Room: Exhibit Hall | Forum 4
Purpose: SRS treatments must be completed in a timely manner. One challenge to these single fraction treatments is equipment failure mid-treatment, especially if immediate repair is not possible and an identical treatment unit is not available. In this work we present the results of converting VMAT SRS AMV and brain metastasis patients from a high definition multi-leaf collimator (HD-MLC) Varian Edge linac with 2.5 mm inner leaf MLC thickness to a Millennium MLC TrueBeam with 5 mm inner leaves.
Methods: A cohort of 5 treated AVM plans (0.13-8.53 cc) and 5 treated brain metastasis (0.7-23.5 cc) plans were used in this work. The plans were put through an in-house MLC converter software developed in Python. The entire plan was converted, as a simulation of worst-case scenario. In practice, this software would be required if a plan was partially treated at which point only the portion of the arc not treated would be converted. Plans were normalized for appropriate coverage which was deemed to be PTV coverage V99.8 to the isodose prescription. New plans were then compared to the clinical plan using several dosimetric parameters including min and max dose, conformity index (CI), gradient index (GI), and V12Gy.
Results: The results of the HD-MLC plan compared to the converted Millennium MLC plan were (median [range]): Conformity Index increasing from 1.3 [1.1-1.9] to 2.3 [1.4-7.3]; Gradient Index decreasing from 4.6 [2.8-9.2] to 4.1 [2.7-6.8]. The V12Gy of the converted plan showed an increase of 2.7 [1.6-11.1] cc. The maximum dose was an average of 13% higher after the conversion.
Conclusion: The coarser MLC leaf plans showed a decrease in dosimetric quality compared to the HD-MLC plans. While the decrease in quality would have a small clinical impact, the impact of not finishing a single fraction treatment within hours would more clinically detrimental.