JRSBRT 6.1, p. 55-65

Purpose: To estimate the combined effect of setup uncertainty on optimal dosimetric margin by analyzing the dose distribution and biological effect in LINAC-based stereotactic radiosurgery (SRS) with dynamic conformal arc (DCA) technique.

Methods: SRS treatment plans were generated from CT scans of the Rando head phantom using four non-coplanar DCA’s with total 480-degrees of arc. A single spherical planning target volume (PTV) of 4 different diameters was placed at the center of the phantom to simulate brain lesions. For each PTV, 5 treatment plans were created using identical dose calculation parameters, each with 5 different dosimetric margins. To simulate the effect of setup uncertainty, the isocenter for each plan was shifted to 13 different positions. A marginal dose of 20Gy in a single fraction with 6MV photon beam was prescribed to 49 different percentage isodose surfaces (%IDS). The plan quality was evaluated using Conformity Index (CI), Gradient Index (GI), EUD-based Tumor Control Probability (TCP), Normal Tissue Complication Probability (NTCP), and uncomplicated biological objective function (TCP x (1-NTCP) =p+).

Results: A +1mm dosimetric margin could result in a much higher p+ compared to 0mm and -1mm dosimetric margins and a smaller GI while achieving an equivalent p+ in a certain range of %IDS compared to +2mm and +3mm dosimetric margins. With 2mm setup error and +1mm dosimetric margin, the %IDS range optimized for each PTV is: around 80%IDS (10mm diameter); 63~70%IDS (20mm diameter); 66~79%IDS (30mm diameter).

Conclusion: This simulation study identified the preferred prescription %IDS for a given setup error and dosimetric margin to achieve an optimal dose distribution and favorable biological effect.

Keywords: stereotactic radiosurgery, dosimetric margin, setup error, tumor control probability, normal tissue sparing, conformity index

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