A Novel Method to Verify Gamma Knife Radioactive Source Count

Background: Regulation requires periodic reconfirmation of thenumber of radioactive sources in Gamma Knife (GK). To comply with regulations,the total dose output (TDO) method, originally designed to verify amount ofradiation received by patients, was adopted as a tool for source security.However, we suspect the TDO method may be unreliable in detecting small numbersof missing sources given that a 2% margin of error is allowed. We propose an alternatemethod using Gafchromic films to identify number of sources present. Method: To test the efficacy of the TDO method, we simulated real-life source loss byplugging up to 5 sources. TDO after plugging 0, 1, 2, 3, 4, and 5 sources weremeasured using GK 4C. Percentage change for each additional source plugged wascomputed. We then proceeded to test the film method. For GK 4C, 2 GafchromicRTQA2 single sided films measuring 10 × 10 inch were first halvedto 5 × 10 inchfilms. Four 5 × 10 inch films were thenadhered to the inner surface of the 18 mm collimator. For GKPerfexion, 2 Gafchromic EBT3 double sided films measuring 10 × 12 inches were combinedand rolled into a cone film and adhered to the collimator during installation.In both cases, the films were exposed to 0.5 Gy. Results: TDO wasinversely related to the number of source plugged. TDO decreased by more than2% after 5 plugs. The film method was able to outline all 201 and 192 Cosources for GK4CandPerfexion respectively. Conclusion: A single lost source is hard todetect using the TDO method given that at least 5 sources had to be lost beforethe change in TDO will exceed the margin of error (±2%). The film method bypassesthis limitation and offers an easy alternative to accurately obtain the numberof sources.