Session Item

The aim of this work was to evaluate CyberKnife patient-specific delivery quality assurance (DQA) using film dosimetry for lung stereotactic body radiation therapy (SBRT).

In this study, 20 lung cancer patient plans were selected to conduct DQA measurements. We performed CyberKnife DQA using solid water slab phantom and Gafchromic EBT-3 film for dose verification. For phantom set-up, four fiducial markers were attached to the top of the phantom for fiducial tracking. Film was placed between the slabs. Then, CT image of the phantom was acquired using a CT simulator. The DQA plan was created by overlaying the deliverable plan on the solid water slab phantom in Multiplan treatment planning system (TPS). The central axis of the device was aligned with planning target volume center. All beams were rearranged and the dose was scaled down to 2 Gy for each plan.  The final dose calculations were performed with Ray Tracing calculation algorithm with high resolution. The DQA plan in TPS was shown in Fig.1. Gafchromic EBT-3 film was irradiated on CyberKnife G4 unit. For the calibration process, Gafchromic EBT3 films were cut into 5 × 5 cm² squares, and were placed at a 1.6 cm depth using a 0° gantry angle, SSD=100 cm, 10 × 10 cm field size, under 6 MV energy. Doses of 50, 100, 200, 300, 400, 500 were applied to obtain the calibration curve. Films were scanned using an Epson 10000XL flatbed scanner after waiting for 24 h to ensure color stabilization. The DQA planning doses were imported as Dicom RTDose  and the calculated and measured doses were analyzed using the PTW Verisoft 6.2 software program. 

In our study, the results were evaluated by applying 3% dose-difference (DD) and 3 mm distance-to-agreement (DTA) limits. Gamma comparison of measured and calculated doses showed good agreement. The gamma-passing rate ranged between 90.1% and 98.6%. 11 plans had an average passing score of greater than 95%. For the other 9 plans it was less than 95%.

Patient-specific DQA for CyberKnife lung SBRT using film was implemented in a convenient way. Also, scaling down of the dose makes the DQA process practical to prevent an additional uncertainty. This study analysis showed that the implementation of film-based DQA dose verification could be conducted truly to improve lungs treatment safety on CyberKnife.