Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame
Introduction
Stereotactic irradiation of extracranial targets is an emerging treatment concept in clinical radiotherapy. Hypofractionated treatment with two to four fractions (10–20 Gy/fraction, normalized to the 65%-isodose), 30–75 Gy/normalized to the 80%-isodose in 5–15 fractions or single dose irradiation (14–24 Gy, normalized to the 80%-isodose) [6], [7], [10], [12], [14], [15], [25] are reported. The dose is given to macroscopic tumor not amenable to surgery or unlikely to be controlled by conventional radiotherapy. The planning target volume (PTV) encloses the tumor with a tight security margin, which accounts for target motion and positioning uncertainty. Similar to intracranial stereotactic radiotherapy the dose is prescribed to a target enclosing isodose and a substantial dose inhomogeneity with higher central and median doses in the target center is desired. Typical targets for extracranial stereotactic treatment are small tumors or recurrences of tumors of the lung (T1/2), lung metastases, liver metastases, abdominal tumors, pelvic recurrences of rectal or cervical cancer and bone metastases of relatively radioresistant primaries. Usually these targets are close to radiosensitive organs at risk as mediastinal structures, the stomach and duodenum, small or large bowel, rectum, urinary bladder, nerves or spinal cord.
Unlike the situation in intracranial stereotactic irradiation or radiosurgery, hypofractionated stereotactic radiotherapy of extracranial targets has to consider motions of the target and organs at risk. Patient movement in the immobilization device due to the lack of sharp fixation and repositioning uncertainty in a fractionated treatment have to be addressed. Evaluation of treatment precision and of risk factors for target deviations is important to define narrow security margins for highly conformal dose distribution inherent to the concept of ‘radiosurgery’ with the use of high single doses.
In our clinic stereotactic irradiation of extracranial targets was started in November 1997 adopting the method from Karolinska Hospital in Stockholm, Sweden. We use the stereotactic body frame (SBF) as well as the concept of fractionation and dose prescription created and described by Lax and Blomgren [6], [7], [14], [15]. According to the Swedish concept the SBF is not only meant to be an immobilization device but is also a philosophy for treatment application: the stereotactic system of coordinates is used as definite reference system for the target position instead of anatomical landmarks such as bony structures or skin markers often used in conventional radiotherapy.
For quality assurance and evaluation of potential pitfalls at the beginning of a newly introduced treatment modality at our institution we modified the original procedure described by Lax and Blomgren. The CT-scan prior to stereotactic irradiation was not only used for verification of the isocenter and target position. Additionally the position of bony structures and of the body outline were analyzed to evaluate the reproducibility of the patient repositioning in the SBF using the stereotactic system of the SBF as external reference system. The reproducibility of bony structures and its relation to target reproducibility was analyzed to evaluate the value of bones for isocenter verification at the linac by port films.
In this study the precision of alignment, the repositioning uncertainty and target mobility during fractionated treatment as well as the positional correlation of soft tissue GTV's with bony landmarks used in portal imaging were measured using CT-simulation and port films. These data were analyzed in relation to the PTV margin chosen. The results are reported from the first 30 consecutive patients with 32 targets, who underwent hypofractionated stereotactic irradiation of extracranial tumors at our department from November 1997 to September 1998.
Section snippets
Materials and methods
The SBF has been described in detail, as well as the methodology for extracranial stereotactic radiotherapy, by Lax et al. [14], [15]. The clinical results from the use of the SBF has been published by Blomgren et al. [6], [7]. The SBF is a U-shaped rigid plastic frame with outer dimensions of 111 cm in length, 51.2 cm in width and 36.3 cm anterior–posterior (Fig. 1). It is commercially available by ELEKTA Instruments Inc. Within the frame a vacuum pillow is attached, which is molded
Alignment
Alignment of the stereotactic coordinate system of the SBF was adequate with median and mean deviations of 0 mm with a standard deviation of 1.4 mm. Maximum longitudinal deviation in cranial direction of 2.9 and of 3.9 mm in caudal direction indicated that in single cases alignment was inadequate. Possible reasons for misalignment are shift of the SBF on the CT-couch due to rapid couch movements at the slice selection procedure or not optimal slice-selection from the CT-scout. Therefore the
Discussion
In this study a comprehensive approach was tried to evaluate the target reproducibility for stereotactic conformal radiotherapy of extracranial lesions using a rigid body frame for immobilization. This means relying on a stereotactic coordinate system of the frame rather than marks on the patient surface for treatment isocenter definition. The system studied was developed by Blomgren and Lax [6], [7], [14], [15] at Karolinska Hospital in Stockholm, Sweden. An external reference system
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