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Small Field Dosimetry: Detector Dependency On Output Factor and Off-Axis-Ratio Measurements for a Novel Radiosurgery System

D Pinnaduwage1*, S Srivastava1 , X Yan1 , S Jani1 , C Jenkins2 , G Weidlich2 , M Bodduluri2 , S Sorensen1 , (1) Barrow Neurological Institute (BNI) St. Joseph's Hospital & Medical Center, Phoenix, AZ, (2) Zap Surgical Systems, Inc., San Carlos, CA


(Monday, 7/15/2019) 7:30 AM - 9:30 AM

Room: Stars at Night Ballroom 1

Purpose: The first clinical ZAP-X system (ZAP Surgical Systems, Inc., San Carlos, CA), a self-shielded stereotactic radiosurgery unit for intracranial treatments was installed at our institution. Here we report detector dependency for output factor (OF) and off-axis-ratio (OAR) beam profile measurements, within the context of small field dosimetry.

Methods: The ZAP-X delivery platform consists of a 2.7MV S-band linear accelerator and a tungsten collimator wheel with 8 diameter sizes ranging from 4mm to 25mm. OARs and OFs were measured using several detectors (PTW 60012 Diode, PTW 60018 SRS Diode, PTW microDiamond, Sun Nuclear EDGE) and using Gafchromic EBT3 film. Deviations in penumbra and field size (FWHM) was investigated. Beam scanning was performed using a PTW MP3-XS 3D water tank at depths=7, 50, 100, 200 and 250mm with SSD=450mm. OFs were measured at dmax=7mm with SAD=450mm. OFs were also acquired with the PTW PinPoint 31014 ionization chamber for comparison. EBT3 films were irradiated at 450mm SAD with an acrylic build-up of 6.5mm.

Results: Excluding the PinPoint chamber, the largest deviation (3.6%) in OF among the different detectors was for the 5mm collimator. OFs for all other collimators were within 2.1%. Compared to other detectors, averaged PinPoint measurements deviated by 3.9%, 10.5% and 16.1% for 7.5, 5 and 4mm collimators, respectively. Compared to the PTW 60012 diode, averaging across all FS’s and depths, FWHM and penumbra deviated by ≤0.07 and ≤0.05mm (SRS Diode), ≤0.04 and ≤0.18mm (microDiamond), ≤0.07 and ≤0.08mm (EDGE) and ≤0.17 and ≤0.04mm (film).

Conclusion: There was no significant detector dependency on FWHM and penumbra for the aforementioned detectors, and any of these could be used for OAR and OF measurements. The pinpoint chamber is not recommended for OF measurements. Gafchromic Film measurements provide an independent verification of the OFs and OARs entered in the treatment planning system.


Radiation Detectors, Stereotactic Radiosurgery, Small Fields


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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