Lyndsay Kubicek, DVM DACVR (Radiation Oncology)
Angell Animal Medical Center
Stereotactic radiation therapy (SRT) is a newer form of advanced radiation technology originating in human medicine that is currently applied in veterinary radiation oncology. Stereotactic radiation uses highly conformal and focused irradiation beams directed precisely at a tumor volume. This technology has several proposed advantages over traditional fractionated radiation therapy including reduced side effects and fewer anesthetic events.1,2
Multiple terms have been used for stereotactic-based radiation that includes stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). In a purest sense, SRS is used when describing a single high dose of radiation delivered to an intracranial lesion, and contrary to its name, it does not involve physical surgery. Stereotactic body radiation is used to describe delivering high dose radiation over 1-5 treatments outside the cranium.
There is a different radiobiology associated with stereotactic radiation compared to traditionally fractionated radiation, in that sparing normal tissue is accomplished by avoidance of normal tissue structures and not by delivering smaller doses of fractions over a course of therapy (fractionation). Dose heterogeneity to a tumor and high delivery dose rates also play an important role in the biology of radiation damage to the tumor.3
There are important underlying principles for the employment of stereotactic radiation therapy. High quality diagnostic imaging is performed real time or close to real time that allows direct visualization of the tumor and allows the delivery of ablative doses of radiation with a high degree of confidence in tumor location. This collectively is called image guided radiation therapy (IGRT). Another principle is the requirement of a target to deliver such ablative doses to; SRT is not delivered to scars with the rare exception in human radiation of post-surgical brain tumors.4,5
In veterinary medicine there are multiple publications for the use of stereotactic radiation therapy such as brain tumors, osteosarcoma, nasal tumors, heart base tumors, injection site sarcomas and oral squamous cell carcinoma.
The use of stereotactic radiation therapy for brain tumors has been documented in multiple studies. One study evaluating all brain tumor types had a median survival time of 13 months following a single dose of radiation. When meningiomas were separated and evaluated the median survival time was 16 months.1 This is similar to another report evaluating 3 doses of stereotactic radiation therapy for canine meningiomas with a reported median survival time of 18 months.6 The side effects were similar to traditionally fractionated radiation with the benefit on only 1-3 treatments compared to 20 treatments.
Single fraction SBRT has been evaluated in appendicular osteosarcoma in two studies. The median survival times were 9.7 and 11.9 months with a fracture rate of 40%, which is slightly above the reported inherent risk of 30% associated with having an appendicular OSA. The median time to fracture was 6 months, or 4.2 months if subchonral bone was involved.7,8
There are two studies evaluating the use of single fraction or 3 fraction doses of stereotactic radiation therapy for non-lymphomatous nasal tumors. The median survival time of one paper was 13 months and the other was 10.4 months excluding nasal osteosarcoma.9,10 Patient’s response rate is similar to definitive therapy and the acute side effects of each protocol were mild. Given the high dose per fraction delivered with stereotactic radiation the potential risk of late terms side effects may be increased, although not seen in either paper. Further studies are underway at multiple hospitals.
Stereotactic radiation therapy can also be used in a palliative setting. A study using 3-5 fraction protocol for 11 cats with injection site sarcomas was recently published. The protocol was well tolerated with very minimal side effects noted. There was a 73% response rate (partial and complete response). The median survival in this patient population was 10 months.11 This offers a new option for owners who do not wish to perform aggressive surgery but maintain tumor control and a good quality of life.
A previously published case report of a dog receiving traditionally fractionated radiation therapy confirmed the radiosensitivity of heart base tumors.12 A proof of principle study was recently published evaluating paralyzation and breath-holding techniques for the delivery of stereotactic radiation therapy to 4 dogs with heart base tumors. This study confirmed the safety and ease of this delivery technique as well as early responses. Traditional delivery schemes for heart base tumors involve 18-20 doses which are less appealing for owners especially when dealing with the heart. Offering 3-5 doses of radiation with similar control is very attractive to owners. The aim of the paralyzation paper was to describe technique and thus only 2 of the 4 patients were out far enough from treatment to describe survival time. However, at the time of writing the 2 patients were out 8 and 13 months both with a measureable response.
Stereotactic radiation therapy is a versatile and emerging technology in both human veterinary oncology. Veterinary studies are underway at multiple institutions further defining the role of this technology. We are happy to announce we will be upgrading our system at Angell Animal Medical Center in Spring 2017 to offer stereotactic radiation therapy.
For more information, please contact Angell’s Oncology Service at 617-541-5136 or firstname.lastname@example.org.