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Elevated Dose to Normal Tissue Resulting From a Cranial Radiosurgery Error

G Ding

G Ding*, Vanderbilt University, Nashville, TN


SU-I-GPD-T-212 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: On December 29, 2010, The New York Times reported a series of radiosurgery accidents (“A pinpoint beam strays invisibly, harming instead of healing”) directly resulting in the death of one patient. It was subsequently found that a jaw setting error on a Varian LINAC with a BrainLab circular cone being used to treat trigeminal neuralgia resulted in patients receiving an elevated dose to normal tissue. In this study the treatment geometry in question was simulated to estimate the elevated dose.

Methods: Using EGSnrc Monte Carlo codes, the Varian LINAC head and BrainLab cone were simulated, with calculated doses benchmarked against measurements. Calculations were based on a realistic treatment plan for right side trigeminal neuralgia delivering 90 Gy using a 6 MV (4 mm cone) beam with 7 arcs. The calculated dose to target and normal tissue with jaws set correctly (5×5 cm²) and incorrectly (10×10 cm²) were then compared.

Results: Although the dose increase to the target with jaws set incorrectly was small, extremely high doses (> 25 Gy) occurred 5 cm away from the isocenter. The volumes of normal tissue receiving doses of 5 Gy, 10 Gy and 20 Gy with correct (incorrect) jaw settings were 3.1 cm³ (1074.0 cm³), 0.89 cm³ (222.8 cm³) and 0.27 cm³ (17.3 cm³), respectively. The mean doses to the right and left eye with correct (incorrect) jaw settings were 0.18 Gy (2.7 Gy) and 0.13 Gy (5.1 Gy), respectively.

Conclusion: The jaw setting error increased the amount of radiation emitted around outside of the cone reaching normal tissue. In this case the leakage dose to normal tissue was up to 30% of the target dose. It is worth noting that point dose measurements at the isocenter might not have detected the error.

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