Session Item

The purpose of the study was to implement the new 2D-array 1600SRS with OCTAVIUS rotational phantom for pre-treatment verification and comparison 112 volumetric modulated arc therapy (VMAT) plans for 56 stereotactic patients. All plans used the flattening filter-free (FFF) photon beam.

The OCTAVIUS detector 1600SRS is a 2D-array with 1521 liquid-filled ionization chambers with an active volume of 2.5 x 2.5 x 0.5mm³ (0.003cm³). The main applications of the OCTAVIUS detector 1600SRS are pre-treatment plan verification in stereotactic body radiation therapy (SBRT) of small fields up to 15 x 15cm². Detector layout in the center area is 6.5cm x 6.5cm with a spacing of 2.5mm. Initially, the new 2D-array 1600SRS was tested. The linearity, dose rate, angle dependence were checked, and cross-calibration for different field sizes was done.

The study investigates verification of 56 VMAT SBRT plans (6FFF) located in lung, bone,or metastatic node. GTV volume ranged from 0.2 to 38.0 cm³. The fraction dose was 8 to 11.5Gy.

The two measurement methods: the 2D perpendicular field by field (PFF) for planar dose distribution with Varian's Portal Dosimetry (PD) and the 3D dose distribution reconstructed from the field by field measurements (RFF) OCTAVIUS detector 1600SRS combine with Octavius Rotation Unit 4D were implemented.

Gamma analysis with 5% of maximum dose threshold, local normalization with Dose Difference (DD) 2%, and Distance-to-Agreement (DTA) 2mm were done with passing rates more than 95%. The 2D gamma analysis for the Portal Dosimetry with ARIA 15.6 (Varian Med. Systems, USA) and 3D analysis for the 1600SRS detector with VeriSoft 8.0.1.0 (PTW, Freiburg) were obtained.

The agreement of the tested parameters with the producer's specifications was demonstrated. For different sizes of fields, the cross-calibration factor was designated. Average value of cross factor for field 3x3cm² was 0.949, for field 4x4cm² was 0.956 and for field 5x5cm² was 0.962. For PD verification plans, there were 10.7% incorrect plans of all. Average volumetric 3D gamma evaluation (L2%/2mm, Th 5%) results was 98.6% (±1.05%) for the Octavius plans and 98.7% (±0.89%) for PD plans. The minimum passing gamma rate for PD verification plans was 92.7%, while this value for OCTAVIUS detector was 95.4%. The percentage difference between the individual fields analyzed based on the two methods ranged from ±3%. The greatest difference was obtained for the field that did not meet the gamma results, and it was 5.3%.

Both dosimetry methods showed comparable results and can be used as an alternative to each other for verification stereotactic plans. The OCTAVIUS detector 1600SRS is more precise, characterized by high resolution and the possibility of absolute measurement. The PD method is faster than the array, but it’s less accurate. If the result of the gamma analysis is lower than 95%, additional verification should be made by using OCTAVIUS detector 1600SRS.