Stereotactic Body Radiation Therapy

In 1951, Dr. Lars Leskell and Borje Larsson introduced the concept of radiosurgery for use in intracranial conditions considered inoperable. Stereotactic radiosurgery (SRS) has been used to treat functional disorders of the brain such as trigeminal neuralgia or arteriovenous malformations, vascular malformations, and intracranial and spinal benign and malignant tumors. The development of stereotactic body radiation therapy (SBRT) began in the early 1990s at the Karolinska Institute (Stockholm, Sweden) and was derived from the techniques and procedures of SRS. Researchers Ingmar Lax and Henric Blomgren at the Karolinska Institute created a body frame to aide in targeting extracranial treatment sites. During this time in Japan, Minoru Uematsu began work on juxtaposing closely a computed tomography (CT) scanner and linear accelerator (linac) into a synthesized “FOCAL”(Fusion Of CT And Linac) unit in lung applications,¹³ leading to the development of performing SBRT without a body frame. By the late 1990s, researchers Robert Timmerman, Lech Papiez, and colleagues initiated a phase 1 trial of SBRT for medically inoperable lung cancer at Indiana University in North America.¹³

The American College of Radiology (ACR) and American Society for Radiation Oncology (ASTRO)define SBRT as “an external beam radiation therapy method used to very precisely deliver a high dose of radiation to an extracranial target within the body, using either a single dose or a small number of fractions.”¹ The similarity between SBRT and SRS is the combining of multiple finely collimated radiation beams and stereotaxy (3D target localization). The multiple radiation beams intersect to deliver an accurate, high dose of radiation to a carefully defined location. The differences between the two radiation treatments include the treatment sites (e.g., SRS involves the head and spine) and the number of fractions utilized (i.e., SBRT one or a small number of fractions, SRS is typically one fraction). SBRT has been used as a treatment for tumors of the abdomen, liver, lung, neck, pancreas, kidney, and prostate. This Technical Brief will focus on SBRT.

Stereotactic Body Radiation Therapy Terms

There are several terms that have been used to describe SBRT. These terms include “stereotactic radiotherapy,” “fractionated stereotactic radiosurgery,” “hypofractionated stereotactic radiosurgery,” and “staged radiosurgery.” Within actual practice the differentiation between these various terms is blurred, with terms commonly used interchangeably. Consensus does not exist for the definition of SBRT with respect to the minimum radiation dose per fraction, or the maximum number and diameter of lesions to be treated.² However, most define SBRT as the treatment of an extracranial lesion with a single or very few (≤5) high-dose fractions.¹⁴ Based on the current working definitions of ACR, the American Society for Radiation Oncology (ASTRO), and American Medical Association (AMA) common procedural terminology (CPT) codes, this Technical Brief will use the term stereotactic body radiation therapy (SBRT).

SBRT Treatment Delivery and Treatment Planning

SBRT is characterized by patient immobilization, target localization and tracking software, limiting normal tissue exposure to high-dose radiation, preventing or accounting for organ motion (e.g., respiratory motion), the use of stereotaxy, and the subcentimeter accuracy of the delivered dose.^3,4 Factors used to determine if SBRT is an appropriate procedure include tumor shape and stage, volume (1–35 cm³),¹⁵ location, histology, invasiveness, and the performance status of the patient. The key components of a SBRT procedure are target delineation,⁵ a simulation study, treatment planning, and treatment delivery. The treatment team includes a radiation oncologist, medical physicist, radiation therapist, and, depending on the body site and indication, a diagnostic radiologist, nurse, anesthetist, and dosimetrist as needed.⁶ The treatment team may also include specialists such as surgeons.

A simulation study is performed with computed tomography (CT) prior to the treatment planning. The CT table matches the treatment table and the dataset is then imported into the treatment-planning system. Treatment planning includes patient marking (e.g., tattoos, fiducials), preplanning imaging, plan development, and patient positioning. Along with CT simulation images, the treatment-planning system may also import and fuse diagnostic magnetic resonance imaging (MRI), positron emission tomography (PET), combined PET/CT, and/or angiography images with the CT simulation images to add functional data to optimize the treatment plan. The treatment team develops a plan for the procedure using software to select the shape, size, intensity, and entry point of the radiation beam to treat the targeted tumor.

SBRT is particularly challenging because of the added complexities introduced by target motion with natural physiologic processes (e.g., respiration).¹⁶ Techniques used to assist in decreasing organ or body motion include full body immobilization (e.g., vacuum pillows),⁵ abdominal compression devices, breath-hold techniques, gating, and tracking methods. The desire to treat lesions outside of the head using the highest precision dose delivery in the setting of fractionated stereotactic radiotherapy has led to the development of image-guided radiotherapy.¹⁰ SBRT devices can use image guidance (kV or MV⁵ x-ray imaging, CT, ultrasound) to intermittently monitor the position of the targeted tumor by tracking bony structures or implanted fiducials. Imaging can also visualize soft tissues (e.g., lung, prostate) without referencing bony structures or fiducials.⁵ Before treatment begins the patient is positioned on the treatment couch with or without an immobilization device and reoriented to the SBRT system.

In order to deliver treatment accurately in accordance with the treatment plan, it is imperative to accurately position the patient on the treatment system. On-board CT images or x-ray images are acquired with the patient positioned on the treatment couch and these images are compared with the treatment plan images to ensure a match between the planning geometry and the treatment geometry. If the geometries do not match, the treatment table is adjusted so that the treatment geometry then accurately aligns with the planned geometry. If the treatment team determines a change in the tumor morphology from imaging results (e.g., CT), the treatment plan needs the capability to be modified for the new tumor morphology.⁵ However, most tumors are not going to change that much between the treatment doses of a SBRT treatment course.⁵ The treatment can then begin.

At present, a medical linear accelerator (linac) is used for the delivery of SBRT. A linac emits x-ray photon radiation with typical energies ranging from 6 to 10 MV for SBRT. The angle of the radiation beam can be changed by either the rotation of the linac gantry or by the movement of a linac mounted to a robotic arm. The treatment table can also be adjusted to allow changes in the angle of the delivery beams.⁵