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.
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.