By: Dr. Ana P. Rebelo, DVM, MS

Lymphoma affects cells of the immune system and is considered a systemic cancer, so chemotherapy has traditionally been the mainstay of treatment. However, another approach that many dog owners, and even some veterinarians, may be less familiar with is half-body irradiation (HBI).

HBI uses radiation to target lymphoma cells throughout the entire body. Treatment is divided between the cranial (front) half and caudal (back) half, typically two to four weeks apart, allowing normal tissues, particularly the bone marrow, time to recover. Because radiation treatment requires the dog to remain completely still and treatment positioning must be precise, sedation or general anesthesia is required for each radiation treatment.

HBI does not replace chemotherapy. Instead, it is incorporated into a chemotherapy treatment plan to target lymphoma cells that may have survived chemotherapy, with the goal of extending remission and survival.

How can HBI benefit dogs with lymphoma?

A 2025 study evaluated dogs with high-grade lymphoma that had achieved complete remission following initial multi-agent chemotherapy. Dogs that subsequently received HBI had a median survival time of 1,924 days (approximately 5.3 years), compared with 566 days (approximately 1.6 years) in dogs treated with chemotherapy alone.

Another study evaluating dogs with high-grade B-cell lymphoma reported similarly encouraging results. Among dogs receiving chemotherapy combined with HBI, the two-year survival rate was 78%.

These findings are particularly exciting because relapse remains one of the greatest challenges in treating canine lymphoma. However, not every dog with lymphoma is expected to benefit equally, and careful patient selection is extremely important.

Does every dog with lymphoma benefit equally?

Probably not. Lymphoma type appears to matter considerably. Dogs with B-cell lymphoma treated with HBI had a median progression-free interval of 2,127 days (5.8 years), compared with 292 days (10 months) for dogs with T-cell lymphoma. This means that half of the dogs with B-cell lymphoma remained in remission for longer than 2,127 days.

Disease status is also important. HBI is primarily used as a consolidation treatment after chemotherapy has achieved complete remission, with the goal of targeting lymphoma cells that may remain but are too few to detect with routine testing.

When is HBI recommended?

For dogs receiving CHOP chemotherapy, HBI is commonly incorporated during the course of the CHOP protocol, rather than waiting until chemotherapy has been completed.

The exact timing and protocol can vary between radiation oncology centers. The important concept is that HBI is used as a consolidation treatment while the dog is still receiving chemotherapy and remains in complete remission.

Before HBI: confirming remission

Even when a dog appears to be in complete remission, lymphoma may still be detectable elsewhere in the body. Therefore, comprehensive restaging is performed within seven days before HBI to confirm there is no evidence of persistent disease.

Restaging typically includes thoracic radiographs, abdominal ultrasound, FNA and cytology of the liver, spleen, and a peripheral lymph node.

This is an essential safety step. HBI should only be performed when the lymphoma is in remission, as persistent disease may reduce its benefit and substantially increase the risk of severe, potentially life-threatening toxicity.

What side effects can occur?

Because HBI treats a large portion of the body, side effects can involve multiple organ systems. One of the most important is bone marrow suppression, which can decrease white blood cells and platelets and increase the risk of infection or bleeding.

Gastrointestinal side effects, including decreased appetite, nausea, vomiting, and diarrhea, may also occur, particularly after treatment of the caudal half of the body. Some protocols have also been associated with lung changes.

Side effects vary by radiation protocol, disease burden, and other treatments being administered. This is why careful patient selection, bloodwork, and close monitoring are essential when using HBI.

If the results are so promising, why isn’t HBI used more often?

HBI requires specialized radiation therapy equipment and expertise, as well as anesthesia, coordination with chemotherapy, comprehensive restaging, and close monitoring.

More importantly, although recent results are very encouraging, the available evidence is still limited. Studies generally include small numbers of dogs and use different radiation doses, chemotherapy protocols, and treatment schedules. Therefore, we cannot assume that every dog will experience the long remissions reported in recent studies.

Still, current evidence suggests that carefully selected dogs, particularly those with high-grade B-cell lymphoma in complete remission, may benefit substantially from HBI.

The take-home message!

Chemotherapy remains the cornerstone of treatment for canine multicentric lymphoma, but HBI may provide an additional opportunity to extend remission in carefully selected dogs.

Recent studies have shown particularly encouraging results in dogs with high-grade B-cell lymphoma that are in complete remission at the time of HBI, with some dogs experiencing prolonged disease control.

HBI is not appropriate for every dog and can have significant side effects. Careful patient selection, confirmation of remission, and close monitoring are essential, while its potential benefits should be weighed against treatment logistics, anesthesia, side effects, cost, and the limitations of the available evidence.

For the right patient, HBI represents a promising and still relatively underrecognized approach to extending remission and survival in canine lymphoma.