Image-guided radiotherapy via daily online cone-beam CT substantially reduces margin requirements for stereotactic lung radiotherapy

Inga S Grills, Geoffrey Hugo, Larry L Kestin, Ana Paula Galerani, K Kenneth Chao, Jennifer Wloch, Di Yan
International Journal of Radiation Oncology, Biology, Physics 2008 March 15, 70 (4): 1045-56

PURPOSE: To determine treatment accuracy and margins for stereotactic lung radiotherapy with and without cone-beam CT (CBCT) image guidance.

METHODS AND MATERIALS: Acquired for the study were 308 CBCT of 24 patients with solitary peripheral lung tumors treated with stereotactic radiotherapy. Patients were immobilized in a stereotactic body frame (SBF) or alpha-cradle and treated with image guidance using daily CBCT. Four (T1) or five (T2/metastatic) 12-Gy fractions were prescribed to the planning target volume (PTV) edge. The PTV margin was >or=5 mm depending on a pretreatment estimate of tumor excursion. Initial daily setup was according to SBF coordinates or tattoos for alpha-cradle cases. A CBCT was performed and registered to the planning CT using soft tissue registration of the target. The initial setup error/precorrection position, was recorded for the superior-inferior, anterior-posterior, and medial-lateral directions. The couch was adjusted to correct the tumor positional error. A second CBCT verified tumor position after correction. Patients were treated in the corrected position after the residual errors were <or=2 mm. A final CBCT after treatment assessed intrafraction tumor displacement.

RESULTS: The precorrection systematic (Sigma) and random errors (sigma) for the population ranged from 2-3 mm for SBF and 2-6 mm for alpha-cradle patients; postcorrection errors ranged from 0.4-1.0 mm. Calculated population margins were 9 to 13 mm (SBF) and 10-14 mm (cradle) precorrection, 1-2 mm (SBF), and 2-3 mm (cradle) postcorrection, and 2-4 mm (SBF) and 2-5 mm (cradle) posttreatment.

CONCLUSIONS: Setup for stereotactic lung radiotherapy using a SBF or alpha-cradle alone is suboptimal. CBCT image guidance significantly improves target positioning and substantially reduces required target margins and normal tissue irradiation.

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