One of the major challenges in cancer treatment is to destroy tumor cells while minimizing damage to healthy tissues. Advances in radiation oncology have made it possible to deliver radiation doses more precisely to the tumor while reducing unnecessary irradiation of surrounding healthy organs. One of the major achievements in this field is radiosurgery (Stereotactic Radiosurgery, SRS). Despite its name, radiosurgery does not usually involve conventional surgery: no incision is made, and the tumor is not surgically removed. Instead, a high dose of radiation is precisely delivered to a small, predefined target.The primary goal of this approach is to deliver radiation to tumor tissue with the highest possible precision while maximally protecting surrounding healthy tissues and critical organs.
What Does “Treatment in Millimeters” Mean?
The term “millimeter precision” does not mean that the exact location of every individual tumor cell can be visualized. Rather, it refers to the ability to determine the location and shape of a tumor with a high degree of accuracy using modern imaging and treatment-planning technologies, and to deliver the radiation dose precisely to the target volume.
Treatment Planning and Ensuring Precision
Treatment planning primarily relies on computed tomography (CT) and magnetic resonance imaging (MRI). When appropriate, PET/CT may also be used to improve assessment of tumor extent and target volume. Based on these imaging studies, the radiation oncologist defines the target as well as the healthy tissues and critical organs that need to be protected. The precision of radiosurgery is not solely attributable to advanced treatment equipment; rather, it is the result of multiple integrated steps throughout the treatment process. For example, image-guided radiation therapy (IGRT) allows the patient’s position and the location of the target to be verified immediately before treatment. This is particularly important when treating small tumors [5].
Accounting for Tumor Motion
Special consideration is required when treating tumors that move with respiration, such as lung tumors. Because the position of a tumor changes during breathing, this motion must be taken into account during treatment planning. Four-dimensional computed tomography (4D-CT) can be used to assess the position of the tumor during different phases of the respiratory cycle. When necessary, other motion-management techniques may also be used, including delivering radiation during a specific phase of respiration or temporarily holding the patient’s breath. The goal of these approaches is to deliver the radiation dose as accurately as possible to the target while minimizing unnecessary irradiation of surrounding healthy tissues.
SRS and SBRT
Stereotactic Radiosurgery (SRS) is commonly used for small, well-defined intracranial lesions. For tumors located elsewhere in the body, Stereotactic Body Radiation Therapy (SBRT) is used. SBRT is a highly precise form of radiation therapy in which a relatively high dose of radiation is delivered to a tumor in a limited number of specially planned fractions.
SBRT may be used for selected tumors of the lung, liver, spine, and other organs [4]. The underlying principle of both approaches is the same: to deliver a high radiation dose as precisely as possible to a small and clearly defined target.
Radiosurgery for Brain Metastases
One of the important applications of radiosurgery is the treatment of brain metastases. In selected patients with a limited number of appropriately sized metastases, each lesion can be treated with SRS. The 2022 ASTRO clinical practice guideline considers SRS a recommended treatment option for appropriately selected patients with 1–4 brain metastases. In certain circumstances, SRS may also be considered for patients with a greater number of metastases, including 5–10 lesions [1,2]. The choice of treatment is not based solely on the number of metastases. Their size, location, total intracranial tumor volume, the patient’s overall condition, and the extent of disease elsewhere in the body are also important considerations. In some cases, particularly when a tumor is large or causes significant mass effect, surgical intervention may be required. One of the important advantages of radiosurgery is its ability to deliver radiation highly selectively to metastatic lesions, thereby reducing unnecessary irradiation of healthy brain tissue.
Radiosurgery for Lung Cancer
An important application of the principles of radiosurgery is the treatment of early-stage non-small cell lung cancer (NSCLC). In this setting, SBRT is frequently used. It is particularly important for patients who are not suitable candidates for surgery for medical reasons. For patients who are medically and technically eligible for surgery, the choice between surgery and SBRT should be individualized [3]. In the treatment of lung tumors, respiratory motion presents a particular challenge. Therefore, the use of 4D-CT and appropriate motion-management techniques is important. This demonstrates that modern radiation therapy requires not only precise knowledge of the tumor’s location, but also an understanding of how its position changes during treatment.
Safety and Precision of Radiosurgery
Radiosurgery is delivered with a high degree of precision. Treatment accuracy is influenced by several factors, including image quality, accurate target delineation, patient positioning, organ motion, and technical treatment parameters. Therefore, the safety of radiosurgery depends not only on the treatment machine itself. Treatment planning and delivery involve a specialized multidisciplinary team that includes a radiation oncologist, medical physicist, radiation therapist, and radiotherapy technologist. Their coordinated work and continuous quality assurance are particularly important when a high radiation dose is delivered to a small target [4].
Radiosurgery and Surgical Treatment
Radiosurgery is not a universal replacement for surgery. In some situations, surgical intervention may be more appropriate—for example, in patients with large tumors or significant mass effect, where rapid tumor removal may be necessary. Conversely, for small tumors located in anatomically difficult-to-access areas, radiosurgery may represent an important treatment alternative. Therefore, in modern oncology, treatment decisions are not simply based on the question of “radiosurgery or surgery.” Instead, the most appropriate approach is determined according to the individual patient’s clinical situation. Factors considered include tumor type, size, location, extent of disease, and the patient’s overall condition.
Future Perspectives
Radiation therapy technologies continue to evolve. One important area of development is adaptive radiation therapy, in which the treatment plan can be modified during the course of treatment to account for changes in the patient’s anatomy [7]. MRI-guided radiation therapy is also advancing and provides the ability to visualize the target and surrounding tissues in greater detail during treatment. These technologies may further improve the ability to individualize treatment and adapt radiation delivery to changes occurring throughout the treatment course.
Conclusion
Radiosurgery is one of the important achievements of modern radiation oncology. Its primary advantage is not simply the use of a high radiation dose, but the ability to deliver that dose with a high degree of precision to the tumor while maximizing protection of the surrounding healthy tissues. Today, radiosurgery plays an important role in the treatment of selected patients with brain metastases, while SBRT is used for selected tumors in various parts of the body, including the lung, liver, and spine. This level of precision is based on modern imaging, accurate treatment planning, verification of patient positioning, and careful consideration of tumor motion. Thus, the concept of “cancer treatment in millimeters” reflects one of the fundamental principles of modern radiation oncology: the more accurately we can determine the location of a tumor and assess changes occurring during treatment, the more precisely and selectively we can deliver radiation to the target while maximizing the protection of healthy tissues.
Author: Nata Toidze, MD, Radiation Oncology Resident
References
- Gondi V, et al. Radiation Therapy for Brain Metastases: An ASTRO Clinical Practice Guideline. Practical Radiation Oncology. 2022;12(4):265–282.
- ASTRO/ASCO/SNO. Treatment of Brain Metastases Guideline. 2021.
- Videtic GMM, et al. Stereotactic Body Radiation Therapy for Early-Stage Non-Small Cell Lung Cancer: An ASTRO Evidence-Based Guideline. Practical Radiation Oncology. 2017;7(5):295–301.
- ASTRO. Quality and Safety Considerations in Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy: An ASTRO Safety White Paper Update. 2022.
- ASTRO. Quality and Safety Considerations in Image Guided Radiation Therapy: An ASTRO Safety White Paper Update. 2022.
- ASTRO. Safety Considerations for Intensity Modulated Radiation Therapy (IMRT): ASTRO Safety White Paper. 2022.
- ASTRO. Adaptive Radiation Therapy Safety White Paper. 2024.
