SU‐E‐T‐224: Considerations for the Proper Treatment of Multiple Cranial Metastases with Single Isocenter Volumetric Modulated Arc Therapy

Abstract

Purpose:To investigate some limitations of single isocenter VMAT for cranial multiple met cases.Methods:A single isocenter VMAT plan (Varian, Eclipse AAA10 commissioned down to 1 cm) was designed for two 7mm diameter spherical targets in a rectangular Solid Water (Gammex) phantom. The targets were separated by a distance of 6cm and the isocenter was centered in one of the targets. The plan was delivered (Varian, Truebeam STx) three separate times with different artificial couch angle errors of 0, 0.5 and 1 degree. The coronal dose distributions were measured with calibrated EBT3 film placed at mid‐phantom. EBT3 film dosimetry was also performed on the delivery of separate multiple arc vmat plans to targets below 6mm in diameter.Results:Measurements of the sup/inf dose profiles through the high dose distributions show no movement of the central axis high dose region and shifts of the high dose region intended for the off‐axis target. For the 1 degree rotation error, the high dose region was shifted 1.04mm from the target. This corresponds to the shift expected from triangulation (60mmxTan(1deg)=1.047mm). Furthermore, a streak of 10% interleaf leakage dose was observed and is likely a Result of the off axis target traveling a wide path such that a long length of MLC is exposed for the whole arc. The calculated dose was about 10% to 15% low compared to that measured on film for a 5mm diameter target.Conclusion:Judicious use of additional margin for off axis targets or limits on the span of multiple mets treated with one isocenter is recommended. The magnitude of the margin should be based on the rotational errors evaluated for the positioning system and the distance of the target from the isocenter. A lower limit of lesion size that can be accurately treated with VMAT should be determined.