ESTRO 2022 - Abstract Book

S1324

Abstract book

ESTRO 2022

1 Centre Intégré de santé et des services sociaux de Laval , Centre Intégré de Cancérologie de Laval, Laval, Canada

Purpose or Objective Estimate the dosimetric impact of different density override approaches for palliative patient plans using a VMAT technique optimized in RaySearch – RayStation treatment planning software. Materials and Methods The data of 10 patients was used for the study. All patients were planned in RayStation (RS) using a GE RT590 CT scan with metal artefact reduction (MAR). The most common density overrides applied in these plans were: metal, metallic rod, artefacts in soft tissue, dental implants and bones. These overrides were applied even if they were outside the PTV. For the dose comparison, all the density overrides were removed except the air in the digestive tracts and the treatment couch. The dose was then recalculated and compared to the original plan. The quantitative metrics used for dose comparison were : mean dose and V95% of the PTV. A qualitative comparison was also investigated using DVHs and the dose difference. Results Overall, no significant difference was obtained when DVHs were compared. The maximum dose difference calculated for the V95% for the PTV (plan with vs. without density overrides) was 0.09±0.04%. The maximum dose difference calculated for the mean dose to the PTV was equal to 0.12±0.08%. For certain cases where the metallic objects were inside the PTV, local dose differences inside the metallic objects of up to 5% were observed. This is due to the density selected by the user versus the saturation density of the CT scan. Conclusion No significant dosimetric improvement was observed when applying density overrides for VMAT palliative cases using metal artefact reduction CT scans. Local differences were observed and further investigation showed that density overrides improved dose calculation locally for high density materials of more or equal to 2.83g/cc (which is the saturation density of our CT scan). We found it is not necessary to override other areas like the teeth and the tissue surrounding a metallic object creating artefacts. Those overrides do not improve dose calculation and only increase the treatment planning time by 15 to 30 minutes depending on the amount of the corrections needed.

PO-1543 Validation of a liquid-filled ionization chambers 2D array for HyperArc plan verification

J. Calvo-Ortega 1 , M. POZO-MASSÓ 1 , S. MORAGUES-FEMENÍA 1 , C. LAOSA-BELLO 2 , A. ZAMORA 2

1 HOSPITAL QUIRÓNSALUD BARCELONA, ONCOLOGÍA RADIOTERÁPICA, BARCELONA, Spain; 2 HOSPITAL QUIRÓSALUD BARCELONA, ONCOLOGÍA RADIOTERÁPICA, BARCELONA, Spain

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

Materials and Methods 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. Results 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.