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Radiation Shielding Evaluation Based On Five Years of Data From A Busy CyberKnife Center

J Yang

J Yang1,2*, J Feng2, (1) Drexel Univ., Philadelphia, PA, (2) Philadelphia Cyberknife, Havertown, pa

SU-E-T-286 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

We examined the adequacy of shielding guidelines using 5-year data from a busy CyberKnife Center.

Data from 1370 patients, treated from June 2006 through July 2011 with a total of 4900 fractions were analyzed to estimate the workload and monitor unit (MU)/dose ratio. Each of the 680,691 delivered radiation beams was simulated with Matlab software to estimate the use factor.

On average, each patient received 3.6 fractions, with average 9.1 Gy per fraction prescribed at the 71.1% isodose line, using 133.7 beams and 6200 MU. Collimator sizes for intracranial patients were usually smaller (7.5 to 20 mm) than for extracranial patients (20 to 40 mm). Eighty-four percent of the total beams exited through the floor. Due to mechanical limit, about 40% of the surrounding wall received no direct beams. For the rest of the wall, we found hot spots that received above-average MU. The distribution of the spots was correlated with the distribution to the nodes for extracranial treatments, and there were fewer hot spots for intracranial treatments. The maximum MU a spot received is less than 0.25% of total MU used for intracranial treatments and less than 1.2% of total MU used for extracranial treatments. The combination of workload, MU/dose ratio and use factor in our practice are about 10-fold less than recommendations in the CyberKnife shielding guidelines.

Conclusion:The current guidelines were found to be adequate for shielding, even in a busy center. There may be a potential to reduce shielding in areas with no or few direct beams. The shielding design using the parameters mentioned requires less shielding in term of meeting integrated dose goals, but CyberKnife treatments can result in high instantaneous dose rates to restricted portions of the surrounding walls. The uneven distribution of use factor may be considered in the vault design.

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