunotherapy, or targeted therapies can shrink the tumour and stop the fluid from building up again. As a lung cancer specialist, I work with a full team to make sure we're addressing both the immediate symptom (the effusion) and the underlying cause (the cancer).

What often surprises patients is how much better they feel once the fluid is drained. Breathlessness improves, sleep becomes easier, and the constant discomfort lifts. The hardest part for many families is the delay in getting to that point of relief.

If you're experiencing delays in your care, or if you're unsure whether your symptoms are being taken seriously, it's always worth exploring your options. Let your medical team know about any new or worsening symptoms. There's often a quick solution that can get you feeling better and moving forward with treatment much faster.

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When to Seek Help

If you or someone you're caring for has been diagnosed with cancer and is now experiencing:

• Increasing difficulty breathing

• Chest pain or heaviness

• Difficulty lying flat

• A persistent cough

...then please speak to your medical team as soon as possible. These symptoms don't have to be endured while waiting for appointments or scans. Pleural effusion can be drained quickly, and relief is often immediate.

You're welcome to request a consultation if you'd like a second opinion or if you're finding NHS waiting times difficult to manage. Private consultations can often be arranged within days, and treatment plans put in place quickly.

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About Dr James Wilson

Dr James Wilson is a consultant oncologist specialising in lung cancer and advanced radiotherapy. He works in full-time private practice in Central London, helping patients access rapid diagnosis, clear treatment plans, and expert care when time matters most.