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Evaluation of IMRS Plans: Developing a Quantitative Means for Analyzing Single Isocenter IMRS Plans

M Oakey*, A Qureshi , D Pearson , University of Toledo, Toledo, OH


(Wednesday, 7/17/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 1

Purpose: Stereotactic Radiosurgery (SRS) is a common treatment method for multiple metastasis in the brain. A common means of evaluating these plans is the Paddick Conformity Index, the Paddick Dose Gradient Index and the RTOG Conformity Index. IMRS (intensity modulated radiosurgery) employs a single isocenter in the treatment of multiple metastases. Evaluating and comparing IMRS plans can become difficult due to contributions from other SRS sites to each other. The purpose of this study is to develop a quantitative index to evaluate the quality of an IMRS plan in place of traditional indices.

Methods: A standard patient was created with seven randomized spherical targets varying in diameters (4mm-20mm). Multiple plans were generated for each size target, prescribing 20Gy in a single fraction, using Raystation. Two methods for evaluating IMRS plans were employed. The first analysis was done by combining the individual PTVs to one large PTV. Then the Paddick indices and RTOG Conformity were applied to the combined dose statistics. The second method involved determining a defined volume to be applied to each target lesion. This volume was the exact area where the conformity and dose fall off would be evaluated.

Results: The values calculated using both analysis methods followed what would be expected of the common properties of the Paddick and RTOG indices. The gradient index was less than three for the larger lesions and trended upward for the smaller lesions. The conformity index was closer to one for larger lesions.

Conclusion: Our preliminary results showed that the two proposed evaluation methods provided an adequate analysis of the quality of an IMRS plan. The first method would be advantageous for a cluster of lesions where the isodose lines cannot be accurately separated. The second method would be more advantageous for lesions that are more spread out.


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