Room: Exhibit Hall
Purpose: To (1)investigate Varian SRS cone planning dose calculation and Qfix Encompass mask system (2) Evaluate novel clinical solutions for intracranial patients with small targets treated with both systems combined.
Methods: Modified body contour was adopted in SRS cone planning for anthropomorphic phantom and clinical patient, in light of three important factors (1) non-heterogeneity correction (non-HC) in Varian cone planning (2) attenuation of the posterior supporting board of QFix mask (3) facilitation of the intra-fraction optical tracking verification - OSMS. Typical 4-arcs plans with SRS cones (single-target 0.6cmx0.6cmx0.8cm, 2000cGyx1) were developed on the regular and modified body contours individually. In-house secondary dose-calc program, Verifier, featured with HC-on and HC-off, was utilized to compare the results with Varian algorithm. Qfix board attenuation was measured using Exradin A14SL chamber and estimated in computer calculation. OSMS data (translational X-Y-Z and rotational Rot-Pitch-Roll) were recorded with during treatment.
Results: On regular body contour, cone planning could lead up to 10% calculation error due to non-HC, caused by radiological depth discrepancy. With modified body contour, Varian cone planning and Verifier HC-off calculation agree within 1%. In-house HC-on dose calculation has result slightly lower than 3.6%, because Verifier HC-on is taking the bony anatomy into account. Beam attenuation from the posterior board of mask can introduce up to 3% difference on dose calculation. The board is included in body contour, as the avoidance region for arcs geometry. Modified contour does not affect the accuracy of OSMS. Data recorded were within clinical tolerance, 1mm and 1 degree, in general agreement.
Conclusion: With the features of mechanical stability, easy dosimetry and fast planning, SRS cones are beneficial, especially when lacking of HD-MLC and IMRT-QA devices. Varian cone planning and Qfix Encompass mask are feasible clinical solutions. Good understanding the TPS algorithm and performing secondary dose-calculations are essential.
Not Applicable / None Entered.