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Modern Review of Use Factors for Linear Accelerators

A Cetnar

A Cetnar*, D DiCostanzo , Ohio State University Medical Center, Columbus, OH


SU-K-205-15 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 205

Purpose: Shielding is crucial in radiation therapy to reduce radiation to safe levels outside of the treatment vaults. Use factors have traditionally been assumed as 0.25 at each of the primary barriers presuming the gantry is directed at each barrier for an equal amount of time. Treatment has dramatically changed in modern therapy with arc-based plans now a standard of care for patient treatment. NCRP 151, published in 2004, lists updated use factors binned in 90 and 45 degree intervals. However, this data has neither been published nor is information about the percentage of treatments involved intensity modulated techniques available. We present use factor data from nine linear accelerators over two years of modern clinical treatments.

Methods: Data was collected using the treatment planning database of patient treatments from nine linacs at a large mid-western cancer center. The data corresponds to the number of monitor units delivered at each gantry angle for clinical treatments during 2015 and 2016. This data was binned to each degree and could be sorted by machine.

Results: Data was analyzed using traditional 90 degree bins at cardinal angles. The average use factor was found to be 0.25±0.05 for each barrier. If the bins are decreased to 45 degrees, the average use factor is 0.13±0.04 per bin. Five degree bins yield an average use factor of 0.014±0.014.

Conclusion: Our results are similar to the accepted use factor assumptions with an average of 0.25 for each primary barrier. However, we find modern use factors are spread out more evenly around all gantry angles instead of being concentrated at cardinal angles. Special procedures, such as total body irradiation, should be taken into consideration for use factor calculation because this can greatly affect the use factors for a vault.

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