Stereotactic refers to a 3-dimensional coordinate system that enables accurate correlation of a tumor visualized in diagnostic images with actual tumor position in the patient’s body. These treatment methods rely on accurate patient positioning, strict quality assurance, on-board imaging and/or rigid immobilization, and highly conformal delivery methods, which result in a steep dose gradient between the edge of the tumor and surrounding normal tissue. This results in a high dose of radiation being received by the tumor and a relatively low dose being received by surrounding healthy tissue.17,18
SRT further minimizes the volume of normal tissue exposed to high-dose radiation by more narrowly defining the treatment volume and using beam arrangements that achieve a high degree of conformality and rapid dose falloff. Stereotactic radiation may be delivered in multiple fractionation schemes, such as a single high-dose fraction (ie, radiosurgery), or in a fractionated manner consisting of multiple treatments. Historically, the term stereotactic radiosurgery indicated treatment of intracranial location, and stereotactic body radiation therapy (SBRT) has been used for extracranial locations. Of note, the terms SRT and SBRT are sometimes used interchangeably.
The practice of SRT depends on precise target definition and radiation delivery. In human medicine, this is defined by accuracy to within 1 mm. First, the patient is immobilized in a treatment-positioning device for CT and/or MRI imaging to allow for precise definition of target and critical structures, which should be repeatable for treatment delivery. Treatment planning software is then used to create a highly conformal treatment plan that maximizes the dose to the target while minimizing dose outside the target and to critical structures. This is typically accomplished using multiple rotational arc and/or IMRT fields and/or specialized attachments to the linear accelerator that finely collimate a beam of radiation (ie, stereotactic cones). For patient setup and radiation treatment, a stereotactic coordinate system and/or on-line image-guidance is used to verify accuracy immediately before and/or during treatment delivery. There are important quality assurance techniques at each of these steps that should be overseen by a qualified medical physicist and/or radiation oncologist to ensure that the desired levels of accuracy are met.
Stereotactic radiation therapy has become more widely available in veterinary medicine and has been described for use in brain tumors, nasal tumors, osteosarcomas, feline oral squamous cell carcinomas, heart base tumors, feline injection site sarcomas, and trigeminal nerve sheath tumors (Figure 3).19-27