Room: Exhibit Hall | Forum 9
Purpose: To estimate the optimal prescription percentage isodose surface (%IDS) while keeping prescribed target dose coverage, conformal dose distribution and favorable biological effect for stereotactic radiosurgery (SRS) of brain tumors using dynamic conformal arcs (DCA).
Methods: SRS treatment plans with prescription dose of 20Gy and 6MV photon beam in a single fraction using DCA technique were generated based on the CT scan of a Rando head phantom. A simulated single spherical brain lesion (PTV) is located at the center of the head with four different diameters: 5mm, 10mm, 20mm, and 30mm. According to previous work, 1mm dosimetric margin was chosen to compensate for up to 2mm setup error with favorable dosimetric and biological characteristics. The setup uncertainty was simulated by moving the isocenter up to 2mm - while holding all other plan parameters constant - in the anterior, superior, and lateral directions in each plan. For each plan, dose was prescribed to different %IDS, ranging from 50~98% in 1% increment, and normalized to the maximum dose point. Target Coverage (%COV), Conformity Index (CI), Gradient Index (GI), EUD-based Tumor Control Probability (TCP), and Normal Tissue Complication Probability (NTCP) were calculated for plan quality comparison.
Results: For all PTVs with 2mm setup error and 1mm dosimetric margin, the smallest %IDS for cases with %COV>99% ranges from 54% (0.5cm diameter PTV) to 79% (3cm diameter PTV). The GI local minimum value also locates between 65% (0.5cm diameter PTV) and 80% (3cm diameter PTV). Between 65 and 79%IDS, TCP*(1-NTCP) decreases as %IDS increases for all PTVs and CI<3.0 for 10, 20, 30mm diameter PTVs.
Conclusion: For up to 2mm setup uncertainty, 65~79%IDS should be chosen for single target SRS based on PTV size together with 1mm dosimetric margin to achieve both good target conformity and biological effect.