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Published on: June 7, 2015
Effects of intrafractional motion on targeting accuracy in frameless single-isocenter multi-target stereotactic
Maximilian Grohmann1,2, Cordula Petersen3, Katharina Hintelmann3
1Department of Radiotherapy and Radiation Oncology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. m.grohmann@uke.de.
Purpose:
This study evaluates the impact of intrafractional motion on targeting accuracy in frameless single-isocenter multi-target stereotactic radiosurgery (SIMT) for brain metastases and examines the necessity of intrafractional six-degrees-of-freedom (6DoF) image-guided corrections.
Methods:
A total of 120 SIMT-SRS treatments and 294 couch kicks were analyzed using positioning data from an intrafractional stereoscopic X‑ray image-guidance system. Deviations exceeding clinical tolerance (0.5 mm/0.5°) triggered repositioning. Initial and final 6DoF shifts were compared. The dosimetric impact of simulated setup errors (0.5-1 mm, 0.5°-1°) was evaluated in 15 selected SIMT-SRS cases comprising 90 metastases. Target coverage and plan quality were assessed using D98%, conformity index, and gradient index. A multivariable regression model was applied to assess the influence of target volume and distance to the isocenter on target coverage.
Results:
Intrafractional motion exceeded tolerance thresholds after 66% of couch kicks, necessitating corrections. Translational and rotational errors decreased from 0.71 ± 0.38 mm and 0.58 ± 0.34° before correction to 0.33 ± 0.19 mm and 0.38 ± 0.18° after correction. Simulated setup errors progressively impaired target coverage and conformity. For 0.5 mm/0.5° errors, D98% decreased by 7.6% ± 2.5%, conformity index declined by 9.5% ± 4.9%, and gradient index increased by 1.2% ± 2.3%. At 1 mm/1°, D98% decreased by 17.3% ± 4.8%, conformity index declined by 26.6% ± 9.2%, and gradient index increased by 1.1% ± 2.6%. Smaller targets (≤ 0.4 cm3) were particularly sensitive. Multivariable analysis identified target volume as the primary predictor of coverage loss.
Conclusion:
Intrafractional motion in frameless SIMT-SRS frequently exceeds clinical tolerance thresholds after couch kicks and can compromise both target coverage and conformity. Even small setup errors degrade plan quality, particularly for small lesions, whereas changes in dose gradient are comparatively modest within the investigated range. Intrafractional 6DoF corrections are therefore essential to reduce geometric deviations and preserve treatment quality.
