Mediastinal lymphoma is a fluid-producing cancer of the lymphoid tissue in the front of the chest, classically presenting as muffled heart sounds, slow-onset labored breathing, and reduced compressibility of the cranial chest in a young to middle-aged cat. Siamese and Oriental breeds are over-represented, and FeLV-positive cats historically carried up to 50 times the risk of FeLV-negative cats (Louwerens et al., 2005, JFMS). Chemotherapy can produce months to years of high-quality remission if started before the cat is exhausted by hypoxia.
Last reviewed: June 2026
What Mediastinal Lymphoma Looks Like
Mediastinal lymphoma is a cancer of T-cell lymphocytes that grows as a large mass in the cranial mediastinum — the space between the lungs in the front of the chest. It is one of three classic anatomic forms of feline lymphoma, alongside alimentary and multicentric disease. The mass itself plus the malignant pleural effusion it produces fill the chest cavity, compress the lungs, and reduce respiratory reserve gradually over days to weeks. Owners often miss early signs because the cat compensates by reducing activity. By the time labored breathing is obvious, both lungs are competing with several hundred milliliters of fluid for room.
Classic Signs
The most common presenting signs are gradually increased respiratory rate at rest, open-mouth breathing during stress or play, sudden episodic panting after minimal exertion, reluctance to lie flat, weight loss, and reduced appetite. A subtle but useful sign: a normally lap-loving cat now refuses to be picked up around the chest, because compression hurts. On exam, heart sounds are muffled, the chest wall feels less springy when gently compressed cranially, and percussion sounds dull. Some cats present with regurgitation if the mass also presses on the esophagus.
Diagnosis and Staging
Thoracic radiographs show a large soft-tissue opacity in the cranial mediastinum displacing the trachea dorsally, plus variable pleural effusion. Thoracic ultrasound is superior to radiographs for confirming the mass and for guiding fluid sampling. The diagnostic test of choice is fluid analysis from thoracocentesis: malignant effusion is a milky to pink modified transudate or chylous fluid loaded with neoplastic lymphocytes. Fine-needle aspirate of the mass and immunohistochemistry confirm T-cell origin in 75 to 90 percent of cases. As described in Withrow & MacEwen's Small Animal Clinical Oncology, complete staging includes FeLV and FIV testing, CBC and biochemistry, abdominal ultrasound, and bone marrow aspirate when peripheral cytopenias are present.
FeLV Status Changes Prognosis
FeLV-positive cats with mediastinal lymphoma historically had poorer outcomes than FeLV-negative cats, with shorter remission durations and earlier relapse. The prevalence of FeLV-positive cases dropped substantially after widespread testing and vaccination in the 1990s, so most mediastinal lymphoma seen now in well-vetted populations is FeLV-negative. Even so, every newly diagnosed cat should be tested. Retrovirus testing and management standards for any FeLV-positive cat are outlined in current retrovirus guidance (AAFP Feline Retrovirus Guidelines, 2020). FIV co-infection does not strongly affect prognosis but should be documented. Young Siamese and Oriental breeds frequently present FeLV-negative with this form, reflecting a distinct genetic predisposition independent of viral status.
Chemotherapy Outcomes
The standard protocol is a CHOP-based multi-drug regimen (cyclophosphamide, doxorubicin or epirubicin, vincristine, and prednisolone) delivered over 19 to 25 weeks. Complete remission is achieved in 65 to 80 percent of cats with mediastinal disease, with median remission durations of 6 to 9 months and median overall survival of 9 to 18 months in published case series. A meaningful minority of FeLV-negative cats achieve durable remissions of 2+ years. Cats that achieve complete remission have substantially better outcomes than partial responders. Side effects of chemotherapy in cats are typically milder than in dogs — most cats maintain quality of life with modest GI upset and intermittent neutropenia rather than nausea and hair loss.
Initial Stabilization
Cats presenting in respiratory distress need fluid drained before anything else. Thoracocentesis at presentation removes 100 to 400 mL of effusion and dramatically improves breathing within minutes. Oxygen supplementation, minimal handling, and avoidance of stress maneuvers are essential — a stressed cat with mediastinal lymphoma can decompensate rapidly. Once stable, a single high-dose dexamethasone or prednisolone injection induces partial response within hours and bridges to formal chemotherapy. Owners pursuing chemotherapy should start within days of diagnosis to capitalize on the steroid-induced shrinkage before resistance develops.
When to See a Vet
Call your vet today if:
- A young to middle-aged cat suddenly tires easily after minimal activity
- Gradual increase in respiratory rate at rest above 35 breaths per minute
- Reluctance to lie flat on the side
- Loss of appetite or weight along with new respiratory effort
- A Siamese, Oriental, or related breed with new exercise intolerance
Go to the ER immediately if:
- Open-mouth breathing or visible abdominal effort with every breath
- Blue, purple, or pale gums
- Sudden collapse or inability to stand
- A cat that cannot lie down comfortably and breathes only when standing
- Respiratory rate above 50 breaths per minute at rest
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Frequently Asked Questions
How is mediastinal lymphoma different from regular lymphoma?
It is the same family of cancer — lymphoma — but localized to the cranial mediastinum. Alimentary lymphoma involves the gut, multicentric lymphoma involves peripheral lymph nodes and spleen and liver, and renal lymphoma involves the kidneys. Mediastinal disease has its own prognosis and presents almost entirely as a respiratory problem rather than the GI or weight-loss picture of alimentary disease. Treatment uses similar chemotherapy protocols but staging and response monitoring differ.
How much does mediastinal lymphoma diagnosis and treatment cost?
Initial vet exam typically runs $50 to $150 in the US. Thoracic radiographs $150 to $400, ultrasound $300 to $600, thoracocentesis with cytology $400 to $900, FeLV/FIV testing $80 to $150, complete staging $500 to $1,200. A full CHOP-based chemotherapy protocol delivered at a specialty oncology practice runs $4,000 to $9,000 over 5 to 6 months. Single-agent doxorubicin protocols are roughly $2,000 to $4,000. Palliative prednisolone alone is $20 to $60 per month. Pet insurance with oncology coverage substantially defrays cost.
Will chemotherapy make my cat sick?
Cats generally tolerate chemotherapy better than dogs and far better than humans. Roughly 15 to 25 percent of cats develop mild GI upset for 24 to 48 hours after a drug; about 5 to 10 percent develop a neutropenic episode that requires antibiotics. Hair loss is uncommon in cats — whiskers may thin but coat is usually preserved. Quality of life during remission is typically very good. The decision to pursue chemotherapy rests on the family's goals, finances, and the cat's general health at diagnosis.
Can prednisolone alone control mediastinal lymphoma?
Single-agent prednisolone induces partial response in many cats and can give 1 to 3 months of comfort. It is an entirely reasonable choice when finances or family logistics rule out full chemotherapy. Cats started on steroids alone, however, develop chemotherapy resistance faster — full chemotherapy after steroid-only treatment has shorter remission durations than chemotherapy started without prior steroids. If chemotherapy is on the table, start it before, or in tandem with, prednisolone.
Is mediastinal lymphoma contagious or hereditary?
Not contagious. FeLV is contagious between cats and is a known cause, but FeLV-negative mediastinal lymphoma in Siamese and Oriental breeds appears to have a genetic component rather than an infectious one. Other cats in the household are not at increased risk simply because one cat has the disease. FeLV testing of all cats in the household at diagnosis is still prudent.
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