RADT-04. DOSE PAINTING WITH THE GAMMA KNIFE LIGHTNING OPTIMIZER TO ESCALATE RADIATION DOSE TO RESIDUAL TUMOR WITHIN BRAIN METASTASIS RESECTION CAVITIES

Abstract

Abstract BACKGROUND The novel Gamma Knife (GK) Lightning optimizer allows simultaneous automated optimization of multiple intracranial radiotherapy targets. We used this optimizer to create a dose-painting treatment plan where an escalated dose was prescribed to residual disease within a brain metastasis resection cavity. We validated this method by measuring dose-delivery accuracy using radiochromic film in an anthropomorphic phantom. METHODS A 15.7cc irregular contour was drawn to represent a brain metastasis resection cavity, a 2mm expansion contour created, and a 1.6cc contour drawn representing a nodule of residual disease. In GammaPlan v11.3.1, three targets were created with overlapping dose matrices. Targets were prescribed 3Gy (2mm expansion), 4Gy (resection cavity), and 5Gy (residual disease) in one fraction. On the GK Icon system, an occipital mold was made, an unexposed film placed in the phantom’s cranium, cone-beam coregistration conducted, and treatment delivered. The film was scanned and calibrated for absolute dosimetry. To evaluate dose-painting accuracy, global gamma index (GI) analyses were performed at various dose and distance tolerances, excluding points below 30% of the maximum dose. RESULTS An 18-minute treatment plan with 40 shots was created and delivered. Prescription isodose lines were 3Gy at 55% (2mm expansion, mean 4.8Gy), 4Gy at 69% (resection cavity, mean 5.2Gy), and 5Gy at 75% (residual disease, mean 6.1Gy). All target volumes had ≥ 99% prescription dose coverage and the maximum dose was 6.9Gy. Paddick conformality indices were 0.79 (2mm expansion), 0.74 (resection cavity), and 0.15 (residual disease). Gamma index pass rate/mean GI/median GI were 73%/0.68/0.55 at 1%/1mm tolerance, 85%/0.58/0.48 at 2%/1mm tolerance, and 97%/0.34/0.28 at 2%/2mm tolerance. An average of 17,883 points were evaluated per analysis. CONCLUSIONS We successfully performed GK dose painting with the Lightning optimizer, verifying dose delivery within clinically acceptable tolerances. Future work is needed to determine optimal dose levels for use in clinical practice.