The Effect of Setup Uncertainty on Optimal Dosimetric Margin in Linac-based Stereotactic Radiosurgery with dynamic conformal arc technique

Purpose: Using a simulation study 1) to estimate the effect of setup uncertainty on optimal dosimetric margin by analyzing dose distribution and biological effect of LINAC-based stereotactic radiosurgery (SRS) with a dynamic conformal arc (DCA) technique; 2) to find the suitable prescription percentage isodose surface (%IDS) for a given the setup error and dosimetric margin to reach an optimal dose distribution and favorable biological effect.

Methods and materials: In this project, SRS treatment plans were made based on Rando head phantom’s computed tomography (CT) scans. Photon beam with 6 megavoltage (MV) energy was used to deliver 20 Gy prescription dose in one fraction. Plans were simulated in a commercial treatment planning system (Brainlab iPlan RT Dose ver. 4.5.4), using four non-coplanar DCAs with total gantry angles of 480 degrees. A single sphere brain lesion with 4 different diameters (5 mm, 10 mm, 20 mm, and 30 mm) was simulated at the center of the head phantom. Five plans each with different dosimetric margins (-1 mm, 0 mm, 1 mm, 2 mm, and 3 mm) were generated for each planning target volume (PTV), which is equal to the volume of the simulated lesion with a uniformly expanded margin. For each plan, the isocenter position was shifted to 15 different locations (in three orthogonal directions each with 0 mm, 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm shift) to imitate the potential setup errors in a fixed multileaf collimator (MLC) shape. To evaluate the plan quality, three dosimetric parameters: Conformity Index (CI), Heterogeneity Index (HI), Gradient Index (GI), and three biological effect parameters: generalized equivalent uniform dose (gEUD)-based Tumor Control Probability (TCP), Normal Tissue Complication Probability (NTCP), and biological objective function p+= TCP x (1-NTCP) were calculated after normalizing the dose-volume histogram for each plan to different %IDSs, ranging from 50%~98% with 1% increment.

Results: With up to a 2 mm setup error, a smaller dosimetric margin results in a smaller GI with lower p+. A larger dosimetric margin results in a larger GI. Compared to 0 mm and -1 mm dosimetric margins, a 1 mm dosimetric margin could result in a much higher p+. Compared to 2 mm and 3 mm dosimetric margins, a 1 mm dosimetric margin could result in a smaller GI while achieving an equivalent p+ in a certain range of %IDS. For a given 2 mm setup error and 1 mm dosimetric margin, an optimal %IDS range could be given by considering CI smaller than 2.5, small GI, and high p+. The %IDS ranges optimized in this simulation study for each PTV were: around 70%IDS (5 mm diameter); around 80%IDS (10 mm diameter); 63%~70%IDS (20 mm diameter); 66%~79%IDS (30 mm diameter). For the 5 mm diameter PTV, the constraint of CI smaller than 2.5 was not satisfied, compromising the dose conformity to achieve a high tumor control probability. For a given 1.5 mm setup error and a 2 mm dosimetric margin, the %IDS ranges for different PTV sizes were: 53%~68%IDS (5 mm diameter); 58%~70%IDS (10 mm diameter); 68%~75%IDS (20 mm diameter); 65%~77%IDS (30 mm diameter). For PTVs with both 5 mm and 10 mm diameters, the constraint of CI smaller than 2.5 was not satisfied, compromising the dose conformity to achieve a high tumor control probability.

Conclusion: This simulation study estimated the effect of setup uncertainty on optimal dosimetric margin for the LINAC-based SRS with the DCA technique. It also recommended the suitable prescription percentage isodose surface (%IDS) for a given setup error and dosimetric margin to reach an optimal dose distribution and favorable biological effect. With 1 mm dosimetric margin and a suitable selection of %IDS between 63%~80% based on PTV size, proper target conformity, TCP and NTCP can still be reached even with up to 2 mm of setup error.