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Absolute Dose Output for a New Self-Shielded Stereotactic Radiosurgery System

S Sorensen1*, S Srivastava1 , D Pinnaduwage1 , S Jani1 , X Yan1 , M Bodduluri2 , (1) Barrow Neurological Institute (BNI) St. Joseph's Hospital & Medical Center, Phoenix, AZ, (2) Zap Surgical Systems, Inc., San Carlos, CA


(Sunday, 7/14/2019) 1:00 PM - 2:00 PM

Room: Stars at Night Ballroom 1

Purpose: To determine the absolute dose (AD) output for the Zap-X (Zap Surgical System, Inc., San Carlos, CA) system by comparing modified TG-21 and TG-51 protocols, film, and TLDs.

Methods: The current treatment planning system for Zap requires absolute output to be calculated at d(max)(~7mm) and 450mm SAD. Both N(D),(w) and N(X) calibration coefficients were measured by an accredited dosimetry calibration laboratory for a PTW 31010 chamber. An f-bracket can be mounted to the collimator where a vendor-provided acrylic hemisphere phantom (AHP) can be attached. The phantom design places the chamber center at 450mm SAD and ~6.5mm from the surface. A similar phantom (AHP2) was designed that puts the chamber at a distance of 500mm and depth of 44mm acrylic with SSD of 456mm. N(X) along with other TG-21 parameters were used to calculate the absolute dose in both phantoms. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate absolute dose utilizing the N(D),(w) and TG-51 factors. The f-bracket also allowed EBT3 film to be placed in a holder at d(max) and 450mm SAD. A calibration curve created from a TrueBeam was used to calculate absolute dose from irradiated films. A single-beam SRS phantom from MD Anderson (Houston, TX), consisting of TLDs at depths of 1.5 and 7.5cm, was irradiated and returned for analysis.

Results: AD measurements from TG-51, TG-21+AHP2, film, and TLD agreed to within ±2% of each other, while the AD of the AHP+TG-21 method was approximately 3% lower.

Conclusion: A TG-51 absolute dose calculation in water is preferred, but may not be practical due to the difficulty in setting up the tank. The TG-21 protocol using AHP2 is an accurate alternative option for routine use. All measurements were within acceptable agreement and provide confidence in the system’s AD output.


Not Applicable / None Entered.


TH- External beam- photons: Development (new technology and techniques)

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