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Volume Effects in Radiosurgical Spinal Cord Dose Tolerance: How Small Is Small?

J Grimm

J Grimm1*, (1) Johns Hopkins University, Baltimore, MD


SU-I-GPD-T-604 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: It is known that a suitably small volume of even predominantly serial structures like spinal cord can tolerate a destructively high radiation dose per fraction. However, exact dose tolerance limits and associated volumes remain unknown. To improve the foundation of the question, this study examines uncertainty of Monte Carlo dose calculations of small volumes of spinal cord.

Methods: Radiosurgical spinal cases for the CyberKnife (Accuray Inc, Sunnyvale, Ca) at Johns Hopkins University are planned with Monte Carlo and 2% uncertainty objective. The seven most recent cases were selected to examine accuracy of the spinal cord maximum point dose calculation in comparison to other volumes, in five fractions. Each of the plans was recalculated 5 times using Monte Carlo with 2% uncertainty objective, and the Dose Volume Histogram (DVH) of all versions were compared. Dx was defined as the dose corresponding to volume x in the spinal cord DVH.

Results: Average normalized standard deviation of Dmax was twice as high as for D0.03cc, four times higher than D0.1cc, and five times higher than D1cc. The standard deviation of all volumes examined were less than the 2% uncertainty objective, but for larger volumes the uncertainty was relatively lower. Among spinal cord contours in 130 recent cases, the average ratio of D0.03cc/Dmax was 93%.

Conclusion: The maximum point dose Dmax has zero associated volume and is the most convenient "suitably small volume." However, in this study Dmax had the highest uncertainty. This study did not consider actual human dose tolerance, for which statistical dose response models have been published, but it provides insight regarding which dosimetric objectives may be most reliable, to help determine true tolerance in future studies.

Funding Support, Disclosures, and Conflict of Interest: Dr. Grimm designed and holds intellectual property rights to the DVH Evaluator software tool which is an FDA-cleared product in commercial use.

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