Purpose: The purpose of this study was to evaluate the effect of target positioning error (TPE) on dose distribution and tumor control probability (TCP) in stereotactic radiosurgery (SRS) plans for metastatic brain tumors of different sizes using the CyberKnife M6.
Methods: The reference plans in SRS were created using the circular cone of the CyberKnife M6 for each spherical planning target volume (PTV) with diameters (Φ) of 5, 10, 15, and 20 mm, contoured on CT images of a head phantom. The dose of 20 Gy per single fraction was prescribed to the 70% isodose lines, covering 95% of the PTV. Subsequently, TPE plans were created by shifting the beam center by 0.1–2.0 mm in three dimensions relative to the reference plans, using the same monitor units and beam arrangements. Dosimetric parameters such as V20Gy of the PTV, conformity index (CI) and generalized equivalent uniform dose (gEUD)-based TCP estimates were evaluated for each plan. Shifts corresponding to the TCP reduction rate of 3% were also evaluated for each PTV size.
Results: The V20Gy, CI and TCP values of the reference plans for Φ5-mm PTV were 95.1%, 0.90 and 81.4%, respectively. The corresponding values for TPE plans with 0.5-mm, 1.0-mm, and 2.0-mm shifts were 79.0%, 0.62 and 79.6%, 62.9%, 0.40 and 72.3%, and 34.2%, 0.12 and 19.5%, respectively, all of which decreased as TPE values increased. The TCP values for TPE plans with 2-mm shifts were 19.5% for φ5-mm PTV, 68.9% for Φ10-mm PTV, 74.3% for Φ15-mm PTV, and 79.0% for Φ20-mm PTV. Shifts corresponding to the TCP reduction rate of 3% were 0.6, 1.1, 1.2, and 1.6 mm, respectively.
Conclusion: The TPE had a significant effect on target dose coverage and TCP in SRS for metastatic brain tumors using the CyberKnife M6, in particular, for small PTV.
Funding Support, Disclosures, and Conflict of Interest: This study was partially supported by Grants-in-Aid for Scientific Research KAKENHI (grant nos. 20K16819) from the Japanese Society for the Promotion of Science.