Abstract
To compare stereotactic radiosurgery (SRS) plan quality metrics of manual forward planning (MFP) and Elekta Fast Inverse Planning™ (FIP)-based inversely optimized plans for patients treated with Gamma Knife®. Clinically treated, MFP SRS plans for 100 consecutive patients (115 lesions; 67 metastatic and 48 benign) were replanned with the FIP dose optimizer based on a convex linear programming formulation. Comparative plans were generated to match or exceed the following metrics in order of importance: Target Coverage (TC), Paddick Conformity Index (PCI), beam-on time (BOT), and Gradient Index (GI). Plan quality metrics and delivery parameters between MFP and FIP were compared for all lesions and stratified into subgroups for further analysis. Additionally, performance of FIP for multiple punctate (<4 mm) metastatic lesions on a subset of cases was investigated. A Wilcoxon signed-rank test for non-normal distributions was used to assess the statistical differences between the MFP and FIP treatment plans. Overall, 76% (87/115) of FIP plans showed a statistically significant improvement in plan quality compared to MFP plans. As compared to MFP, FIP plans demonstrated an increase in the median PCI by 1.1% (p<0.01), a decrease in GI by 3.7% (p< 0.01), and an increase in median number of shots by 74% (p< 0.01). TC and BOT were not statistically significantly different between MFP and FIP plans (p>0.05). FIP plans showed a statistically significant increase in use of 16 mm (p< 0.01) and blocked shots (p< 0.01), with a corresponding decrease in 4 mm shots (p< 0.01). Use of multiple shots per coordinate was significantly higher in FIP plans (p<0.01). The FIP optimizer failed to generate a clinically acceptable plan in 4/115 (3.5%) lesions despite optimization parameter changes. The mean optimization time for FIP plans was 5.0 min (Range: 1.0 - 10.0 min). In the setting of multiple punctate lesions, PCI for FIP was significantly improved (p<0.01) by changing the default low-dose/BOT penalty optimization setting from a default of 50/50 to 75-85/40. FIP offers a significant reduction in manual effort for SRS treatment planning while achieving comparable plan quality to an expert planner-substantially improving overall planning efficiency. FIP plans employ a non-intuitive increased use of blocked sectors and shot-in-shot technique to achieve high quality plans. Several FIP plans failed to achieve clinically acceptable treatments and warrant further investigation.
Highlights
In light of the development of this new dose optimizer, the motivation for this study is to investigate and evaluate the quality of treatment plans produced using Fast Inverse PlanningTM (FIP) as well as understand any improvements in treatment planning time
For all patients in Group 1, with non-punctuate lesions, the FIP dose optimizer was able to generate clinical plans for 111 of 115 (96.5%) lesions that were included for analysis
For lesions in close proximity to each other with overlap of the low dose region, the first run of optimization generated a solution with suboptimal Gradient Index and selectivity
Summary
Manual forward planning (MFP) has been used where the planner manually places shots within the tumor to shape a desired isodose distribution. In this process, multiple parameters need to be determined by the planner, including the number, location, collimator settings, and relative weights of each of the shots. In the simplest of examples, one individual shot has 65,536 possible beam shapes given different selections for sector collimator size. The IP tool uses well-established metrics such as coverage, selectivity, and Gradient Index (GI) at a predetermined isodose level as well as a Beam-On Time (BOT) penalization to develop a plan solution. Its clinical use has been limited as it has not been able to consistently outperform manual planners with significant experience and still universally requires manual editing[14,15]
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