Stereotactic Body Radiation Therapy (SBRT) is an advanced, non-surgical method of delivering highly focused radiation treatments to tumors outside of the brain and spine. This technique uses sophisticated technology to target cancerous tissue with extreme precision. SBRT is also referred to as Stereotactic Ablative Radiotherapy (SABR), reflecting its goal of delivering a high enough dose of radiation to destroy the tumor, similar to a surgical ablation.
Core Principles and Mechanism of SBRT
The foundation of SBRT is the precise delivery of a very high radiation dose to a small, defined target while rapidly dropping the dose just outside the tumor boundary. This accuracy is achieved using advanced imaging technologies, such as four-dimensional computed tomography (4D CT), which maps the tumor’s movement as the patient breathes. Factoring in this motion ensures the radiation beams consistently hit the target throughout the treatment session.
SBRT uses hypofractionation, meaning the total radiation dose is divided into a significantly smaller number of treatments, or fractions, compared to standard radiation. SBRT regimens usually consist of only one to five treatments, instead of the typical 20 to 40 sessions. Each session delivers a much larger dose per fraction, often ranging from 6 to over 20 Gy, which is several times higher than conventional daily doses.
The high dose per fraction directly damages the DNA of cancer cells and induces indirect tumor cell death through unique radiobiological effects. This includes damage to the tumor’s surrounding vascular bed, which cuts off the blood supply and starves the tumor of nutrients and oxygen. This combination of direct cell killing and vascular disruption contributes to the high rate of tumor control seen with this ablative approach. The shortened schedule also limits the time cancer cells have to repopulate between treatments.
Medical Conditions Treated
SBRT is a standard treatment for several tumor types, especially for patients who are not suitable candidates for surgery due to other health conditions.
- Early-stage, non-small cell lung cancer (NSCLC), offering a highly effective, non-invasive alternative to surgical resection.
- Oligometastatic disease, where only a few small metastases are present.
- Small, isolated growths in the liver, including primary liver cancers or metastases.
- Painful or growing metastases in the spine and other bones, often providing rapid pain relief and structural control.
- Localized prostate cancer, where the ability to complete treatment in just a few sessions is a major advantage.
The Patient Treatment Process
The SBRT process begins with simulation, a detailed planning stage that involves meticulous mapping of the tumor and surrounding healthy organs. The patient undergoes specialized CT and often MRI scans while positioned in the exact setup that will be used during treatment. This imaging data allows the clinical team to define the tumor volume and calculate the optimal angles and intensities for the radiation beams.
A crucial part of simulation is creating custom-fitted immobilization devices, such as vacuum bags or body molds, to ensure the patient remains perfectly still during each session. These devices minimize movement and guarantee the planned radiation dose is delivered exactly where intended. For tumors that move with breathing, specialized techniques like abdominal compression or real-time tracking of internal markers may be used.
The actual treatment delivery is relatively quick, typically lasting between 30 and 60 minutes per session. Before the radiation beam turns on, the patient’s position is verified using real-time image-guided radiation therapy (IGRT) scans. This imaging confirms the tumor’s location, allowing for small, last-minute adjustments to maintain sub-millimeter accuracy. The radiation machine then rotates around the patient, delivering multiple beams that converge on the tumor.
SBRT vs. Conventional Radiotherapy
The fundamental difference between SBRT and conventional external beam radiation therapy (EBRT) lies in their fractionation schedules and dose distribution. Conventional EBRT typically delivers small daily doses of 1.8 to 2.0 Gy over many weeks, often totaling 20 to 40 treatment sessions. This highly fractionated approach is designed to allow healthy tissues time to repair themselves between treatments while cancer cells accumulate damage.
In contrast, SBRT employs ultra-high doses per fraction over just one to five sessions, leveraging the tumor’s unique biological response to this ablative dose. SBRT planning results in a much steeper dose fall-off profile, meaning the radiation intensity drops dramatically just outside the tumor boundary. This steep gradient minimizes the dose received by nearby healthy organs and structures, allowing the delivery of much higher total doses to the tumor without increasing toxicity. While conventional EBRT is used for a broad range of curative and palliative purposes, SBRT is primarily reserved for localized tumors where the goal is to deliver a potentially curative, ablative dose in a short period.