SU‐FF‐T‐49: A Study of the Transit Dose for the Leksell Gamma Knife Model 4C Using Diode and GafChromic Film in Air and Phantom

H. Mota, C. Austerlitz, H. Gay, R. Allison, C. Sibata

Research output: Contribution to journalArticlepeer-review


Purpose: To evaluate the undesirable dose to the patients due to uncollimated beam occurring in the Gamma Knife Model 4C before and after helmet docking, using diode and GafChromic film. Material and Method. The Leksell Gamma Knife model 4C was used for this study. The transit dose was measured along the Z axis (Leksell coordinate system) in air and inside the Elekta plastic phantom using diode and EBT GafChromic film. The detectors were calibrated against an Exradin‐A19 0.007 cm3 ionization chamber calibrated for AAPM‐TG‐21 protocol. The electrometer was the Max4000 (Standard Imaging). A Microtek scanner 1800f was used to scan the films. For the measurement in air, a 30 cm long plastic tube was mounted in air along the Z axis inside the helmet allowing the diode to be moved in all its extension, including the cap. The film were sandwiched in two half plastic cylinder mounted inside the helmet. A special cassette made of Mix‐D (density 0.99 g cm−3) was constructed to allow the diode slide inside the phantom along the z axis. The diode measurements were done for different exposure times, to obtain the dose for time zero by extrapolation. Results. The highest transit dose was observed in the region inside the helmet, with distribution approximately constant of 5.70 ± 0.12 cGy/shot. Inside the helmet the dose reaches another plateau of 4.72 ± 0.02 cGy/shot (10 to 15 cm from the isocenter). When the diode was positioned at the inner surface of the phantom, no significant difference from the in‐air measurements was obtained. The film results presented higher values since they were obtained with several shots and includes scatter radiation. Conclusion. The results show that a significant dose may result for several shots and confirm the values published for the model C, obtained with different methodology.

Original languageEnglish
Pages (from-to)2411-2412
Number of pages2
JournalMedical physics
Issue number6
StatePublished - Jun 2007


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