TU‐C‐T‐617‐06: Importance of Pre‐Fraction Helical CT Isocenter Verification in Extracranial Stereotactic Radiosurgery

Purpose: To quantify the impact of pre‐fraction helical CT isocenter verification vs. setup based on planning CT in fractionated extracranial stereotactic radiosurgery. Method and Materials: Treatment plans (Elekta PrecisePlan) and pre‐fraction isocenter verification helical CT scans for 12 patients (40 fractions) were recovered from treatment plan archives. All structures were contoured by a single physician at the time of treatment. Each plan was imported into a customizable treatment plan analysis suite (CERR). Using CERR, pre‐fraction isocenter verification CT scans were fused with the original treatment plan using the external body frame as a reference. The original planned dose distribution was then translated from original treatment plan isocenter to pre‐fraction verification isocenters in each fraction. Dose and volume parameters for pertinent structures were automatically extracted using both registration methods (planned or pre‐fraction scans) for the original treatment plan and for all subsequent fractions. All patients were treated using the pre‐fraction verified isocenter rather than pre‐calculated body frame fiducials as per our institutional policies. Results: GTV volumes on pre‐fraction CTs varied from original planned GTV volume (64%–203%, mean=101.8+/−26.5%) largely due to helical sampling of a mobile target. Using the external body frame as the only setup reference would have resulted in geographic misses (<80% coverage of 95% of GTV) in 7/40 (17.5%) fractions. Pre‐fraction isocenter verification resulted in improved D95 GTV coverage (88–102%, mean=99.3% +/−2.4%) with no geographic misses. Conclusion: The current RTOG protocol (0236) evaluating extracranial stereotactic radiosurgery does not require pre‐fraction CT tumor position verification. Our institutional policy is to verify isocenter/tumor position prior to each fraction via CT. Although helical scanning artifacts are present, pre‐fraction CT‐based isocenter verification may provide more consistent tumor coverage than setup to planned body frame fiducials. Conflict of Interest: Support for this research was provided in part by Elekta, Inc.