Dose Painting With the Gamma Knife Lightning Dose Optimizer: Technical Description and Validation of Dose Delivery

Abstract

Abstract Objective: The recently introduced Gamma Knife (GK) Lightning (Elekta) fast inverse planning dose optimizer allows concurrent optimization of multiple targets, but the optimizer’s use for generating a simultaneous integrated boost (SIB) plan has not been described and validated for accuracy of dose delivery. Here, we describe a method for creating an SIB using the GK Lightning optimizer and conduct validation of dose delivery. Materials and Methods: Radiochromic film was positioned in an anthropomorphic phantom. A 15.7-cm3 irregular contour was drawn to represent a brain metastasis resection cavity, a uniform 2-mm radial-expansion contour created, and a 1.6-cm3 contour drawn representing a nodule of residual disease within the cavity. Targets were prescribed 3 Gy (2-mm expansion), 4 Gy (cavity), and 5 Gy (residual disease) in 1 fraction. Within the GammaPlan Lightning optimizer, “beam-on time” and “low-dose” settings were iteratively adjusted to create a clinically acceptable plan. Treatment was delivered using the GK Icon system. The film was scanned and calibrated for absolute dosimetry. Global gamma index analyses were performed at various dose and distance tolerances. Results: An 18-minute treatment plan with 40 shots was delivered. Prescription isodose lines were 3 Gy at 55% (2-mm expansion), 4 Gy at 69% (resection cavity), and 5 Gy at 75% (residual disease). All target volumes had greater than or equal to 99% prescription dose coverage and the maximum dose was 6.9 Gy. Paddick conformality indices were 0.79 (2-mm expansion), 0.74 (resection cavity), and 0.15 (residual disease). Gamma index pass rate, mean, and median values were 77%, 0.68, and 0.54 at 1%/1-mm tolerance, 85%, 0.58, and 0.49 at 2%/1-mm tolerance, and 97%, 0.34, and 0.28 at 2%/2-mm tolerance. Conclusion: We successfully created an SIB plan with the GK Lightning optimizer, verifying dose delivery within clinically acceptable tolerances. Future work is needed to determine optimal dose levels for use in clinical practice and determine what disease entities may benefit from an SIB.