Stereotactic radiotherapy of extracranial targets: CT-simulation and accuracy of treatment in the stereotactic body frame

Background and purpose: Evaluation of set-up accuracy and analysis of targe t reproducibility in the stereotactic body frame (SBF), designed by Blomgre n and Lax from Karolinska Hospital, Stockholm. Different types of targets w ere analyzed for the risk of target deviation. The correlation of target de viation to bony structures was analyzed to evaluate the value of bones as r eference structures for isocenter verification. Materials and methods: Thirty patients with 32 targets were treated in the SBF for primary or metastatic peripheral lung cancer, liver metastases, abd ominal and pelvic tumor recurrences or bone metastases. Set-up accuracy and target mobility were evaluated by CT-simulation and port films. The contou rs of the target at isocenter level, bony structures and body outline were compared by matching the CT-slices for treatment planning and simulation us ing the stereotactic coordinates of the SBF as external reference system. T he matching procedure was performed by using a 3D treatment planning progra m. Results: Set-up accuracy represented by bony structures revealed standard d eviations (SD) of 3.5 mm in longitudinal, 2.2 mm in anterior-posterior and 3.9 mm in lateral directions. Target reproducibility showed a SD of 4.4 mm in longitudinal, 3.4 mm ap and 3.3 mm in lateral direction prior to correct ion. Correlation of target deviation to bones ranged from 33% (soft tissue targets) to 100% (bones). Conclusion: A security margin of 5 mm for PTV definition is sufficient, if CT simulation is performed prior to each treatment to correct larger target deviations or set-up errors. Isocenter verification relative to bony struc tures is only safe for bony targets but not for soft tissue targets. (C) 20 00 Elsevier Science Ireland Ltd. All rights reserved.