Abstract
PurposeTo develop a knowledge‐based planning (KBP) model that predicts dosimetric indices and facilitates planning in CyberKnife intracranial stereotactic radiosurgery/radiotherapy (SRS/SRT).MethodsForty CyberKnife SRS/SRT plans were retrospectively used to build a linear KBP model which correlated the equivalent radius of the PTV (req_PTV) and the equivalent radius of volume that receives a set of prescription dose (req_Vi, where Vi = V10%, V20% … V120%). To evaluate the model’s predictability, a fourfold cross‐validation was performed for dosimetric indices such as gradient measure (GM) and brain V50%. The accuracy of the prediction was quantified by the mean and the standard deviation of the difference between planned and predicted values, (i.e., ΔGM = GMpred − GMclin and fractional ΔV50% = (V50%pred − V50%clin)/V50%clin) and a coefficient of determination, R2. Then, the KBP model was incorporated into the planning for another 22 clinical cases. The training plans and the KBP test plans were compared in terms of the new conformity index (nCI) as well as the planning efficiency.ResultsOur KBP model showed desirable predictability. For the 40 training plans, the average prediction error from cross‐validation was only 0.36 ± 0.06 mm for ΔGM, and 0.12 ± 0.08 for ΔV50%. The R2 for the linear fit between req_PTV and req_vi was 0.985 ± 0.019 for isodose volumes ranging from V10% to V120%; particularly, R2 = 0.995 for V50% and R2 = 0.997 for V100%. Compared to the training plans, our KBP test plan nCI was improved from 1.31 ± 0.15 to 1.15 ± 0.08 (P < 0.0001). The efficient automatic generation of the optimization constraints by using our model requested no or little planner’s intervention.ConclusionWe demonstrated a linear KBP based on PTV volumes that accurately predicts CyberKnife SRS/SRT planning dosimetric indices and greatly helps achieve superior plan quality and planning efficiency.
Highlights
Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) are advanced and highly precise forms of radiation therapy
For the 40 training plans, the R2 for the linear fit between req_PTV and req_vi was 0.985 Æ 0.019 for isodose volumes ranging from V10% to V120% (Table 3); for V50% R2 = 0.995 and for V100% R2 = 0.997 (Fig. 1)
As to the prediction accuracy analysis using the fourfold crossvalidation method, the results were very similar to the above: the average absolute prediction error for gradient measure (GM) was 0.39 Æ 0.23 mm, and for fractional brain V50% was 0.12 Æ 0.08
Summary
Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) are advanced and highly precise forms of radiation therapy. They have been clinically used to treat intracranial tumors and functional abnormalities of the brain.[1,2,3,4] In contrast to conventional fractionated radiation therapy; SRS/SRT delivers one or a few fractions of large ablative dose to a relatively smaller target volume with sub-millimeter target localization accuracy.[5,6,7] The normal tissue sparing for the surrounding brain tissues is achieved by a very steep dose falloff outside the target regions. Favorable treatment results for brain tumors, for example, meningioma have been obtained using SRS/ SRT.[4,8,9,10,11,12,13,14,15] The dedicated machines designed for effective SRS and SRT include Gammaknife, conventional linear accelerator, and CyberKnife, which is a compact, image-guided linear accelerator with a robotic manipulator
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