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Imaging the Effective Point of Measurement of a Dosimeter Inside a Phantom

S Cyriac1*, J Liu2, E Calugaru2, J Chang2,3, (1) KIMS Cancer Care and Research Center Pvt Ltd, Trivandrum, KL, IN, (2) Northwell Health and Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, Lake Success, NY, (3) Hofstra University, Hempstead, NY


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: develop an imaging method for identifying the effective point of measurement (EPM) of a detector inside the dosimetry phantom.

Methods: Leksell Gamma Knife (LGK)-solid water (SW) phantom at our institution is designed for PTW-31010 ionization chamber for reference dosimetry. When PTW-31010 is inserted into LGK-SW phantom, the center of its sensitive volume is at (x,y,z) = (100,100,100) of Leksell Coordinate System (LCS) in units of millimeters. However,(100,100,100) might not be the EPM for other detectors, particularly in the z-direction for diode detectors that have much smaller sensitive volumes. Hence, we developed a strategy for identifying the z-LCS coordinate of EPM for four (PTW-TN60008, TN60016, TN60017, and PTW-60019) diode detectors using the CBCT of Gamma Knife Icon unit. For each detector, a CBCT image was acquired and the flat face/tip of the detector was identified. The z-LCS coordinate of EPM was then determined based on the published distance between EPM and the flat face/tip. For validation, a plan was generated to measure the output of 4-mm collimator for PTW-TN60017 at various couch locations along z-axis.

Results: PTW-60019, z-LCS coordinate was at z=96.5. Since EPM of this detector is 1mm from tip, the exact EPM location was z(eff) 96.5+1=97.5, or 2.5mm from the EPM of PTW-31010. Applying the similar calculation, z-LCS coordinate was at z(eff)=98.4, 98.8, and 97.8 for respectively PTW-TN60008, TN60016, and TN60017. Center of the measured profile for PTW-TN60017 was located at z=97.7, only 0.1 mm difference from z(eff)=97.8 for this diode identified using the CBCT.

Conclusion: simple and effective imaging method was developed to determine the EPMs of diode detectors when inserted into the existing LGK-SW phantom. With the acquired information, each diode can be positioned at the focal point of the machine for accurate dosimetry, without any modification or customization of the phantom.

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Funding Support, Disclosures, and Conflict of Interest: We are grateful for the financial support from the Union for International Cancer Control (UICC), 31-33 Avenue Giuseppe Motta 1202 Geneva - Switzerland.


Dosimetry, Gamma Knife, Cone-beam CT


TH- Radiation Dose Measurement Devices: Development (new technology and techniques)

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