Optimizing GammaPlan treatment time

Dershan Luo1, John Eley2, Wayne Newhauser3, Paul Brown4

1Houston, United States 2UT Graduate School of Biomedical Sciences at Houston, Houston, Texas, USA 3Dept of Physics and Astronomy,Louisiana State University,Baton Rouge,Louisiana, 4Dept. of Radiation Oncology, UT M.D. Anderson Cancer Center, Houston, Texas, USA

Keywords: dose delivery, gamma knife, dose planning, technique, radiosurgery

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     The creation of a radiosurgical plan should be of the highest quality yet created for efficient radiation delivery.
     Our purpose was to determine when the treatment time of a GammaPlan can be improved without compromising the plan quality.
     The clinical plans of 188 patients with 476 brain metastases and/or surgical cavities treated on our Leksell Gamma Knife PERFEXION within a six-month period were evaluated. Patients with other diagnosis, e.g. acoustic neuroma or meningioma, and those metastatic lesions whose dose grids were overlapping, were not included.
      Each plan was prescribed by one of the six radiation oncologists using GammaPlan 8.3.1/9.0.0. All plans were normalized to 20Gy prescription (Rx) dose and 2.5 Gy/min doserate. Since the location of a lesion and a patient’s head size cannot be altered to improve TX time, quantification was attempted by placing a single shot at various locations inside a large and a small head, and treatment time was analyzed under these experimental conditions.
     The relationship between the normalized TX time and TX volume was investigated. A curve was empirically determined where about half of the lesions have lower normalized TX time than the nominal time predicted by the curve. The spread of TX time observed ranges from about 1.8 to 0.3 times the predicted. This range includes the effects of head size, which contributes to 12% of time variation, and lesion location, which can result in as large as 50% of time difference. After discounting these two quantifiable factors, the remaining spread in TX time is due to the shape of the lesion and the design of the plan, i.e., number of shots, shot size, block pattern, and even shot location, with some effect from neighboring lesions even on non-overlapping grid. After satisfying other quality metrics, a lower planned TX time indicates that the plan is likely to have reached an optimum, while a much higher planned TX time, as determined by the upper threshold, also established by this study, may signify the need for a better plan.
     This was a single center study of 6 radiation oncologists.
     A good plan has high coverage, high conformity, low gradient index, and low treatment time.
     Knowing the bounds and nominal spread of these metrics helps one determine if an optimum radiosurgery plan has been reached.


Project Roles:

D. Luo (), J. Eley (), W. Newhauser (), P. Brown ()