🧠 Vet Guide to Pituitary Macroadenoma in Canine Cushing’s Disease: Diagnosis & Radiation Therapy in 2025 🐶

By Dr Duncan Houston, BVSc

For many dogs diagnosed with Cushing’s disease, the underlying cause is a small, hormone-secreting tumor in the pituitary gland. However, in approximately 10–20% of these cases, the pituitary tumor grows large enough to compress nearby brain structures—this is called a pituitary macroadenoma. These tumors not only cause hormonal imbalances but can also trigger neurologic symptoms due to their size. In this guide, I’ll walk you through what to expect in 2025 for dogs diagnosed with this serious condition. 📊

What Is a Pituitary Macroadenoma? 🧬

The pituitary gland, located at the base of the brain, regulates many of the body's hormones. When a tumor in this gland becomes large enough—typically ≥10 mm in diameter—it’s categorized as a macroadenoma.

• 🧠 Causes pressure on nearby brain tissue
• 🐶 Occurs in ~10–20% of dogs with pituitary-dependent Cushing's disease
• 📈 May be silent at first, growing slowly over time

Although macroadenomas are generally benign (non-spreading), their location and size can still create dangerous and painful effects.

Symptoms: When to Suspect a Macroadenoma 🕵️

Dogs with pituitary macroadenomas often have already been diagnosed with Cushing’s disease for several months and may be stable on medication. Signs to watch for include:

• 🔄 Changes in behavior or alertness
• 🍽️ Poor appetite (rare in dogs with Cushing’s)
• 🚶‍♂️ Circling, pacing, or walking into objects
• ⚡ Seizures
• 😴 Listlessness or lethargy

Note: Any dog with Cushing’s disease who suddenly develops neurologic signs or stops eating should be evaluated for a possible macroadenoma. This is especially important in brachycephalic breeds (e.g., boxers, pugs), which tend to develop larger pituitary tumors. 🧪

How Is a Macroadenoma Diagnosed? 📸

Imaging is the only way to definitively diagnose a macroadenoma. Options include:

1. MRI (Magnetic Resonance Imaging)

• 🧲 Gold standard for viewing soft tissue, especially the pituitary gland
• ⚕️ Provides detailed tumor size and location

2. CT Scan (Computed Tomography)

• 🔬 More widely available but less detailed than MRI
• 🧠 Still useful in many veterinary hospitals

Both procedures require general anesthesia, and costs range from $1,500–$3,000 depending on the facility. ❗

Blood tests cannot distinguish macro from microadenomas, although researchers are currently developing promising new diagnostic assays.

When Is a Tumor “Too Big”? 📏

Veterinary professionals currently consider a pituitary tumor to be a macroadenoma if it measures:

• ➕ 10 mm or larger (standard threshold)
• 📉 Some institutions like UC Davis use 8 mm as their threshold for concern

At 10 mm or more, the tumor may begin to affect brain function—even in the absence of symptoms. Imaging helps determine whether intervention is necessary. 🧠

Treatment Options for Pituitary Macroadenomas 💉

Unfortunately, surgical removal is not feasible in dogs due to the gland’s deep location at the brain’s base. However, modern radiation therapy offers a powerful alternative. 💥

Radiation Therapy

• 🛠️ Targets the tumor to reduce its size and relieve brain pressure
• 📅 Usually involves 2–3 sessions per week over 4–6 weeks
• 💵 Costs can range from $4,000–$7,000
• ✅ Up to 70% of dogs show improvement, often within 1–2 months

Risks and Side Effects

• 👂 Hearing loss
• 💧 Reduced tear production (dry eye)
• 🎨 Loss of skin or hair pigment in irradiated areas

Very large tumors (25 mm+) may be too advanced for radiation to be effective. Early detection is essential. 🕒

Post-Therapy Considerations

While radiation may shrink the tumor and relieve neurologic signs, more than 50% of dogs will still need medication for Cushing’s. Radiation is not typically curative for hormone production. ⚠️

Should Every Dog with Cushing’s Be Imaged? 🤔

This is a hotly debated topic in veterinary medicine. On one hand:

• 💵 Imaging is expensive and involves anesthesia risks
• 📉 Only 10–20% of dogs with Cushing’s develop macroadenomas

On the other hand:

• 🔍 Early imaging can identify large or fast-growing tumors
• 📆 Tumors ≥10 mm may benefit from prompt radiation
• 📸 Tumors 7–9 mm warrant follow-up imaging in 12–16 months

Current guidance (including UC Davis) recommends imaging if possible, especially for tumors >8 mm. 🩻

Prognosis: What to Expect 📈

According to the landmark Theon/Feldman study:

• 🧠 Dogs with mild neurologic signs treated with radiation had prolonged survival
• ⏱️ Average survival time was 5 months after the onset of severe neurologic symptoms
• 🎯 7 out of 24 dogs survived >1 year post-radiation

Early diagnosis and treatment significantly improve outcomes, but dogs with severe neurologic compromise may not benefit as much from therapy. ⛑️

Final Thoughts from Dr Duncan Houston 🧑‍⚕️

Pituitary macroadenomas add a complex layer to managing canine Cushing’s disease. While many dogs with microadenomas thrive on hormone therapy alone, a macroadenoma can lead to significant neurologic dysfunction and discomfort. Thankfully, radiation therapy offers hope and relief for many dogs—especially when tumors are caught early.

If you notice new or worsening symptoms in a dog being treated for Cushing’s, don’t delay. Talk to your vet about imaging options and whether a referral to a veterinary neurologist or oncologist is appropriate.

Need guidance or a second opinion? Visit Ask A Vet or download the Ask A Vet app for 24/7 access to licensed veterinary support. 🐶📲