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Design of a Daily QA Constancy Phantom for Cyberknife M6 Robotic Radiosurgery System

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R Holla

R Holla1*, B Pillai2 , D Khanna3 , (1)Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Amrita University, India(2) Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Amrita University, India (3) Karunya University, Coimbatore, Tamilnadu


SU-I-GPD-T-536 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Performing daily QA on Cyberknife system includes routine linac output constancy and constancy check for energy. Placing the ionization chamber at 80cm SAD as specified by the equipment supplier for Cyberknife system is difficult since the system is non isocentric. However, birdcage assembly, an assembly and detector mount for holding the ionization chamber at a fixed distance of 80CM from the the radiation source can be used to monitor the output.But the measurement chamber is placed in air which is not suitable for 6MV photon beams. Hence a new phantom has been designed which can be fixed to birdcage assembly to monitor output and energy constancy in the medium (Polystyrene)

Methods: A polystyrene block of 5cm thickness is designed to fit in the distal end of the birdcage and this block is drilled to accommodate 0.13cc(PTW, Germany) ion chamber at the dmax depth of 1.5cm. The source to the chamber distance(SCD) is exactly 80cm when the phantom is placed in the birdcage assembly. Two more polystyrene phantom blocks of 3.5cm and 5 cm thickness are cut to fit inside the birdcage assembly. These 3 blocks gives us the measurements at depths of 1.5cm, 5cm and 10cm. Primary calibration has been done in water. Standard readings in the polystyrene phantom are obtained by comparison with water measurements. Measurement at 1.5cm depth corrected for temperature and pressure represents the output constancy of the Linac. The ratios of the readings 5cm and 10cm depths to 1.5 cm depth represent the constancy of beam quality.

Results: Daily measurements are performed with this phantom developed in-house.

Conclusion: Since the phantom is designed to fit inside the birdcage assembly, reproducibility of the setup is found to be very good therefore the readings will truly reflect the performance of the machine.

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