Introduction
Stereotactic radiotherapy today is a guideline-recommended treatment for malignant or benign tumors as well as neurological functional or vascular disorders. Stereotactic radiotherapy has therefore become a standard procedure in the radiation oncology community, which is also used outside of clinical trials and outside of specialized university centers. Despite the highest level of evidence in randomized controlled trials and despite the existence of national and international practice recommendations, there is no generally accepted definition of stereotactic radiotherapy, especially since accurate image guidance has largely replaced stereotactic frames. Similarly, there are no generally accepted and comprehensive quality requirements for stereotactic radiotherapy.
This lack of consensus was identified by the German Society for Radiation Oncology (DEGRO) and therefore the DEGRO board mandated the DEGRO working group for radiosurgery and stereotactic radiotherapy to establish a statement about definition and quality requirements of stereotactic radiotherapy. Previous published guidelines of the DEGRO working group for radiosurgery and stereotactic radiotherapy [
1‐
4], the European Society of Radiation Oncology (ESTRO) [
5], the American College of Radiology (ACR), the American Society for Radiation Oncology (ASTRO) [
6,
7] and the United Kingdom Consortium for Stereotactic Radiotherapy [
8], as well as international literature and national and international recommendations formed the basis. In 2018, a statement about the definition and general quality requirements for stereotactic radiotherapy was established and published by the DEGRO society. This consensus statement was amended to the current version in collaboration with the German Society for Medical Physics (DGMP) working group for physics and technology in stereotactic radiotherapy, taking into consideration the results of an expert review on technological quality requirements for stereotactic radiotherapy [
9].
Definition for stereotactic radiotherapy
There are three types of stereotactic radiotherapy, which differ with regard to the spectrum of indication, fractionation and quality requirements: (1) stereotactic radiosurgery (SRS) as a treatment of intracranial malignant or benign tumors and functional or vascular disorders with one single irradiation fraction, (2) fractionated stereotactic radiotherapy (FSRT) of intracranial malignant or benign tumors and functional or vascular disorders as well as (3) stereotactic body radiotherapy (SBRT) of extracranial malignant or benign tumors and functional or vascular disorders.
Generally, stereotactic radiotherapy is defined as
1.a method of percutaneous external beam radiotherapy, in which
2.a clearly defined target volume
3.is treated with high precision and accuracy
4.with a biologically high radiation dose
5.in one single or a few fractions
6.with locally curative intent.
Details to (1): Stereotactic radiotherapy can be performed using either linear accelerators or dedicated stereotactic radiotherapy devices (e.g., Gamma Knife [Elekta AB, Stockholm, Sweden], CyberKnife [Accuray Inc., Sunnyvale, CA, USA], Edge [Varian Inc., Palo Alto, CA, USA] and Versa HD [Elekta AB]), which fulfill the minimal technological quality requirements as listed below.
Details to (2): The target volume is clearly defined and not characterized by a diffuse infiltration into critical serial organs-at-risk. For tumors, the target volume is confined to the macroscopic tumor and a small surrounding volume of potential microscopic tumor spread.
Details to (3): All substeps of stereotactic radiotherapy are systematically optimized and appropriate quality assurance measures have been implemented to achieve high precision and accuracy of stereotactic radiotherapy. From a clinical perspective, this includes disease staging, the interdisciplinary discussion for indicating stereotactic radiotherapy, the choice of optimal pretreatment imaging with appropriate spatial and temporal resolution for target volumes and organs-at-risk definition and highly conformal radiation treatment planning, frame-based and/or image-guided radiation delivery with active or passive motion management during treatment and close follow-up including dedicated imaging for evaluation of the treatment outcome, preferably by the treating institution. From a medical physics perspective, additional and more sophisticated quality assurance procedures and often tighter acceptance limits are necessary for stereotactic radiotherapy as compared to conventional fractionated radiotherapy.
Details to (4): Due to extreme hypofractionation, radiotherapy doses in stereotactic radiotherapy are biologically at least as high, often higher, as compared to radical radiation therapy doses using conventional fractionation.
Details to (5): Stereotactic radiotherapy doses are delivered in a few (maximum 12) fractions. A risk-adapted adjustment of the fractionation and the total dose based on the volume and location of the target is essential.
Details to (6): The primary goal of stereotactic radiotherapy is long-term local tumor control for cancer therapy or the destruction of cell and/or tissue function for functional or vascular indications with minimal risk of side effects. In most cases, this local tumor control or the destruction of cell and/or tissue function results in a higher-ranking clinical goal, e.g. symptom control or prognosis improvement of the underlying disease.