Session Item

To validate the use of the PTW 1600 SRS 2D array for verification of HyperArc SRS plans.

The PTW 1600 SRS array was sandwiched between 3 cm of PTW polystyrene slabs (1600 SRS assembly). The assembly was CT scanned and imported into the Eclipse. 6X FFF photons beams from a TrueBeam linac equipped with a 120 MLC were used.

The angular response of the array was investigated using 6 plans consisting of a 3×3 cm2 static field calculated and irradiated on the 1600 SRS assembly (G: gantry; T: table, VARIAN IEC scale): G0T0, G45T0, G90T45, G135T0, G180T0 and G90T45. To evaluate the impact of the array angular response in a clinical scenario, a plan (VERIF plan) consisting of four non coplanar dynamic conformal arcs was designed on the 1600 SRS assembly to treat a 2 cm-diameter target in the center of the array. This plan had the G and T angles of the HyperArc technique (G: 180.1 to 179.9 and T0; G: 179.9 to 0.0 and T45; G: 0.0 to 180.1 and T315; and G: 180.1 to 0.0 and T270). The dose measured by the central detector of the array was compared with the planned dose.

Twelve clinical HyperArc plans (20 targets) were mapped to the 1600 SRS assembly such that the center of each target was virtually located at the center of the array. Film-based measurements were also done for each target by replacing the array by a plate containing a EBTXD radiochromic film. Film plane was located at the same depth as the effective measurement plane of the array. Finally, measurements were performed for each target by mapping the clinical HyperArc plan to the PTW Octavius 4D (O4D) phantom, including the 1600 array.

For each type of measurement, the coronal planar dose measured was compared to the planned one. 2D gamma index analysis using 3% local, 1 mm and a cut off of 20% of the maximum dose, were used.

1) A lack of isotropic response up to 16% was observed (Table 1). However, the VERIF plan resulted in a dose difference < 1% respect to the expected dose (Table 1).

2) Table 2 shows the gamma passing rates obtained for the 3 kinds of measurements.

The PTW 1600 SRS assembly seems adequate to verify SRS plans, as discrepancies in gamma passing rates obtained respect to the film-based measurements were generally within 5%. The O4D method, however, revealed bad results (pass rates < 90%), as compared to the film-based measurements.